Polymastiidae (Porifera: Demospongiae) of the Nordic and Siberian Seas

Polymastiidae (Porifera: Demospongiae) of the Nordic and Siberian Seas are revised and compared with the related species of the North Atlantic based on the morphological data from the type and comparative material and the molecular data from fresh samples. Twenty species from six polymastiid genera are recorded. Two species, Polymastia svenseni from Western Norway and Spinularia njordi from the Norwegian Sea, are new to science. One species, Polymastia andrica, is new to the Nordic Seas and two species, Polymastia cf. bartletti and P. penicillus, are new to the Scandinavian Coast. Distribution of the polymastiids in the North Atlantic and Arctic is discussed and the allegedly wide distribution of Spinularia sarsii and S. spinularia is questioned.

Vast marine areas north of Russia, known as the Siberian Seas, are characterized by an inhospitable environment, with, for the most part, shallow depths, a strong influx of fresh water from the great Siberian rivers and considerable temperature fluctuations between winter, when the sea surface is covered with ice and the water temperature sinks below zero, and the warm season, when the surface water layers may be heated (Coachman & Aagaard, 1974). The impact of the freshwater inflow, the ice movements and the summer heating is especially severe in the large shallow-water areas along the coast. The bottom here comprises spacious plains covered with mud and clay (Herman, 1974;Weber, 1989). Due to the unstable environment, both in the water body and on the seabed, the biodiversity, especially the diversity of sessile macrobenthos, of these areas is considerably poorer than along the coasts of the Nordic Seas (Golikov & Scarlato, 1989). On the contrary, in the offshore areas of the Siberian Seas the salinity is more stable, a branch of the Atlantic current brings warm water to the deep (Coachman & Aagaard, 1974), and the northern coast of large offshore archipelagos, e.g. Severnaya Zemlya and New Siberian Islands, is characterized by the rock cliffs and steep slopes running to great depths (Herman, 1974;Weber, 1989). These areas are oases hosting a relatively rich bottom fauna, particularly some diverse sponge communities (Golikov et al., 1990). The studies of the sponge fauna in the Siberian Seas were started by Fristedt (1887) and Levinsen (1887) and continued by Rezvoj (1924Rezvoj ( , 1928, Gorbunov (1946) and . The latter study until now remains the most comprehensive description of the Arctic sponge species, although some data presented there are obviously out of date and need a serious revision based on the modern taxonomic concepts. Among the northern seas of Russia, the White Sea, a large semi-isolated, brackish gulf of the Barents Sea, stands out for its peculiar hydrological conditions affecting the biodiversity. The deep waters of the White Sea, where the temperature is below zero all the year round, are inhabited predominantly by Arctic species. Conversely, the shallow depths, where seasonal fluctuations of the water temperature are considerable, host opportunistic Atlantic species (Babkov & Golikov, 1984). The exploration of the White Sea sponge communities begun by Merejkowsky (1878) was continued by Swarczewsky (1906),  and Ereskovsky (1993aEreskovsky ( , b, 1994aEreskovsky ( , b, 1995a. However, these unique communities still need further studies based on up-to-date approaches. Among all diverse sponge taxa inhabiting the Nordic and Siberian Seas the family Polymastiidae Gray, 1867 is one of the most common. Despite the polymastiids never reaching such large sizes as, for example, the astrophorid species do , they are subdominants of shallowwater hard bottom communities in some Norwegian fjords (Svensen, personal communication), in the White Sea (Plotkin et al., 2005) and Laptev Sea (Golikov et al., 1990). In the deep waters common polymastiids such as Tentorium semisuberites (Schmidt, 1870) and Radiella spp. are often the most frequently recorded macrobenthic species (Barthel & Tendal, 1993;Witte, 1996). Polymastiidae were described in all studies on the Nordic and Russian sponge faunas (see the references above) and these records were summarized by , who listed eight polymastiid species for the Greenland Sea, 12 species for the Norwegian and Barents Sea, four species for the White Sea and eight species for the Siberian Seas and the Arctic Ocean. The White Sea list was appended by one more polymastiid by Ereskovsky (1993b), while Plotkin (2004) provided the re-descriptions of all these species and proposed some changes in their taxonomy.
Meanwhile, the records of most species presented by  and Plotkin (2004) were based on non-type material that may question their identification. Furthermore, the polymastiids of the Scandinavian Coast and Svalbard have been never properly revised. Additionally, rich sponge samples recently taken from the poorly studied underwater mountains and vents in the Greenland and Norwegian Sea must be examined. Finally, recently recovered sponge phylogenies based on molecular data (e.g. Morrow et al., 2012Morrow et al., , 2013Redmond et al., 2013;Morrow & Cárdenas, 2015) challenge the traditional taxonomy based on morphology. Particularly they question the generally accepted concept of the relationships between the polymastiid genera  as well as between the Polymastiidae and other families (Hooper & Van Soest, 2002). A monotypic order, Polymastiida Morrow & Cárdenas, 2015, is established for the polymastiids, and a homoplasy of most morphological characters traditionally used in the taxonomy of this family and a non-monophyly of four genera from 15 polymastiid genera altogether known  are revealed (Plotkin et al., 2016b).
The aim of the present study is to revise the polymastiid fauna of the Nordic and Siberian Seas based on morphological examination of the type material and other historical collections as well as on both morphological and molecular data from fresh material. We also provide a key for identification of the polymastiid species in the area of study (Appendix 1). The area covered by the study comprises the Scandinavian Coast from the Swedish Western Coast and Southern Norway to the Norwegian-Russian border, Russian Coasts (including the White Sea) from the border to the easternmost point, Icelandic Coast, Southern and Eastern Coasts of Greenland, offshore archipelagos Svalbard, Franz Josef Land, Novaya Zemlya, Nordenskjold and Severnaya Zemlya, offshore areas of the Greenland, Norwegian, Barents, Kara, Laptev, East-Siberian and Chukchi Seas and adjacent areas of the Arctic Ocean. We also compare the Nordic and Siberian sponges with individuals from the British Isles, Canadian Atlantic Coast and some other regions in order to explore the dispersal of the species.

M A T E R I A L S A N D M E T H O D S
The study was based on historical and fresh material stored in 14 museums (Table 1). Altogether more than 1700 sponge individuals were studied (Online resource 1). The architecture of their skeletons was examined under light microscope on histological sections prepared on a precise saw with a diamond wafering blade after embedding of sponge fragments in epoxy resin as described by Boury-Esnault et al. (2002), Vacelet (2006) and Boury-Esnault & Bézac (2007). Spicules were examined under light microscope and SEM after their isolation from organic matter in nitric acid following standard procedures. The number of specimens used for spicule measurements is given in the corresponding section of the description of each species. The number of spicules of each category measured in one specimen is indicated as N. Measurements are presented as minimum -mean -maximum, unless otherwise indicated. Table 1. List of museums whose collections were used in the present study.

Museum acronym Museum title and affiliation
Genetic synapomorphies and autapomorphies of the species were defined in the 5 ′ -end barcoding region of cytochrome oxidase subunit I (CO1) and the region coding the RNA of the large ribosomal subunit (28S rDNA) from helix B10 to helix E19. The sequences, the alignments and the respective phylogenies were presented by Plotkin et al. (2016b). GenBank accessions are indicated in Online resource 1, this study. Alignments and the respective phylogenetic trees are deposited in TreeBase and available at http://purl.org/ phylo/treebase/phylows/study/TB2:S18487 (see Matrix M34248 and Tree Tr91844 for CO1, Matrix M34250 and Trees Tr91846 -Tr91847 for 28S rDNA, complete dataset, and Matrix M34256 and Tree Tr91856 for 28S rDNA fragment D1-D19 demonstrating intragenomic polymorphism). Generalized phylogeny reconstructed from the concatenated dataset is presented in Figure 1, while the main apomorphies are indicated in Online resources 2 (for CO1) and 3 (for 28S rDNA), this study. Apomorphies in 28S rDNA were defined only within the unambiguously aligned parts of the matrix (positions 1 -449, 492 -577, 585 -667, 685 -940 and 949 -2155 in the alignment). Based on the phylogenies recovered by Plotkin et al. (2016b) we accept the abandonment of Radiella Schmidt, 1870. However, we stick to the traditional taxonomy of other genera  even, if they are not monophyletic in these phylogenies, until a new classification of Polymastiidae is built.  Plotkin et al. (2016b). Complete 28S rDNA alignment is used. The original trees are available at http://purl.org/phylo/treebase/phylows/study/TB2:S18487. Branches corresponding to different individuals of the same species are collapsed. The species from the Nordic and Siberian Seas are highlighted. The following species from Plotkin et al. (2016b) are renamed according to the classification accepted in the present study: Polymastia sp. 1 as Polymastia svenseni, Radiella hemisphaerica as Polymastia hemisphaerica, Radiella sarsii as Spinularia sarsii, Radiella sp. as Spinularia njordi and Sphaerotylus sp. 2 as Sphaerotylus renoufi.
Genus Spinularia Gray, 1867 S. njordi sp. nov. S. sarsii   comb. nov. S. spinularia (Bowerbank, 1866) Genus Tentorium Vosmaer, 1887 T. semisuberites (Schmidt, 1870) Genus Weberella Vosmaer, 1885 W. bursa (Müller, 1806) Description of taxa Family POLYMASTIIDAE Gray, 1867 diagnosis Sponges of encrusting, massive, globular, hemispherical, discoid, columnar or pedunculate body shape. Oscula are often located at the summits of papillae or, sometimes, directly on the surface of the main body. Assortment of spicules comprises at least two size categories of smooth monactines. Tracts of principal monactines radiating from the sponge base or forming a reticulation constitute the main choanosomal skeleton or the innermost layer of the cortex. Auxiliary choanosomal skeleton comprises smaller spicules, freescattered or grouped in little bundles, which may be smooth monactines, smooth or acanthose oxeas, raphides in trichodragmata or astrotylostyles. A complex specialized cortical skeleton is developed to a greater or lesser degree, composed of at least a palisade of smooth tylostyles, subtylostyles, or oxeas and/or exotyles. A fringe of extra-long monactines may be present at the edge of the body where it is in contact with the substrate.

diagnosis
Polymastiidae of encrusting, massive, globular, hemispherical or discoid body shape, always bearing papillae with oscula at the summits. Main choanosomal skeleton composed of tracts of principal monactines radiating from the sponge base or forming a reticulation. Auxiliary choanosomal skeleton comprises smaller monactines, free-scattered or grouped in little bundles. Cortical skeleton constituted at least by a superficial palisade of small smooth tylostyles or subtylostyles and an internal layer of larger monactines lying obliquely to the surface and may include middle layers. A fringe of extra-long monactines may be present at the edge of the body.

discussion
Polymastia Bowerbank, 1862, with its currently accepted assortment of species , is not monophyletic as was suggested by Plotkin et al. (2012) based on morphological data and confirmed by Plotkin et al. (2016b) based on the CO1 and 28S rDNA phylogenies (see also Figure 1, this study). In both phylogenies the type species of Polymastia, P. mamillaris (Müller, 1806), formed a strongly supported clade with only five other species of this genus, P. andrica de Laubenfels, 1949, P. arctica (Merejkowsky, 1878, P. grimaldii (Topsent, 1913), P. uberrima (Schmidt, 1870) and P. thielei Koltun, 1964, along with Trichostemma hemisphaericum Sars, 1872 was in fact the type species of Trichostemma Sars, 1872 accepted as Radiella hemisphaerica at the time of the study by Plotkin et al. (2016b). However, no morphological synapomorphies of this clade could be defined. In the 28S rDNA tree a pair of unidentified species Polymastia sp. 1 and Polymastia sp. 2 was the sister to the Polymastia-clade with a strong Bayesian support. In the CO1 tree a trio of unidentified species Polymastia sp. 1, Polymastia sp. 2 and Polymastia sp. 3 was the sister to the Polymastia-clade, although with a weak support. All other Polymastia spp. including four species described in the present study, P. boletiformis (Lamarck, 1815), P. bartletti de Laubenfels, 1942, P. nivea (Hansen, 1885 and P. penicillus (Montagu, 1814), fell in the clades with the species of other genera in both molecular trees.
In the present study based on these phylogenies Trichostemma is regarded as a junior synonym of Polymastia, Polymastia sp. 1 is described as P. svenseni sp. nov., Polymastia sp. 2 is described as an unidentified species and Polymastia sp. 3 is not considered because it occurs outside the area covered by the study. Meanwhile, for the sake of taxonomic stability until a new classification of Polymastiidae is built, we retain the allocation of P. boletiformis, P. bartletti, P. nivea and P. penicillus to Polymastia, though it contradicts the molecular phylogenies. Consequently, the diagnosis of Polymastia (see above) is emended accordingly.
description External morphology Cushion-shaped sponges covering the substrate and occupying up to 6 cm 2 ( Figure 2A). Surface strongly hispid, covered with sediment, with up to several tens of cylindrical or flattened, greyish or whitish papillae which are 1-12 mm long and 1-5 mm wide. In preserved sponges exhalant and inhalant papillae do not differ in size or shape.

Anatomy
Choanosome in alcohol yellowish or greyish, dense. Main choanosomal skeleton composed of radiating tracts (88 -417 mm thick) of principal spicules crossing the cortex and forming a surface hispidation reinforced with exotyles ( Figure 2B). Ascending tracts also form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered small spicules, being especially abundant in the subcortical area. Cortex in alcohol light-coloured, firm, not detachable. Cortical skeleton constituted by a superficial palisade (116 -232 mm thick) of small spicules, a middle layer (40 -272 mm thick) of collagen fibres with low density of spicules and an internal layer (56 -170 mm thick) of tangentially arranged intermediary spicules ( Figure 2C). Skeleton of the papilla walls composed of two layers only, the superficial palisade and the internal tangential layer. Single small and intermediary spicules reinforce the bulkheads separating aquiferous canals and vestibules in the papillae.

discussion
Before our study, Polymastia andrica was recorded only from the type locality, the Gulf of St. Lawrence (de Laubenfels, 1949). We have identified as P. andrica a sponge from Newfoundland and 10 Norwegian individuals based on their morphological similarities with the material from the type locality (although the exotyles in the Norwegian specimens are shorter than those in the Canadian sponges) and the identity of CO1 from the Newfoundland specimen and the Norwegian specimens. Polymastia andrica is morphologically very similar to P. arctica and P. mamillaris, but differs from these two by the presence of exotyles. Additionally P. andrica differs from P. arctica by the absence of threads with buds at the summits of the inhalant papillae and by the absence of size difference between the inhalant and exhalant papillae. All genetic data obtained support the discrimination between P. andrica and P. mamillaris based on morphology. The morphological differences between P. andrica and P. arctica are only confirmed by the CO1 data, but not by 28S rDNA.

Anatomy
Choanosome in life orange, dense. Main choanosomal skeleton composed of radial, or longitudinal tracts (170-460 mm thick) of principal spicules branching in the subcortical area, crossing the cortex and forming a surface hispidation ( Figure 3C). Ascending tracts also form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered bundles, each of two to five small spicules, being especially abundant in the subcortical area. Cortex in life cream-coloured, firm, not detachable. Cortical skeleton constituted by a superficial palisade (180 -310 mm thick) of small spicules, a middle layer (90 -180 mm thick) of collagen fibres with low density of spicules and an internal layer (160 -250 mm thick) of tangentially arranged intermediary spicules ( Figure 3D). Skeleton of the papilla walls composed of two layers only, the superficial palisade and the internal tangential layer. Single intermediary spicules reinforce the bulkheads separating aquiferous canals and vestibules in the papillae.
Genetic data CO1 sequences obtained from five individuals of Polymastia arctica are identical, but these individuals differ in 28S rDNA (Matrix M34250 in TreeBase) and, moreover, three of them exhibit a polymorphism in this gene (Matrix M34256 in TreeBase). By both genes P. arctica is closely related to P. andrica and P. grimaldii (see the synapomorphies in the Genetic data section for P. andrica above). 28S rDNA of these three species displays a high level of intraspecific and intragenomic polymorphism, while the CO1 data are consistent (Plotkin et al., 2016b). In this gene P. arctica has two autapomorphies (Online resource 2, p. 1). Apart from them, P. arctica differs from P. andrica by 7 bps, from P. grimaldii by 11 bps and from the type species of Polymastia, P. mamillaris, by 28 bps in CO1 (Matrix M34248 in TreeBase). occurrence ( Figure 3E) Literature data: Norwegian Coast: Troms and Finnmark (73 -182 m) (as Polymastia mammilaris - Arnesen, 1918). Norwegian and Barents Sea (as P. mamillaris - . White Sea (as Rinalda arctica -Merejkowsky, 1878-Merejkowsky, , 1880as P. penicillus -Swarczewsky, 1906;as P. mamillaris -Koltun, 1966

discussion
Polymastia arctica is morphologically very similar to P. andrica and P. mamillaris. The main feature distinguishing P. arctica from the latter two is the presence of threads with buds at the summits of some inhalant papillae (Arnesen, 1918;Plotkin & Ereskovsky, 1997), although the budding intensity in the populations displays a considerable seasonal fluctuation with some individuals stopping bud formation in the warmest period (Plotkin & Ereskovsky, 1997). Additionally P. arctica differs from P. andrica by the absence of exotyles and from P. mamillaris by the relatively thicker middle cortical layer and the presence of spicules in the bulkheads separating aquiferous canals in the papillae. Some minute differences between these three species in the shape of spicules were also reported, e.g. principal spicules usually being fusiform subtylostyles in P. arctica and strongyloxeas in P. mamillaris (Plotkin & Boury-Esnault, 2004), but our study has revealed instability of this character. All genetic data obtained support the discrimination between P. arctica and P. mamillaris based on morphology. The morphological differences between P. arctica and P. andrica are only confirmed by the CO1 data, but not by 28S rDNA.
Polymastia cf. bartletti de Laubenfels, 1942 (Figure 4) type material Holotype ( In life the surface is brown and the papillae are yellowish ( Figure 4A). In alcohol both the surface and the papillae have become whitish. Swedish sponge 12 × 9 × 0.7 mm in size, with one cylindrical papilla which is 12 mm long and 1.8 mm wide ( Figure 4B). Surface and papilla are whitish in alcohol.

Anatomy
Choanosome in alcohol whitish, dense. In both sponges studied main choanosomal skeleton composed of tracts of principal spicules ( Figure 4C). The tracts, 71-135 mm thick in the middle of the body, radiate towards the base and the cortex. Ascending tracts also form a framework of the papilla skeleton. Examination of the auxiliary choanosomal skeleton and the cortex in the Swedish individual was not possible because of its small size. In the Canadian sponge the auxiliary choanosomal skeleton comprises small and intermediary spicules, most free-scattered, some in bundles of three to seven. Cortex dense, but friable, not detachable. Cortical skeleton constituted by a superficial palisade (106-166 mm thick) of small spicules, which is overlapped with an inner layer (203-286 mm thick) of criss-cross intermediary spicules ( Figure 4D).

discussion
Before our study Polymastia bartletti was known only from the type locality, the Foxe Basin (de Laubenfels, 1942). We have identified as P. bartletti a specimen from Newfoundland based on its external and anatomical similarities with the original description, and a specimen from Sweden based on the similarities of its external features and DNA with the Newfoundland sponge. But we cannot exclude that the Swedish individual may in fact represent another species since its spicules in all categories are slightly shorter than those in the Canadian individual, and the sequences of the phylogenetic markers from these sponges are not completely identical. More careful morphological examination and genetic studies of larger material are required to check this assumption. Polymastia bartletti is morphologically very similar to the NE Atlantic species, P. nivea. Discrimination between these two species is based mainly on their large genetic difference. In its turn P. nivea was often confused with P. robusta Bowerbank, 1862 andP. boletiformis (e.g. Koltun, 1966). Polymastia nivea and P. boletiformis in fact differ considerably both in morphology (Plotkin, 2004;Plotkin et al., 2012;present study) and genetics (Plotkin et al., 2016b;present study), while the status of P. robusta is questionable present study). The records of P. robusta from the Canadian Atlantic (e.g. Lambe, 1896;Whiteaves, 1901) may indicate P. bartletti, but the respective material should be re-examined to test this assumption.

External morphology
Sponges cushion-shaped, covering the substrate or massive ( Figure 5A). The largest individuals may occupy up to 100 cm 2 . Surface smooth, sometimes covered with sediment, with cylindrical or conical papillae. In living sponges both the surface and the papillae bright orange or yellow. Inhalant papillae 6-18 mm long and 2-5 mm wide. About 2-3 inhalant papillae per 1 cm 2 of the surface. Exhalant papillae 16-36 mm long and 3 -6 mm wide, with well visible oscula at the summits. A sponge may bear 1-6 exhalant papillae.

Anatomy
Choanosome in life slightly darker than cortex, crumbly. Main choanosomal skeleton composed of tracts of principal spicules 1282 alexander plotkin et al.
forming a reticulation or meanders ( Figure 5B, C). Ascending tracts form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered principal spicules. Cortex leather-like, easily detachable. Cortical skeleton constituted by a superficial palisade (80-250 mm thick) of small spicules and an internal layer (169-420 mm thick) of criss-cross principal spicules ( Figure 5C). Aquiferous cavities connected with ostia in the surface and separated by bundles of intermediary spicules lie in a space (125-400 mm thick) between the cortex and the choanosome. Both cortical layers extend to the walls of papillae. Each papilla bears several inhalant canals, and in exhalant papilla there are also one to three exhalant canals located midmost. Bulkheads separating the canals are reinforced with free-scattered principal spicules.

discussion
Polymastia boletiformis has a confused taxonomic history.
Since the description of Alcyonium boletiforme from an unknown locality by Lamarck (1815) this name had not been in use until Topsent (1933) reported that Lamarck's type material and Polymastia robusta Bowerbank, 1862 from the British Isles were the same species. Although A. boletiforme was an older name than P. robusta Topsent (1933) relegated the former to a synonym of the latter, and this was followed by most of the later authors (e.g. Arndt, 1935;Alander, 1942;Cabioch, 1968;Boury-Esnault, 1987) except for Burton (1959a) who retained the name P. boletiformis. In the recent literature (Van Soest et al., 2000;Van Soest, 2001Morrow et al., 2012;Plotkin et al., 2012) the name P. boletiformis was, however, prioritized instead of P. robusta following the principle of priority (Article 23 .1 in Anonymous, 1999).
However, Plotkin (2004) demonstrated clear morphological distinctions between P. euplectella and P. robusta, a radial choanosomal skeleton and three size categories of spicules in the former against a reticulate skeleton and two spicule categories in the latter. The present study has confirmed these differences by genetic data. Meanwhile, we have revealed the strong similarities between P. euplectella and Reniera nivea, relegating the former to a synonym of the latter (see Description of Polymastia nivea below). We can now assume that all records of P. robusta/P. boletiformis to the North and North-East from Nordmøre Coast in Norway very probably indicate P. nivea. Moreover, the present study has shown that P. bartletti, a Canadian species morphologically very similar to the Arctic-Scandinavian P. nivea, differs greatly from the latter as well as from the European P. boletiformis by genetics. We can therefore assume that the records of P. robusta/P. boletiformis from Canada may indicate P. bartletti (see the description of this species above). Finally, we have examined one of the dry syntypes of P. robusta BMNH 1930.7.3.20 and found that its choanosomal skeleton is radial as distinct from the commonly accepted definition of P. boletiformis (Boury-Esnault, 1987;Van Soest et al., 2000;Plotkin et al., 2012), but the condition of the syntypes prevents us from more detailed study. Unfortunately we have not examined P. bulbosa and P. ornata, and therefore we cannot conclude whether these two are separate species or conspecific with P. boletiformis, P. robusta or some other species.
Thus, for the moment, we gather under the name P. boletiformis South European, British and South Scandinavian Polymastia with intensive orange or yellow colour, a smooth surface with differentiated exhalant and inhalant papillae, a reticulate choanosomal skeleton and two spicule categories. These morphological similarities are confirmed by the genetic identity of the British and South Scandinavian individuals.
Radiella grimaldii (Burton, 1959a: 13;Koltun, 1964, p. 149   polymastiidae of the nordic and siberian seas 1285 papillae flattened, leaf-shaped, or sometimes cylindrical, up to 9 mm in length. Basal surface smooth, sometimes even sleek, attached to a small substrate only by a central point. Marginal fringe of extra-long spicules preventing sinking of the sponge into the sediment may be reduced in some individuals.

Anatomy
Choanosome in life pale orange or beige, firm ( Figure 6G). Main choanosomal skeleton composed of tracts (65 -655 mm thick) of principal spicules radiating from sponge base and dividing into two to four thinner tracts, which cross the upper cortex and form a surface hispidation ( Figure 6G, H). Ascending tracts also form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered small spicules, especially concentrating below the upper cortex. Cortex in life whitish, firm, not detachable ( Figure 6G). Skeleton of the upper cortex constituted by a superficial palisade (170 -210 mm thick) of small spicules, a middle layer (100 -180 mm thick) of collagen fibres with low density of spicules and an internal layer (100 -140 mm thick) of tangentially arranged intermediary spicules ( Figure 6I). Skeleton of the basal cortex (520-700 mm thick) formed by the peripheral tracts of principal spicules running parallel to the surface overlapped by a superficial palisade of small spicules and an inner confused mass of intermediary spicules ( Figure 6J). Marginal fringe composed of bundles of extra-long spicules (exotyles) embedded into the cortex. Skeleton of the papilla walls composed of the superficial palisade and the internal tangential layer. Both inhalant and exhalant papillae with single central canals.
Genetic data CO1 sequences obtained from three individuals of Polymastia grimaldii are identical. 28S rDNA available only from one of these individuals is polymorphic (Matrix M34256 in TreeBase). By both genes P. grimaldii is closely related to P. andrica and P. arctica (see the synapomorphies in the Genetic data section for P. andrica above). 28S rDNA of these three species displays intraspecific and intragenomic polymorphism, while the CO1 data are consistent (Plotkin et al., 2016b). In CO1 P. grimaldii has one autapomorphy (Online resource 2, p. 1). Apart from the latter, this species differs from P. andrica by 12 bps, from P. arctica by 12 bps and from the type species of Polymastia, P. mamillaris, by 31 bps in CO1 (Matrix M34248 in TreeBase).

discussion
Polymastia grimaldii was a key species in a long discussion on the relationships between a broadly acknowledged genus Polymastia and two genera with uncertain status, Radiella Schmidt, 1870 and Trichostemma Sars, 1872. The latter two names were since Schmidt (1880) often regarded as the synonyms for the same genus, but there were some debates about which of them should be considered as the senior name (see Discussion on Polymastia hemisphaerica (Sars, 1872) below) until Boury-Esnault (2002) relegated Trichostemma to a synonym of Radiella following the principle of priority (Article 23 .1 in Anonymous, 1999). Radiella/Trichostemma was usually distinguished from Polymastia by a radial growth pattern (a sponge attached to the substrate by a small point of the basal surface), the presence of a basal cortex distinct from the upper cortex and the presence of a fringe of extra-long monactines at the boundary between the upper and basal surface Plotkin et al., 2012). All these features are displayed by Polymastia grimaldii, but at the same time this species possesses numerous papillae and a three-layered cortex including a middle layer of collagen fibres that rather resemble the type species of Polymastia, P. mamillaris, than Radiella spp. or Trichostemma spp. (Boury-Esnault, 1987;Plotkin, 2004;Plotkin et al., 2012). Based on the similarities between P. grimaldii, P. mamillaris and other Polymastia spp. several early authors identified some sponges with evident distinctive features of Radiella/Trichostemma as P. penicillus (Vosmaer, 1882;Hansen, 1885;Fristedt, 1887;Levinsen, 1887) or P. mamillaris (Vosmaer, 1885). It was Topsent (1913) who established a new species, Trichostemma grimaldii, for the sponges combining the features of Radiella/Trichostemma and Polymastia. But, after a time, he re-considered the generic allocation of this species transferring it to Polymastia (Topsent, 1927a). In the same manner Koltun (1964) initially placed grimaldii in Radiella, but two years later  relegated it to a subspecies of Polymastia mamillaris. The uncertainty about the taxonomic affinities of P. grimaldii was perfectly expressed by Boury-Esnault (1987, p. 44): 'P. grimaldii may be considered as a step on the evolutionary line which starts at Polymastia advancing to Trichostemma'.
This uncertainty was recently resolved by the phylogenies reconstructed from CO1 and 28S rDNA datasets (Plotkin et al., 2016b), where P. grimaldii formed a clade with Radiella hemisphaerica (formerly Trichostemma hemisphaericum, the type species of Trichostemma), P. mamillaris (the type species of Polymastia) and four other Polymastia spp. At the same time two species of Radiella grouped with the type species of Spinularia Gray, 1867, S. spinularia (Bowerbank, 1866), outside the Polymastia-clade (see Discussion on the genus Spinularia below). Consequently, grimaldii and hemisphaerica are now affiliated with Polymastia.
A fringe of extra-long spicules, 4-9 mm wide, developed at the sponge edge separating the upper and basal surface.
Other sponges hemispherical or discoid, up to 65 mm in diameter, with the marginal spicule fringe up to 13 mm in width. Upper surface whitish or cream-coloured in life, knobbly, with up to 30 conical papillae. In living sponges the papillae with well visible oscula. Under sampling and preservation the papillae stretch and the oscula contract. Basal surface shaggy or hispid, attached to a small substrate.

Anatomy
Choanosome in life cream-coloured, firm. Main choanosomal skeleton of radiating tracts (240 -370 mm thick) of principal spicules dividing into two to three thinner tracts, which cross the cortex and form a surface hispidation ( Figure 10C). Ascending tracts also form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered bundles, each of two to five small spicules, being especially abundant in the subcortical area. Cortex in life whitish, firm, not detachable. Cortical skeleton constituted by a superficial palisade (120-150 mm thick) of small spicules, a middle layer (65 -105 mm thick) of collagen fibres with low density of spicules and an internal layer (85 -155 mm thick) of
tangentially arranged intermediary spicules ( Figure 10D). Skeleton of the papilla walls composed of two layers only, the superficial palisade and the internal tangential layer.
Since Boury-Esnault (1987) P. mamillaris and P. grimaldii are, however, recognized as two separate species. Later, based on a careful comparison between the holotypes of Spongia mamillaris and S. penicillus and additional material Morrow & Boury-Esnault (2000) demonstrated that P. mamillaris and P. penicillus were different species too. According to this study most of the previous records of P. mamillaris from the British Isles, French, Spanish and Portuguese Coasts in fact represent P. penicillus characterized by a twolayered cortex and spicules in all size categories being tylostyles. Polymastia mamillaris distributed only along the Swedish Coast is distinguished by a three-layered cortex (with a middle layer of collagen fibres), principal spicules being strongyloxeas and intermediary spicules being styles. Furthermore, Plotkin & Boury-Esnault (2004) proved that Polymastia arctica (originally placed in Rinalda) commonly synonymized either with P. mamillaris or with P. penicillus was actually a valid species distributed in the White and Barents Sea and distinguished by the bud formation on the papillae, relatively thick middle and intermediate layers in the cortex and the presence of spicules in the bulkheads separating canals in the papillae. Now we can finally confirm the morphological differences between P. arctica, P. grimaldii, P. mamillaris and P. penicillus by genetic evidence (Plotkin et al., 2016b;present study). Moreover, we have revealed that P. andrica erected by de Laubenfels (1949) for the Canadian records of P. mamillaris is a valid species distributed from Canada to the Norwegian Coast and Svalbard, differing from the similar Polymastia spp. by genetics and morphologically distinguished by the presence of extra-long cortical spicules (exotyles) (see the respective description above).
Other sponges cushion-shaped, covering the substrate and occupying up to 20 cm 2 . Surface smooth, free of sediment, with up to 30 papillae ( Figure 11E, F). Colour of the surface in life pale orange or pale yellow, sometimes whitish, in alcohol always becoming whitish. Papillae of most living individuals cylindrical, 8-60 mm in length and 2-8 mm in diameter, semitransparent with well-visible spicule network, oscula not visible ( Figure 11E). Some sponges with much smaller (2 -6 mm in length and 1-4 mm in diameter) papillae of the same colouration as the surface ( Figure 11F).

Spicules
(Measurements based on four specimens, individual variation presented in Table 4

discussion
Describing his new species, Reniera nivea from the Norwegian Sea, Hansen (1885, p. 5) apparently confused individuals with quite different morphology, pyriform sponges ('isolated individuals' in his interpretation, depicted in pl. I figure 6a in his paper) and encrusting sponges with papillae ('collections of individuals on plates' depicted in pl. I figure 6b).  synonymized Hansen's sponges bearing papillae with Polymastia robusta and pyriform sponges with Quasillina brevis. Later P. robusta was synonymized with P. boletiformis (Topsent, 1933; see also Discussion on the latter species above in the present paper). Meanwhile, a new species P. euplectella strongly resembling Hansen's sponges with papillae was described from the Barents Sea by Rezvoj (1927).  relegated P. euplectella to a synonym of P. robusta, but Plotkin (2004) resurrected the status of P. euplectella. Based on the re-examination of the type material of both P. euplectella and R. nivea (except for Hansen's pyriform sponges which are considered as lost) we can conclude that they belong to the same species, which gets the name Polymastia nivea in accordance with the principle of priority (Anonymous, 1999). This species is distinguished from P. boletiformis by its pale colouration, longer papillae, all without visible oscula, longitudinal choanosomal skeleton, overlapping spicule layers in the cortex and the presence of three spicule categories. Meanwhile, P. nivea is morphologically very similar to P. bartletti (see the description of the latter above). Genetic data clearly indicate that P. bartletti, P. boletiformis and P. nivea are three separate species (Plotkin et al., 2016b).

Anatomy
Choanosome in alcohol firm, cream-coloured. Main choanosomal skeleton composed of longitudinal or radial tracts (125 -360 mm thick) of principal spicules fanning under the cortex, crossing the cortex and forming a fine surface hispidation ( Figure 13C). Auxiliary choanosomal skeleton comprises free-scattered intermediary spicules. Cortex in alcohol whitish, firm, not detachable. Cortical skeleton constituted by a superficial palisade (110-140 mm thick) of small spicules and an internal layer (350 -500 mm thick) of tangentially arranged intermediary spicules ( Figure 13C). Skeleton of the papilla walls is a framework of the tracts ascending from the choanosome and covered with the cortical layers.

discussion
Polymastia penicillus was for a long time confused with the sympatric species P. mamillaris (Johnston, 1842;Bowerbank, 1862;Gray, 1867;Vosmaer, 1885;Boury-Esnault, 1987) and P. grimaldii (Vosmaer, 1882;Fristedt, 1887;Levinsen, 1887) until Morrow & Boury-Esnault (2000) demonstrated that these three were separate species (see the detailed taxonomic history in Discussion on P. mamillaris above), that is now confirmed by genetic data (Plotkin et al., 2016b;present study). Morphologically P. penicillus is distinguished by a surface smoother than in P. mamillaris and P. grimaldii and by a two-layered cortex against the three-layered cortex in the latter two species. At the same time P. penicillus and P. mamillaris share the encrusting growth pattern and the presence of just three spicule categories that differentiate them from P. grimaldii. Minor differences between P. penicillus and P. mamillaris in spicule shape emphasized by Morrow & Boury-Esnault (2000) appear to be unstable.
Polymastia svenseni sp. nov. (Figure 14) type material etymology Named after Erling Svensen, a Norwegian underwater photographer, who has discovered a large population of this species in Stavanger.

External morphology
Both holotype and paratype cushion-shaped, removed from a rock cliff. Surface, for the most part, smooth, cream-coloured in alcohol, with sparse rests of sediment and papillae, which may be conical tapering towards the summits, cylindrical or leaf-shaped expanding towards the summits. Holotype 105 × 52 × 13 mm, with 157 papillae 2-13 mm long and 1.5 -5.5 mm ( Figure 14A). Paratype 67 × 44 × 12 mm, with 76 papillae 2 -15 mm long and 1.4 -6.7 mm wide. Other sponges cushion-shaped, occupying up to 250 cm 2 of the substrate ( Figure 14B). Surface cream-coloured or whitish in life, smooth, partly covered with sediment, with up to 400 papillae tapering towards the summits. Few papillae with visible oscula at the summits.

Anatomy
Choanosome in life pale orange or yellowish, firm. Main choanosomal skeleton composed of tracts (112 -321 mm thick) of large spicules radiating from the base and ending in the cortex ( Figure 14C). Ascending tracts also form a framework of the papilla skeleton. Auxiliary choanosomal skeleton comprises free-scattered large and small spicules. Cortex in life whitish, firm, not detachable. Cortical skeleton constituted by a superficial palisade (220 -240 mm thick) of small spicules, a middle layer (130-150 mm thick) and an internal layer (100-125 mm thick), both composed of criss-cross large spicules lying loosely in the middle layer and much more condensed in the internal layer ( Figure 14C). Skeleton of the papilla walls composed of two cortical layers, the superficial palisade and the internal layer.

type material
Lectotype (designated herein, see figure 1c in Plotkin, 2004 and Figure 15A  description

External morphology
Lectotype globular, about 50 mm in diameter, attached to small stones ( Figure 15A). Surface velvety, pale beige with sparsely scattered small brown stains, bearing 18 papillae. The papillae crater-shaped (very short and wide) or conical, 1.5 -5.5 mm long and 1.7 -9.2 mm wide at base, most with well-visible oscula at the summits. Paratype ZIN RAS 10638 irregularly ovoid, 81 × 60 mm in diameter, split across, removed from the substrate ( Figure 15B). Surface velvety, pale brown with darker stains, bearing 23 wart-like small papillae, the largest with oscula at the summits. Paratypes ZIN RAS 10639 are small, massive sponges on stones or removed from the substrates ( Figure 15C -H). Surface, for the most part, velvety or knobbly, porous in some individuals, brownish, with one or few conical or crater-shaped papillae, all with oscula at the summits. Some paratypes with minute hispidation around the base. Other sponges massive, fistshaped, globular or ovoid, up to 250 mm in diameter ( Figure 15I). Surface knobbly, velvety or smooth, with wellvisible ostia. Colour in life whitish, cream-coloured or beige, occasionally with large dark brown spots. In alcohol the colouration darkens and the ostia contract. Up to 30 papillae, all with oscula at the summits, crater-shaped in life and often stretching under sampling.  10.5-11.2-12.3 mm, N ¼ 300.

polymastiidae of the nordic and siberian seas
Genetic data CO1 sequences obtained from five individuals of Polymastia thielei are identical. 28S rDNA was obtained only from one of these sponges. By both genes P. thielei is closely related to morphologically quite different P. hemisphaerica (Plotkin et al., 2016b). The synapomorphies and differences between these species are described above in the Genetic data section for P. hemisphaerica.

discussion
Before Koltun (1964) established Polymastia thielei the sponges with the characteristic features of this species had usually been identified as P. uberrima because of some external similarity between these species (e.g. Hansen, 1885;Thiele, 1903;Lundbeck, 1909;Topsent, 1913). Polymastia thielei also displays some similarities in body shape and architecture of the cortex with Weberella bursa (Müller, 1806). The main distinctive features of P. thielei are much smaller number of papillae and their conspicuous crater-like shape. Weberella bursa differs from both P. thielei and P. uberrima by the pale colouration of both the cortex and the choanosome, much thinner cortex, the reticulate choanosomal skeleton and the presence of just two size categories of spicules. Polymastia uberrima is distinguished by the presence of a marginal hispid collar absent in both P. thielei and W. bursa and the regular radial choanosomal skeleton, while in P. thielei the choanosomal skeleton is less regular, with a tendency to meandering and anastomosing. The morphological differences between P. thielei, P. uberrima and W. bursa are clearly confirmed by the genetic data.

type material
Holotype of Polymastia uberrima ( Figure 17A): ZMUC-DEM-395, Iceland, precise locality unknown. Lectotype (designated herein, Figure 17B, C) and two paralectotypes of Polymastia infrapilosa Topsent, 1927: MOM 04-1449, SE of Halifax, Nova Scotia, Canada, 44810 ′ N 62827.5 ′ W, 75 m, Scientific campaigns by Albert the 1st of Monaco, station 3425, 13.08.1913. Herein P. infrapilosa is Fig. 16. Polymastia thielei, distribution: black star, locality of lectotype; white stars, localities of paralectotypes; white diamond, locality of MOM 04-0851 identified as P. uberrima by Topsent (1913) and re-identified as P. thielei herein; white circles, our data.  Figure 17A). Surface, for the most part, smooth, cream-coloured, with about 20 papillae and a minutely hispid greyish edging, 5-6 mm wide, on the undamaged side. Most papillae wart-like, weakly developed, but some conical, up to 5 mm long and 7 mm wide at base, with visible oscula at the summits. Lectotype and paralectotypes of Polymastia infrapilosa massive, removed from the substrates. Surface, for the most part, smooth, cream- coloured, with a minute hispidation, 6-7 mm wide, along the undamaged edge and conical, occasionally cylindrical, papillae bearing oscula at the summits. Lectotype 42 × 38 × 21 mm, split across, with 49 papillae, which are 2-12 mm long, 2-8 mm wide at base and 1-2 mm wide at summit ( Figure 17B, C). Paralectotypes 36 × 30 × 16 mm, with 23 papillae, and 37 × 29.5 × 10 mm, with 30 papillae. The latter with several surface protuberances, each bearing one to three buds, 2-3 mm in diameter. Other sponges massive or globular, occupying up to 100 mm 2 of the substrate. Except for a marginal collar, the surface smooth or velvety, whitish in life, but often darkening in alcohol, with up to 150 conical or cylindrical papillae, in life all with visible oscula. The marginal collar is a rough, hispid or even shaggy area of various widths ( Figure 17D).

Anatomy
Choanosome yellowish in life, but may become dark brown in alcohol, dense. Main choanosomal skeleton composed of tracts (207 -571 mm thick) of principal spicules radiating from the sponge base and occasionally ramifying under the cortex ( Figure 17E, F). Most tracts end in the cortex except for the marginal area where they protrude above the surface. Some tracts ascend to the papillae ( Figure 17F). Auxiliary choanosomal skeleton comprises small bundles of intermediary spicules, especially abundant in a subcortical area (423-1484 mm thick). Cortex in life cream-coloured, firm, not detachable. Cortical skeleton includes a superficial palisade (201 -257 mm thick) of small spicules, a middle layer of loosely and confusedly lying bundles of intermediary spicules (672 -1437 mm thick) and an internal layer (181-326 mm thick) with a high concentration of criss-cross intermediary spicules. In the areas under the papillae the middle layer contains aquiferous cavities connected with the surface ostia ( Figure 17F). The papillae walls reinforced with the ascending choanosomal tracts and covered with the superficial palisade and internal cortical layer. Several exhalant and inhalant canals running into the papillae are separated by bulkheads reinforced with the ascending tracts and free-scattered intermediary spicules.

Spicules
(Measurements based on nine specimens, individual variation presented in Table 6) Principal spicules -styles to subtylostyles, straight, fusiform.

discussion
Polymastia uberrima exhibits some similarity with P. thielei in external morphology, but is well-distinguished by the presence of a marginal more or less hispid collar. This collar may be destroyed during sampling, and that might be the reason why some early authors did not distinguish between P. uberrima and P. thielei (von Marenzeller, 1878;Hansen, 1885;Thiele, 1903;Lundbeck, 1909;Topsent, 1913;Boury-Esnault, 1987). Furthermore, P. uberrima differs greatly from P. thielei by the architecture of the cortex and choanosomal skeleton (see Discussion on P. thielei above) and these morphological distinctions between the two species are confirmed by genetic data. At the same time P. infrapilosa established by Topsent (1927b) for Polymastia possessing the marginal collar in fact has no differences from P. uberrima and hence is regarded as a junior synonym of P. uberrima.

External morphology
Encrusting sponge, about 12 mm in diameter and 5 mm thick, removed from the substrate ( Figure 19A). Surface strongly hispid, covered with sediment, bearing a conical papilla, 16.5 mm long and 4.6 mm wide at base, with a small osculum at the summit.

Anatomy
Choanosome in alcohol pale orange, firm. Main choanosomal skeleton composed of radiating tracts (92 -106 mm thick) of principal spicules crossing the cortex and forming a surface hispidation reinforced with exotyles ( Figure 19B). Auxiliary

Genetic data
The Norwegian Polymastia sp. described above is genetically very close to P. svenseni and an unidentified Canadian Polymastia (Plotkin et al., 2016b). The relationships of this trio with other polymastiids and the synapomorphies between the Norwegian Polymastia sp. and P. svenseni are described in the Genetic data section for the latter species above. Three synapomorphies in CO1 distinguish the Norwegian Polymastia sp. and the Canadian Polymastia sp.

discussion
The sponge described above is placed in Polymastia based on the CO1 and 28S rDNA phylogenies. Meanwhile, it cannot be identified as any known Polymastia spp. It strongly resembles Polymastia andrica by the presence of four categories of spicules and a surface hispidation formed by the tracts of principal spicules crossing the cortex and reinforced with exotyles. However, in the molecular phylogenies Polymastia sp. is closely related to the morphologically quite distinct P. svenseni, while P. andrica falls in another subclade. We assume that Polymastia sp. may be a species new to science, but postpone the formal erection of this species until more material in addition to the single tiny individual becomes available.     . Phylogenies based on CO1 and 28S rDNA confirmed that Quasillina is a polymastiid (Plotkin et al., 2016b; Figure 1 in this study), but no molecular data on Ridleia are available so far.

type material
Lectotype of Polymastia brevis (designated herein, Figure 21A): BMNH 1910. 1.1.5 -6 (dry specimen), Shetland, UK, precise locality unknown, 109 -164 m. Paralectotypes of Polymastia brevis: BMNH 1910. 1.1.5 -6 (six intact specimens and several fragments, all dry), BMNH 1898. 5.7.59 (two specimens in alcohol) and BMNH 1910. 1 description External morphology Lectotype of Polymastia brevis has an irregularly ovoid main body, 25 mm in diameter and 28 mm high, sitting on a stalk, 11 mm high and 7 mm in diameter and bearing a well developed cylindrical papilla, 5 mm long and 3 mm in diameter, with an osculum at the summit ( Figure 21A). Paralectotypes smaller, some with weakly developed papillae, others lacking papillae and bearing oscula directly on the body summits. Most other sponges columnar, up to 35 mm high and up to 8 mm in diameter, with no segregation between main body and stalk ( Figure 21B). Few sponges with ovoid or pyriform main bodies sitting on stalks. Papillae, if present, are weakly developed. Colour in life pale orange or yellowish. Syntypes of Quasillina richardi are small fragments, one on a stalk lacking the major part of the main body, the other being a residual of the main body.

Anatomy
In life choanosome is an intensive yellow or orange, unstructured, semi-fluid mass without any spicule tracts. This mass is often washed away under preservation. Choanosomal skeleton comprises free-scattered bundles of small spicules ( Figure 21C). Cortex in life pale orange or pale yellow, leatherlike. Cortical skeleton comprises a superficial palisade (150-200 mm thick) of bouquets of small spicules and two inner overlapping layers, a layer of criss-cross large spicules (270-300 mm thick) located midst in the cortex and an internal layer (35 -80 mm thick) of longitudinal tracts of large spicules ( Figure 21D). Skeleton of the papilla wall is the same as the cortical skeleton ( Figure 21E).

discussion
Quasillina brevis is recorded in a wide geographic area, but there is some doubt whether all these records indicate the same species. Topsent (1913) established a new species of Quasillina, Q. richardi, for the sponges from the Northern Norwegian Sea based on their difference from Q. brevis from more southern areas (British Isles, French Coast, Azores). In the former species small spicules were bent in the distal part, while in the latter they were straight. However, the correlation of the spicule shape in Quasillina to geography was questioned by several records of both Q. brevis and Q. richardi from the same localities, e.g. from Iceland (Burton, 1959a) and the Swedish Western Coast (Alander, 1942). In our material from the Nordic Seas we have found both individuals with bent and straight small spicules, without any correlation to geography. Therefore, until molecular data on the British, South European and Azorean Quasillina become available and comparable with our data on the Nordic sponges, we follow  and Plotkin (2004) and conclude that Q. richardi should be regarded as a junior synonym of Q. brevis.

diagnosis
Polymastiidae of hemispherical, dome, cushion-like or button-like body shape, always with papillae. Spicule assortment comprises smooth monactines in at least two size categories and exotyles with ornamented distal extremities, which may be rough, spined, granulated, tuberculated or wrinkled, often with knobs varying from spherical to hemispherical, fungiform, umbrelliform or lobate. Main choanosomal skeleton composed of radial or longitudinal tracts of principal monactines. Auxiliary choanosomal skeleton comprises free-scattered, small and/or intermediary monactines, and, occasionally exotyles. A superficial cortical palisade composed of either exotyles with sparse small monactines or small monactines reinforced with sparse exotyles. An inner layer of criss-cross intermediary monactines may also be present.

polymastiidae of the nordic and siberian seas 1307
Sphaerotylus borealis (Rezvoj, 1928, p. 78, figures 4 & 5; description (according to Plotkin et al., 2016a) External morphology Sponges thickly encrusting or cushion-shaped, the largest up to 100 cm 2 . Surface shaggy, silted with sediment making it dirty greyish or brownish in colour. Up to 50 cylindrical or conical papillae, whitish in life, but usually becoming pale yellow, brownish or pinkish in alcohol. On soft bottoms living sponges are often completely covered by sediment with only erect papillae protruding above ( Figure 23A). On hard bottoms the sponges may contract the papillae. After sampling and fixation the papillae always considerably contract and invaginate into the surface hispidation. Oscula not visible in preserved sponges.

Skeleton
Choanosome in life orange, dense. Main choanosomal skeleton composed of longitudinal tracts of principal spicules which cross the cortex and make up a dense and thick surface hispidation ( Figure 23B). Auxiliary choanosomal skeleton comprises small, occasionally intermediary, spicules often arranged in bundles, 3-7 spicules each. Cortex in life whitish, dense, not detachable. Cortical skeleton composed of a 115 -120 mm thick superficial palisade of small spicules and an internal layer ( 210 mm thick) of tangentially arranged intermediary spicules ( Figure 23B). In areas around papillae these layers are separated by an intermediate, aspicular zone (about 100 mm thick). Exotyles cross the cortex and join the surface hispidation. Walls of papillae lack the tangential cortical layer. Single intermediary spicules scattered both in the walls and in the bulkheads between canals.
Exotyles slender, 5100 -6117-7520 mm long, usually with weakly developed or completely reduced proximal tyles. Shafts 13.8-17.2-20 mm in maximum diameter. Distal knobs (14. 1-19.9 -27.0 mm in diameter) usually irregularly fungiform or umbrelliform ( Figure 23C), more rarely hemispherical or spherical, occasionally with short protuberances on the edges, sometimes slightly displaced along the shaft or comprising several swellings. Surface of the knobs and the

Genetic data
Of the three individuals of Sphaerotylus borealis, from which genetic data were obtained, two sponges differ neither by CO1, nor by 28S rDNA. The third individual is distinguished by two bps in 28S rDNA (Matrix M34250 in TreeBase), but no CO1 was obtained from it. In both CO1 and 28S rDNA phylogenies S. borealis does not group with any of the congeners (Plotkin et al., 2016b
Surface velvety, knobbly, dark brown in colour, with up to 30 papillae ( Figure 25A). Papillae of living sponges whitish or pale yellow in colour, conical, with small, scarcely visible oscula at the summits. In alcohol-preserved specimens the papillae may be considerably contracted looking like tubercles, while their colour does not change much.

Skeleton
Choanosome in life yellowish or pale orange, dense. Main choanosomal skeleton composed of radial or longitudinal tracts of principal spicules which enter the cortex ( Figure 25B). Auxiliary choanosomal skeleton comprises small and intermediary spicules usually scattered singly or sometimes arranged in small groups. Besides these spicules some individuals possess exotyles between the choanosomal tracts ( Figure 25C). Cortex in life whitish, firm, not detachable. Cortical skeleton composed of a superficial palisade ( 110 mm thick) of small spicules, an internal layer (about 170 mm thick) of tangentially arranged intermediary spicules and a middle layer (180-190 mm thick) with a low concentration of spicules ( Figure 25D). Exotyles cross the cortex forming a dense superficial layer with their distal knobs rising above the palisade. Papillae walls without internal cortical layer. Single intermediary spicules scattered both in the papillae walls and in the bulkheads between the canals.

discussion
Sphaerotylus capitatus is a well-defined species and identification of it usually causes no difficulties. Meanwhile, molecular data on a larger set of species are required for reconstruction of the relationships between S. capitatus and its congeners.

diagnosis
Polymastiidae of discoid, hemispherical, lenticular or cushionlike body shape with a shaggy or minutely hispid surface and one to 15 weakly developed exhalant papillae. Main choanosomal skeleton composed of longitudinal or radial tracts of principal monactines crossing the cortex. Auxiliary choanosomal skeleton comprises free-scattered small and/or intermediary Fig. 26. Sphaerotylus capitatus, distribution: black star, type locality; black diamond, data from Topsent (1913); white diamond, data from ; white circles, our data. polymastiidae of the nordic and siberian seas 1311 (sub)tylostyles and may also include raphides in trichodragmata. Cortical skeleton may, in addition to the superficial palisade of small tylostyles, include extra spicule layers. Basal cortex, if present, reinforced with the peripheral tracts of principal spicules lying parallel to the surface. A spicule fringe is always present at the edge of the body.

discussion
Taxonomic history of Spinularia is closely related to Radiella and Trichostemma. Spinularia was established by Gray (1867) Stephens (1915) who defined the main feature distinguishing this genus from Polymastia, Halicnemia and Radiella, the presence of raphides (filiform oxea-like spicules) packed in trichodragmata. In the same paper Stephens stated that the type species of Rhaphidorus Topsent, 1898, R. setosus Topsent, 1898, which also possessed raphides in trichodramata, is synonymous with S. spinularia, and, consequently, Rhaphidorus was synonymized with Spinularia. The actions performed by Stephens were acknowledged by most subsequent authors Alander, 1942;Lévi, 1993;Plotkin et al., 2012). Since Tethea spinularia was the type species of Spinularia, Radiella sol was designated as the type species of Radiella (Boury-Esnault, 2002). Trichostemma was established by Sars (1872) for his new species T. hemisphaericum Sars, 1872, although these names first appeared in a faunistic list three years earlier (Sars, 1869). Schmidt (1880) considered T. hemisphaericum to be a synonym of Radiella sol and, consequently, Trichostemma to be a synonym of Radiella. However, most subsequent authors recognized T. hemisphaericum and R. sol as different species, although they agreed that Trichostemma and Radiella were synonymous, and there were debates on which of these names preceded until Boury-Esnault (2002) acknowledged the synonymization of Trichostemma with Radiella based on the principle of priority (for references and other details see Discussion on Polymastia hemisphaerica above).
The relationships between Polymastia, Spinularia, Radiella and Trichostemma were re-considered by Plotkin et al. (2016b). Based on the CO1 and 28S rDNA phylogenies Radiella sarsii and Radiella sp. (here described as Spinularia njordi sp. nov., see below) forming a strongly supported clade with Spinularia spinularia (Figure 1) were transferred to Spinularia. Conversely, Trichostemma hemisphaericum, falling into a clade with the type species of Polymastia (P. mamillaris), was transferred to Polymastia. Consequently, Trichostemma Sars, 1872 was regarded as a junior synonym of Polymastia Bowerbank, 1862 (Plotkin et al., 2016b). These actions are followed in the present study, and the definition of Spinularia is extended to cover the species lacking raphides in trichodragmata (see Diagnosis above). Spinularia australis Lévi, 1993, a species from New Caledonia originally placed in Spinularia, perfectly fits with this diagnosis. It possesses raphides like S. spinularia and, at the same time, resembles S. sarsii by body shape and the presence of basal cortex. We also agree that Rhaphidorus is a synonym of Spinularia, although we question the synonymization of R. setosus with S. spinularia (for details see Discussion on S. spinularia below).
Meanwhile, the status of the type species of Radiella, R. sol, remains uncertain. Its type material is lost  and no fresh material is available. The age-old non-type specimen identified as R. sol by Schmidt (1880) and re-described by  resembles Polymastia hemisphaerica by its relatively narrow marginal spicule fringe (Plotkin & Janussen, 2008;Plotkin et al., 2012). But it does not match the drawing in the original description of R. sol (Schmidt, 1870: pl. 4, figure 6), which displays a wider marginal fringe rather resembling Spinularia sarsii (Plotkin & Janussen, 2008;Plotkin et al., 2012). However, regardless of the relationships of R. sol with P. hemisphaerica and S. sarsii, Radiella is abandoned since both Polymastia and Spinularia are older names (Plotkin et al., 2016b).

description
External morphology Holotype and both paratypes hemispherical, with a minutely and unevenly hispid surface edged with a narrow spicule fringe and bearing a single weakly developed papilla with an osculum at the summit. Holotype 16-17 mm in diameter and 9 mm in height, attached to a volcanic porous concretion ( Figure 27A). Surface dirty grey, with a whitish papilla. Paralectotype ZMBN 098041 17 -18 mm in diameter and 6 mm in height, attached to a basalt piece ( Figure 27B). Surface dirty grey, with a whitish papilla. Paratype ZMBN 098038 was, before dissection, 11 mm in diameter and 4 mm in height, attached to small gravels. Surface was brownish, with a yellowish papilla located in a small depression. Other sponges hemispherical, lenticular or irregular, up to 24 mm in diameter. Surface, for the most part, hispid, occasionally shaggy, dark brown or dark grey because of the covering sediment. A single papilla and the surrounding area on the body top, which may be gently depressed, are smooth and pale in colour. Marginal spicule fringe up to 3 mm wide in the largest individuals.

Anatomy
Choanosome in alcohol light or dark brown, dense. Main choanosomal skeleton composed of tracts (96 -310 mm thick) of principal spicules radiating from the basal area, where the sponge is attached to the substrate, occasionally ramifying, crossing the cortex and protruding above the surface ( Figure 27C). At the body edge the protruding tracts form a prominent fringe. Some tracts ascend to the papilla walls. Auxiliary choanosomal skeleton comprises freescattered small and intermediary spicules, the latter occasionally gathered in small bundles. Cortex in alcohol whitish or pale brown, firm, not detachable. Cortical skeleton comprises a superficial palisade (217 -300 mm thick) of small spicules. In the central area of the upper cortex there is also an internal layer (116 -494 mm thick) of criss-cross intermediary spicules. Under the papilla the palisade and the internal layer are separated by an aquiferous cavity ( 268 -810 mm in diameter).

discussion
Spinularia njordi resembles the type species of Spinularia, S. spinularia, by the encrusting growth pattern, the consequent absence of the basal cortex and the relatively small marginal fringe composed of the spicules of the same category as those forming the main choanosomal tracts. These features distinguish S. njordi and S. spinularia from other Spinularia spp. At the same time S. njordi differs from S. spinularia by the shaggy surface, the presence of an additional cortical layer made of intermediary monactines and the absence of raphides in trichodragmata in the choanosome. These features rather resemble S. sarsii. Based on the absence of raphides S. njordi and S. sarsii were provisionally allocated to Radiella, as Radiella sp. and R. sarsii respectively (Plotkin et al., 2016b). However, based on the 28S rDNA phylogeny and the identity of the CO1 5 ′ -end barcodes from these species and S. spinularia, they were transferred to Spinularia (Plotkin et al., 2016b). Spinularia njordi is established as a new species primarily based on its autapomorphies in 28S rDNA (see the Genetic data above).
Spinularia sarsii  ( Figure 29) description External morphology Lectotype discoid, flattened, with hispid, grey upper and basal surfaces and a marginal spicule fringe ( Figure 29A, B). Body 9 mm in diameter (excluding the fringe) and 6 mm thick. Upper surface bears four tiny papillae with oscula at the summits ( Figure 29A). Basal surface without any trace of the substrate ( Figure 29B). Fringe 1.5 -4 mm wide. All paralectotypes externally resemble the lectotype. Paralectotype BMNH 1887. 5.2.38.2 is a body sector with a 3.0 -3.5 mm wide fringe and without papillae. Paralectotype BMNH 1887. 5.2.40 discoid, 9 mm in diameter, with a convex, smooth basal surface and a flattened, hispid upper surface bearing a single papilla damaged at the summit ( Figure 29E, F). Fringe 1.5-2.5 mm wide. One of the paralectotypes BMNH 1887. 5.2.61 flattened, irregular, 6 × 8 mm, with hispid upper and basal surfaces and without visible papillae. Fringe 1-2 mm wide. The other paralectotype BMNH 1887. 5.2.61 discoid, flattened, 4 mm in diameter, with an almost smooth basal surface and a hispid upper surface bearing a single papilla. Fringe 0.7 -3.0 mm wide. Paralectotype BMNH 1887. 5.2.66 discoid, 5.0 -5.5 mm in diameter, with a convex, almost smooth basal surface and a flattened, strongly hispid upper surface lacking visible papillae ( Figure 29C, D). Fringe 1.3 -2.6 mm wide. Other sponges discoid, lenticular, hemispherical, occasionally irregular, up to 13 mm in diameter ( Figure 29G, H). Upper surface flattened, hispid or shaggy, covered with sediment, bearing a single papilla, more rarely few, small papillae with oscula at the summits ( Figure 29G). Basal surface often convex, smooth, attached to the substrate with a central point ( Figure 29H). A small substrate may be plunged into the basal cortex. Some individuals are free of any substrates. Width of the marginal fringe may reach half of body diameter.

Anatomy
Choanosome in alcohol light or dark brown, dense. Main choanosomal skeleton composed of tracts (119 -283 mm thick) of principal spicules radiating from the central basal point, crossing the upper cortex and protruding above the surface ( Figure 29I). Some tracts ascend to the papilla walls. Auxiliary choanosomal skeleton comprises free-scattered bundles of intermediary spicules. Cortex in alcohol whitish, firm, not detachable. Skeleton of the upper cortex comprises a superficial palisade (about 300 mm thick) of small spicules, a middle space (340 -405 mm thick) with low concentration of spicules except for ascending choanosomal tracts and an internal layer (100 -110 mm thick) of criss-cross intermediary spicules. Skeleton of the basal cortex formed of peripheral tracts of principal spicules running parallel to the surface and free-scattered single small spicules. Extra-long spicules (exotyles) composing the marginal fringe are embedded in the tracts.

Genetic data
Certain genetic differences are revealed between the morphologically very similar Spinularia sarsii from Norway and Spinularia cf. sarsii from Mozambique. Data obtained from two Norwegian individuals are identical in both genes studied. Moreover, the CO1 sequences from these individuals are identical to those from the type species of Spinularia, S. spinularia, and S. njordi and display one synapomorphy distinguishing them from other polymastiids including the Mozambican S. cf. sarsii (Online resource 2, p. 5). In 28S rDNA the Norwegian S. sarsii differs from S. spinularia by 6 bps and from S. njordi by 12 bps (Matrix M34250 in TreeBase) and shares with them two synapomorphies Fig. 29. Spinularia sarsii, type series, habitus: (A) lectotype BMNH 1887. 5.2.38.1, view from above; (B) the same, bottom view; (C) paralectotype BMNH 1887. 5.2.66, view from above; (D) the same, bottom view; (E) paralectotype BMNH 1887. 5.2.40, view (Topsent, 1892, Cape Verde (1209 -1417 m) , Madeira (2380 -3118) , Moroccan Coast/Saharan Upwelling (851 -2142 m) Boury-Esnault et al., 1994). Mediterranean Sea: Iberian Sea (1020 -1580 m) . Arctic Ocean, Greenland Sea and Norwegian Sea (800-2892 m) (Gorbunov, 1946;Koltun, 1964Plotkin, 2004). Barents Sea,Kara Sea and Laptev Sea (from 145 m and deeper)    . Radiella sarsii perfectly fitted with this definition (Plotkin, 2004;Plotkin et al., 2012). The marginal spicule fringe was also typical of another genus, Spinularia, but the latter was distinguished from Radiella by the presence of raphides in trichodragmata . However, based on the CO1 and 28S rDNA phylogenies, Plotkin et al. (2016b) transferred R. sarsii to Spinularia (for details see Discussion on the genus Spinularia and S. njordi above). The other problem with Spinularia sarsii is the allegedly cosmopolitan distribution reported for this species. Its type localities are such remote regions as Azores and Australia . Besides these S. sarsii is recorded from quite remote localities in the North Atlantic (Newfoundland (Topsent, 1892), Azores (Topsent, 1892, Cape Verdi , Madeira , Moroccan Coast Boury-Esnault et al., 1994), West Mediterranean Sea ), Fig. 30. Spinularia sarsii, distribution: black stars, type localities; white squares, data from Boury-Esnault et al. (1994); black trefoils, data from Burton (1959b); white triangles, data from Topsent (1892; white hearts, data from ; white circles, our data from the Nordic Seas; black circle, our data from Mozambique. polymastiidae of the nordic and siberian seas 1317 Arctic Ocean, Greenland and Norwegian Sea (Gorbunov, 1946;Plotkin, 2004) and Indian Ocean (Mozambique (Plotkin et al., 2016b), Zanzibar (Burton, 1959b), Saya de Malha (Dendy, 1922), South off Maldives (Burton, 1959b)). This list may also be appended with the records of Radiella sol from the Barents, Kara andLaptev Sea by Koltun (1964, 1966) and Trichostemma sol from the Norwegian -Greenland Sea by Barthel & Tendal (1993), who at that time regarded Radiella sarsii to be a synonym of Radiella sol, which was in fact originally described from the Mexican Gulf (Schmidt, 1870). Furthermore, Van Soest et al. (2016 assumed that Suberites alveus Hansen, 1885 and Suberites conica Hansen, 1885 from the Norwegian Sea were very probably conspecific with Spinularia sarsii, just displaying variation of the body shape resembling a hive-like cone in S. alveus and a flattened cone in S. conica. The cosmopolitan distribution of Spinularia sarsii is questioned by Plotkin et al. (2016b) and the present study based on genetic data. In the 28S rDNA phylogeny morphologically very similar S. sarsii (former Radiella sarsii) from the Norwegian Sea and S. cf. sarsii from the Mozambican Coast (former Radiella cf. sarsii) do not group together, although they fall in the same clade (Plotkin et al., 2016b). Furthermore, the Mozambican sponge is distinguished from the congeners in CO1, whereas the sequences of this phylogenetic marker obtained from the Norwegian S. sarsii, S. spinularia and S. njordi are identical. Based on these data the records of S. sarsii from the northern and southern hemisphere are assumed to represent two different species. However, herein we do not formally establish a new species for the Mozambican individual because the locality is outside our study area and more material is required for a careful morphological description. At the same time there are no genetic data on S. sarsii from North Atlantic regions other than the Norwegian Sea, while the comparison of the Norwegian S. sarsii with the type material from Azores reveals no morphological differences. Neither can we check whether Suberites alveus and Suberites conica from the Norwegian Sea are indeed conspecific with Spinularia sarsii because the type material of the first two species is unfortunately lost. Therefore, for the moment we consider all records of Spinularia sarsii from the northern hemisphere as one species.
Spinularia spinularia (Bowerbank, 1866) (    polymastiidae of the nordic and siberian seas 1319 description External morphology Lectotype of Tethea spinularia cushion-shaped, 23 × 15 × 12 mm, removed from the substrate ( Figure 31A). Surface rough, beige in the central area and grey in the periphery, bearing 11 wart-like papillae, most with oscula at the summits. The most intact paralectotype cushion-shaped, 9 × 8 × 5 mm, removed from the substrate. Surface minutely hispid, covered with grey sediment in the periphery and clean, pale orange in the central area around a single wart-like papilla. Two paralectotypes are fragments of cushion-shaped sponges with minutely hispid surface, without papillae, removed from the substrates. The smallest paralectotype is a tiny encrust on a stone. Holotype of Rhaphidorus setosus cushion-shaped, about 12 mm in diameter, considerably damaged, attached to a stone. Scandinavian sponges cushion-shaped, occupying up to 9 cm 2 of the substrate ( Figure 31B). Surface minutely hispid, occasionally smooth and free of sediment in the central area and more hispid and covered with sediment in the periphery, bearing up to 15 papillae. Many individuals with tiny spicule fringes at the body edges. Papillae small, usually wart-like, with oscula at the summits.

Anatomy
Description is based on the lectotype of Tethea spinularia and Scandinavian individuals. Anatomy of the holotype of Rhaphidorus setosus is not studied because of its considerable damage. Choanosome in life pale orange, dense. Main choanosomal skeleton composed of tracts (72-380 mm thick) of principal spicules radiating from the base, crossing the cortex and protruding above the surface ( Figure 31C). Some tracts ascend to the papilla walls. Auxiliary choanosomal skeleton comprises free-scattered bundles of small spicules spread all over the body, bundles of principal spicules concentrated in the basal area and lying perpendicular to the main tracts, and trichodragmata (dense packs, 69 -391 long and 56-453 mm wide) of raphides concentrated in the subcortical area ( Figures 31D & 32C -D). Cortex in life whitish, creamcoloured or pale brown, firm, not detachable. Cortical skeleton comprises a superficial palisade (368 -643 mm thick) of small spicules and an inner space (256-1128 mm thick) with low concentration of spicules, both reinforced with the ascending choanosomal tracts. Papilla wall covered with a superficial palisade.
Genetic data 28S rDNA was obtained from five individuals of Spinularia spinularia, while CO1 was sequenced only from four of them. The CO1 sequences are identical, but a polymorphism is revealed in 28S rDNA. Two subspecies groups, one comprising three individuals and the other two individuals, differ by two bps in this gene (Matrix M34250 in TreeBase). The synapomorphies and distinctions of S. spinularia from the congeners are described in the Genetic data sections for S. njordi and S. sarsii. occurrence ( Figure 33) Literature data: East Greenland (237 m) (Fristedt, 1887), Ireland (1001 m) (Stephens, 1915). Shetland (Bowerbank, 1866(Bowerbank, , 1874. Swedish Western Coast (50 m) Alander, 1942). Norwegian Coast: Rogaland, Hordaland (70 m and deeper) Alander, 1942

discussion
Since Bowerbank (1866) established Tethea spinularia for several sponges from Shetland the validity of this species was not disputed except for the proposal by Schmidt (1880) to synonymize it with Halicnemia patera not supported by the subsequent authors. Meanwhile, there was some disagreement on what genus it should be placed in. Gray (1867) established for T. spinularia a new genus Spinularia. However, Schmidt (1870) and Fristedt ( , 1887 proposed to place T. spinularia in Radiella, while Hanitsch (1894) allocated it to Polymastia. Spinularia was resurrected by Stephens (1915) defining the main distinguishing feature of its type species, the presence of raphides in trichodragmata. Based on this feature she also synonymized an Azorean species Rhaphidorus setosus Topsent, 1898 with S. spinularia. Both actions were encouraged by most subsequent authors Alander, 1942;Plotkin et al., 2012). However, examination of the type and comparative material of S. spinularia and the holotype of R. setosus has revealed that in the latter both the 1320 alexander plotkin et al.
principal spicules and the raphides are longer than in S. spinularia. Moreover, the raphides in R. setosus bear umbrelliform or subspherical distal ornaments, whereas in S. spinularia they are harpoon-shaped. Based on these differences we assume that R. setosus may be accepted as a separate species of Spinularia, S. setosa, although this should preferably be tested further with molecular data.

diagnosis
Polymastiidae of columnar or globular body shape, always with papillae. Main choanosomal skeleton constituted by longitudinal or radial tracts of principal monactines. Skeleton of the upper cortex comprises a palisade of small monactines. Skeleton of the lateral cortex may be either the same palisade or a dense layer of criss-cross principal or intermediary spicules.

discussion
In the CO1 and 28S rDNA phylogenies Tentorium Vosmaer, 1887 is not monophyletic (Plotkin et al., 2016b; Figure 1 in this study). The type species, T. semisuberites and T. papillatum Kirkpatrick, 1908 do not group together and, moreover, in the 28S rDNA tree the Arctic T. semisuberites and the Antarctic T. cf. semisuberites are not sisters. At the same time these phylogenies are unable to reconstruct the relationships between Tentorium spp. and other polymastiids, and therefore no alternative classification is proposed. Until more molecular data on a larger set of species become available, we recognize Tentorium as a valid genus, but emend its diagnosis proposed by Boury-Esnault (2002)

Genetic data
In the CO1 phylogeny Tentorium semisuberites is the sister to the clade of Spinularia spp., but in the 28S rDNA it has no close relations (Plotkin et al., 2016b). At the same time T. semisuberites possesses just one autapomorphy in CO1 distinguishing it from all other polymastiids (Online resource 2, p. 5). Moreover, an intraspecific polymorphism in both genes is observed in this species. The two individuals, from which the data were obtained, differ by two bps in CO1 (Matrix M34248 in TreeBase) and four bps in 28S rDNA (Matrix M34250 in TreeBase).
polymastiidae of the nordic and siberian seas

diagnosis
Polymastiidae of massive or globular body shape, with a smooth surface always bearing papillae. Spicule assortment restricted to two size categories of smooth monactines. Main choanosomal skeleton is a reticulation formed by tracts of principal monactines. Auxiliary choanosomal skeleton comprises free-scattered small monactines. Cortical skeleton composed of a palisade of small tylostyles or subtylostyles and an internal layer of criss-cross principal monactines separated by a middle layer with aquiferous cavities.

discussion
Weberella is morphologically well-defined, but the relationships between Weberella spp. are unclear because for the moment the genetic data are only available from the type species, W. bursa.
Weberella bursa (Müller, 1806) ( Figure 36) synonyms and citations description External morphology Massive, fist-shape, or occasionally globular sponges occupying up to 100 cm 2 of the substrate ( Figure 36A, B). Surface smooth, white or pale cream both in life and alcohol, with up to 200 papillae, all with oscula at the summits. The papillae conical, 2 -8 mm long, up to 8 mm wide at base and 2 mm wide at summit.

Anatomy
Choanosome crumbly, pale yellow in life, but becoming slightly darker in alcohol. Main choanosomal skeleton composed of reticulating tracts (47-254 mm thick) of principal spicules ( Figure 36C). Some tracts ascend to the cortex and the papillae. Auxiliary choanosomal skeleton comprises freescattered small spicules. Cortex both in life and alcohol whitish, firm, but detachable. Cortical skeleton includes a superficial palisade (246-337 mm thick) composed of tufts of small spicules, a middle layer (513 -1011 mm thick) with aquiferous cavities separated by the ascending and radiating choanosomal tracts and an internal layer (100-233 mm thick) of obliquely lying small spicules ( Figure 36D). The cortical palisade and the internal layer stretch to the papillae 1324 alexander plotkin et al.

walls.
Each papilla has a large central exhalant canal and numerous small inhalant canals in the periphery. Bulkheads separating the canals reinforced with the ascending tracts of principal spicules and free-scattered small spicules.

Genetic data
Two individuals of Weberella bursa, from which the genetic data were obtained, differ neither by CO1, nor by 28S rDNA. In the phylogenies based on these genes W. bursa is the sister to morphologically quite distinct Polymastia cf. conigera (a British species not covered by the present study), although the Bayesian support for this relationship is weak (Plotkin et al., 2016b). These species share two unique synapomorphies in CO1 (Online resource 2, p. 3, Matrix M34248 in TreeBase) and three synapomorphies in 28S rDNA, of which one is unique and two are also shared by Sphaerotylus borealis (Online resource 3, p. 3, Matrix M34250 in TreeBase). The difference between W. bursa and P. cf. conigera is 13 bps in CO1 (Matrix M34248 in TreeBase) and five bps in 28S rDNA (Matrix M34250 in TreeBase). The distinctions between W. bursa and S. borealis are described in the Genetic data section for the latter species.
occurrence morphological differences are confirmed by the molecular phylogenies. At the same time the sister relationships between W. bursa and Polymastia cf. conigera as well as the sister relationships between this pair and S. borealis revealed in the molecular phylogenies, but insufficiently supported, need more studies on a larger set of species. Koltun (1964 put Weberella in synonymy with Polymastia, that was not accepted further . Polymastia bursa (Müller, 1806) sensu Koltun should not be mixed with Polymastia bursa (Schmidt, 1862) sensu von Lendenfeld (1898). The latter is taxon inquirendum originally described as Suberites bursa Schmidt, 1862 from the Adriatic Sea (see .

Diversity of species
Altogether 20 species from six polymastiid genera were recorded in the Nordic and Siberian Seas (Table 8). Of these two species, Polymastia svenseni and Spinularia njordi, are new to science, one species, Polymastia andrica, is new to the Nordic Seas and two species, P. cf. bartletti and P. penicillus, are new to the Scandinavian Coast. The sponge from Western Norway herein identified as Polymastia sp. may potentially be another new species, but more material is required to check this. The new findings listed above were mainly done based on molecular data. Polymastia svenseni and S. njordi are distinguished by genetic autapomorphies, but exhibit no clear morphological autapomorphies. Polymastia cf. bartletti is morphologically very similar to P. nivea and therefore these species can be separated only based on their considerable genetic distinctions. Polymastia andrica exhibits just one clear morphological distinction, the presence of exotyles, and several distinctions in CO1 from the sibling species P. arctica, although the relationship between these two needs more studies taking into account the intragenomic polymorphism of their 28S rDNA, which probably may indicate a gene flow through hybridization between these species (Plotkin et al., 2016b). Polymastia penicillus was identified based on its both morphological and genetic distinctions from the congeners. Polymastia sp. is morphologically very similar to P. andrica, but considerably differs from the latter by both CO1 and 28S rDNA. Moreover, based on the 28S rDNA phylogeny Polymastia sp. is a sibling of morphologically distinct P. svenseni.
In addition to the species described in our study, one more polymastiid was recorded in the Nordic Seas -Polymastia paupera Fristedt, 1887 found east off South Greenland. However, Boury-Esnault (1987) suggested that this was a suberitid. We agree with her opinion after examining the holotype of this species (Swedish Museum of Natural History, Stockholm, Type-1207). It is a sponge piece without papillae. The skeleton is made of tylostyles with lobate tyles (one size category) located obliquely to the surface. Cortex is not differentiated. None of these traits are found in the polymastiids.

patterns of distribution
Of all the species studied, 10 species, Polymastia andrica, P. grimaldii, P. hemisphaerica, P. thielei, P. uberrima, Quasillina brevis, Sphaerotylus capitatus, Spinularia sarsii, Tentorium semisuberites and W. bursa, have an amphi-Atlantic boreoarctic distribution ranging from Nova Scotia, Newfoundland and Labrador at the Canadian Atlantic Coast and north-eastwards over the Nordic Seas and along the coasts of Greenland, Iceland, Scandinavia and Russia up to the Arctic Ocean and the Siberian Seas (Table 8). In the south-western parts of this area the occurrence of these species is usually limited to the depths below 100 -200 m, while in the north-east most of them spread to shallow waters (Figure 38). The distribution of two species, P. hemisphaerica and S. sarsii, is limited to the deep-sea (to 150 m for the former species and to 300 m for the latter) in all regions. The prevalence of the amphi-Atlantic boreoarctic species in the Nordic and Arctic faunas was earlier demonstrated on several demosponge genera, e.g. Geodia Lamarck, 1815 from the family Geodiidae Gray, 1867 (Cárdenas et al., Fig. 37. Weberella bursa, distribution: black square, data from Boury-Esnault et al. (1994); white triangle, data from ; black crosses, data from Vosmaer (1885); white circles, our data. Type locality is unknown.

2013) and
Thenea Gray, 1867 (Cárdenas & Rapp, 2012) from Theneidae Carter, 1883, and on hexactinellids, e.g. Asconema Kent, 1870 from Rossellidae Schulze, 1885 (Tabachnick & Menshenina, 2007). Meanwhile, it is still unclear how far southwards in the central North Atlantic and along the European and African coasts the amphi-Atlantic boreoarctic polymastiids may be found. The records of Q. brevis and W. bursa from Azores (Topsent, 1892, the entrance to the Gibraltar Strait and the Mediterranean Sea Boury-Esnault et al., 1994) as well as the records of T. semisuberites from Azores (Topsent, 1892 and the Bay of Biscay (Topsent, 1892) need verification with molecular data. The allegedly cosmopolitan distribution of S. sarsii is already questioned based on the observed genetic differences between the Norwegian and Mozambican sponges, but a thorough comparison between the Norwegian, Azorean and NW African individuals is still required.
The distribution of other polymastiid species studied is narrower (Table 8). Four species, Polymastia arctica, P. nivea, Sphaerotylus borealis and Spinularia spinularia may be regarded as NE Atlantic high-boreoarctic. The known occurrence of Sphaerotylus borealis is limited to Iceland in the south-west and the eastern Kara Sea in the north-east. The distribution of Polymastia arctica and P. nivea is limited to the Norwegian Coast and Russian Coast of the Barents and White Sea. The former species has never been recorded to the south-west from Central Norway (Sør-Trøndelag), while the latter has been found up to Southern Norway (Vest-Agder). Spinularia spinularia is widely distributed along the Scandinavian Coast, East Greenland Coast and around the British Isles, while its records from Azores (Topsent, 1898) are considered as a separate species, S. setosa, based on distinct differences in morphology. Atlantic high-boreoarctic species were earlier recorded among other demosponge families, e.g.  , Tetilla sibirica (Fristedt, 1887) from Tetillidae Sollas, 1886, and Thenea abyssorum Koltun, 1964 from Theneidae (Cárdenas & Rapp, 2012). A few records of the sponge species distribution limited to the Arctic Ocean and Siberian Seas, e.g. demosponges Hemimycale rhodus (Hentschel, 1929), Cladorhiza arctica Koltun, 1959 andPseudosuberites sadko Koltun, 1966, should be verified on the additional material. It seems that the Arctic sponge fauna is predominantly composed of the species dispersed from the Atlantic. Two species studied, Polymastia boletiformis and P. penicillus, represent the southern boreal component in the Scandinavian sponge fauna. They are quite common in the shallow depths along the European coasts ( Figure 38), with their northernmost records from Møre and Romsdal in Norway for P. boletiformis and the British Isles and the Swedish Western Coast for P. penicillus. The occurrence of the southern boreal species along the Scandinavian Coast was earlier recorded for other sponge families, e.g. a calcarean Clathrina coriacea (Montagu, 1814) from Clathrinidae Minchin, 1900(Rapp, 2006, demosponges Characella pachastrelloides (Carter, 1876) and Poecillastra compressa (Bowerbank, 1866) from Pachastrellidae Carter, 1875 and Thenea muricata (Bowerbank, 1858) from Theneidae (Cárdenas & Rapp, 2012).
comprehensive material from adjacent waters can be included. It is recommended that further research on the biogeography of polymastiids should be based on an integrative approach including detailed morphological studies and analyses of a larger set of phylogenetic markers.

S U P P L E M E N T A R Y M A T E R I A L
The supplementary material for this article can be found at https://doi.org/10.1017/S0025315417000285