Phenotypic plasticity refers to the capacity of an organism’s phenotype to vary in response to changes in environmental conditions, without any change in the individual genotype. Sea urchins (Echinoidea) are well-known for their morphological and behavioural plasticity in response to changing habitats or trophic environments. Phenotypic plasticity has been little studied in directly developing species, in which low levels of phenotypic plasticity are usually expected as a consequence of high levels of genetic differentiation among populations and local genetic adaptation. In the present work, we report a significant plasticity in the direct-developing and brooding sub-Antarctic species Ctenocidaris (Eurocidaris) nutrix (Echinoidea, Cidaridae) through morphological and trophic analyses of gut contents and δ13C and δ15N isotopic compositions. Molecular data (COI mtDNA) confirm that the different phenotypes of C. nutrix, the short-spined C. nutrix nutrix and the long-spined C. nutrix longispina, are a single species restricted to sub-Antarctic waters. As formerly demonstrated in broadcasting echinoid species, morphological plasticity appears to be mainly linked to depth and swell exposure, specimens from exposed and shallow sites showing shorter spines and larger apical systems compared to specimens from sheltered or deep sites. Significant differences in the diet and trophic niche of the different phenotypes suggest that prey type and food diversity may also be a factor promoting distinct phenotypic responses, both in the feeding behaviour and morphology of echinoids.

The leptomedusa Dichotomia cannoides is reported for the first time from the Red Sea, extending its known distribution beyond the western Atlantic and western Pacific Ocean. Five specimens were documented in the northern Gulf of Aqaba, and one was collected for molecular analysis. In situ photography was used to extract diagnostic characters and natural habitus. DNA barcoding of the mitochondrial COI gene showed 98.03% identity with D. cannoides from Florida. Phylogenetic analysis confirmed its placement within D. cannoides, distinct from related leptothecate taxa. This finding increases the number of Leptothecata species recorded in the region to 15. The delayed detection of D. cannoides in the Red Sea may be due to its small size, limited seasonality or a recent introduction via ballast water or hull fouling. The species remains known only from its medusa stage. We further discuss how environmental DNA may help uncover its life cycle and those of other hydrozoans.

The family Yoldiidae encompasses protobranch bivalves with notable diversity in deep-sea habitats, with Yoldiella Verrill and Bush (1897) as the most speciose genus. In Brazilian waters, 11 species of Yoldiella have been recorded, including Yoldiella lapernoi Benaim and Absalão (2011) and Yoldiella paranapuensis Benaim and Absalão (2011), two species with similar shell morphology and overlapping geographical distributions. This study re-evaluated the taxonomic distinction between these two Yoldiella species using a combination of morphometric approaches. Size and growth rate comparisons were conducted using analysis of covariance and Bayesian model selection. The shell outline was compared using elliptical Fourier descriptors. The results did not support the current separation of these taxa. Instead, they reveal that the observed morphological differences are attributable to ontogenetic variation, indicating Y. paranapuensis as a juvenile stage of Y. lapernoi. This finding supports the synonymy of these two nominal species and highlights the importance of ontogenetic context in taxonomy.

We report on the collection of two phaeodarian protists from the deep Fram Strait (Arctic Ocean). Specimens were collected using high-volume plankton pumps in the Long-Term Ecological Observatory HAUSGARTEN in 2021. The 18S sequences from our specimens formed two distinct clusters in a phylogenetic tree: one in the family Aulacanthidae and one in the genus Auloscena. The vast majority of phaeodarian species that have been taxonomically described do not have publicly available sequence data (≥95%), so our specimens likely represent species that have been described morphologically but not sequenced. Phaeodarians are an understudied group, despite their abundance and importance for carbon and silica flux. Further exploration will likely reveal a more thorough characterization of biodiversity in the deep Arctic Ocean.

Specimens of shrimp Heptacarpus maxillipes (Thoridae) infected by parasitic barnacle Sylon hippolytes (Rhizocephala) were found for the first time along the Kuril Islands at a depth of 241–670 m. It is the second host from the genus Heptacarpus. Total number of hosts of S. hippolytes is currently 24. Carapace length of the infected shrimps ranged from 3 to 10 mm. Hosts had one or two parasitic externae of S. hippolytes. The size of the externa ranged from 1.5 to 9.0 mm in length and from 1.0 to 4.0 mm in height. Retinacula were not found. A single colleteric gland was present. Receptacles were absent. Regeneration of the ovary in the ovigerous externa was not observed. Sylon hippolytes produces only one larval generation. Larvae are hatched as small cyprids. Phylogenetic reconstruction confirmed the belonging of our specimens to the monophyletic clade of S. hippolytes with genetic distances between subclades from 6.96 to 8.4%. High values of intraspecific distances in S. hippolytes from different areas may point to cryptic speciation within this rhizocephalan.

The first report of Ophiophragmus luetkeni occurred in the British Virgin Islands; however, it was also recorded in Brazil, the United States Virgin Islands, and Trinidad and Tobago, yet its occurrence in Colombia was previously lacking. Between 2023 and 2024, four specimens were collected from sandy and muddy substrates in Cispatá Bay, Colombian Caribbean. Taxonomic identification was conducted through morphological observations and microstructural analysis using Scanning Electron Microscopy. A comparative table for Ophiophragmus species recorded in Colombia are also provided. This new record increases the number of Ophiophragmus species in Colombia to three, contributing to the country’s marine biodiversity and expanding the knowledge of O. luetkeni distribution.

Microscopic epibionts are important components of an intertidal ecosystem. However, because the epibionts are established on habitats provided by basibiont (host) organisms, the epibionts are affected by both the characteristics of basibionts and the ambient environmental conditions. Here, we hypothesised that variations in the epibiont community were affected by the mobility, size, and surface roughness of the basibiont organisms, as well as by environmental conditions, which was tested over a one-month period in spring. Epibionts growing on 16 basibiont species belonging to Gastropoda, Bivalvia, Polyplacophora, and Echinoidea were collected from a rocky shore in Niigata, Japan. Most of the epibionts collected were diatoms, and the highest cell density of the epibionts was recorded on the surfaces of the limpet Cellana toreuma. The epibiont community changed significantly from April to May and was also shaped by the characteristics of the basibionts. The results indicated that basibionts with sessile, large, or smooth surfaces had higher taxonomic richness, Simpson diversity, and cell density of the epibionts than those with mobile, small, or rough surfaces. Multivariate analysis of the epibiont community confirmed the importance of these basibiont characteristics and the survey month. Six groups of epibiont communities were identified based on their contrasting sample communities, and each had its own indicator species. The results indicate that both environmental changes from April to May and changes in basibiont species promote changes in the epibiont community in this coastal region.

The oceanic anglerfishes (Lophiiformes: Ceratioidei) are among the most diverse and ecologically distinctive groups of deep-sea fishes, exhibiting some of the most extreme morphological adaptations in the marine realm. Despite their remarkable biological traits, they remain among the most understudied vertebrates globally, primarily due to the logistical challenges associated with accessing their natural habitats. Here we report the diversity of oceanic anglerfishes collected during the AMAZOMIX scientific cruise, conducted in 2021 off northern Brazil, including the Amazon River plume, across depths ranging from the surface to approximately 1200 m. A total of 34 ceratioid specimens, representing six families and eight genera, were collected. Seven species were identified: Melanocetus johnsonii (Melanocetidae), Haplophryne mollis (Linophrynidae), Lophodolos acanthognathus, Oneirodes carlsbergi (Oneirodidae), Centrophryne spinulosa (Centrophrynidae), Gigantactis vanhoeffeni and Rhynchactis leptonema (Gigantactinidae). Haplophryne mollis, L. acanthognathus, C. spinulosa, and R. leptonema are reported for the first time in Brazilian waters. This study provides morphological and distributional remarks for all identified species and presents an updated checklist of deep-sea anglerfishes recorded in Brazilian waters.

Crystal cells in echinoderms have been described in the literature as a type of coelomocyte (immune cell) containing a crystalline structure. Their putative function has been widely ascribed to osmoregulation but this assertion was never robustly tested. In the present paper, a review of crystal cells and crystalline structures provides evidence supporting a different identity and function. First, the same microcrystals can be found either free or encapsulated by a cell membrane (with or without a structure resembling a nucleus). Specifically, they are typically non-encapsulated when found inside tissues of internal organs and encapsulated when free floating in the hydrovascular or perivisceral fluids. Although usually individually packaged, microcrystals were also observed encapsulated in groups of up to four, with or without other particles. Their morphological features, coupled with their chemical and optical properties, match that of microcrystals of uric acid, described in other phyla, including chordates. Two pathways of excretion of these crystallised by-products were evidenced: rejected with sea water out of the respiratory tree and expelled via transrectal coelomoducts among coelomocyte aggregates. Overall, the present synthesis strongly supports that ‘crystal cells’ historically described as a distinct type of coelomocyte in holothuroids are phagocytes that have engulfed uric acid microcrystals generated as waste by metabolic activities.

Four new species of Poecilosclerida (Porifera, Demospongiae) assigned to the genera Latrunculia and Iophon are described from South Africa and Namibia, located in the Namaqua ecoregion. The Porifera occurring along the continental shelf within this ecoregion are relatively well-known, with 76 species formally described in previous literature. Of these, 35 species belong to the Order Poecilosclerida. Additionally, Latrunculia (Aciculatrunculia) biformis is reported from the continental shelf on the west coast of South Africa, extending its range further northwards into the South Atlantic. DNA barcoding and molecular phylogenetic analyses were employed to ensure accurate taxonomic assignment and designation of new species.

The present account describes a new species of alpheid shrimp, Alpheus madhusoodanai sp. nov., belonging to the brevirostris group, collected from the Cochin estuary, the south west coast of India. This represents the first species of alpheid shrimps described from the estuary. The morphological and molecular characteristics of the new species are compared with those of its closely related congeners. The newly described species is separated from its morphological congener A. rapax, by its wider major chela and longer merus of the first cheliped. Molecular data also confirmed the delimitation of A. rapax with A. madhusoodanai sp. nov. Habitat and distribution details are also discussed, highlighting the potential for further taxonomic exploration in the Cochin estuary and the importance in uncovering its hidden biodiversity.

The annelid genus Diopatra is a well-known example of marine ecosystem engineering, as it creates tubes in coastal sediments all around the world. In the Amazon coast, this annelid is common in intertidal estuarine areas and protected beaches. However, there are no data for the Amazon coast regarding studies on the meiofauna associated with Diopatra sp. tubes. Therefore, the present study characterized, for the first time, the meiofauna community found on a muddy-sandy tidal flat of the Amazon coast in areas with and without the presence of Diopatra sp. tubes. Samples were collected in February 2014 in two different areas: (1) an area in which Diopatra sp. tubes were present, and (2) an area without tubes. A total of 13 major meiofaunal groups were found, with Nematoda as the dominant group. Overall, a significant increase in meiofauna density and richness of the meiofauna was observed in the area with the presence of Diopatra sp. tubes. While no large aggregations of Diopatra sp. tubes were observed in the study region, the presence of even a single tube had significant effects on the environmental conditions available to the meiofauna community. The present findings add knowledge about the presence of the bioconstructor in coastal areas and reinforce the role of tube-building polychaetes as ecosystem engineers.

Studying social behaviour of mobile and cryptic marine mammals is challenging, particularly for small species like harbour porpoises whom in many areas tend to be challenging to initially spot and then gain repeated observations. Recently, there has been a rise in the number of observations of harbour porpoises occurring in atypical group sizes that are larger than the typical size of two to three individuals. This behaviour, in conjunction with the evolving availability and usability of Unmanned Aerial Vehicle (UAV) technology, is supporting and improving the capture of information on social behaviours. Around Shetland, Scotland, harbour porpoises are sighted from land via opportunistic and systematic surveys year-round with atypical group sizes also observed. To capture information on group size, opportunistic and systematically collected UAV footage were collated and analysed. This revealed sexual approach behaviours in typical group sizes of small groups (two to four individuals), as well as atypical group sizes of medium groups (five to nine individuals), large groups (10 to 19 individuals), and aggregations (≥20 individuals) occur in multiple bays around Shetland. This is the first insight into group sizes and behaviours for the region which was recently designated as an Important Marine Mammal Area (IMMA). Further insight into these social behaviours and group size (e.g. seasonality, site fidelity) could aid harbour porpoise conservation and management within the IMMA and elsewhere.

This study provides the first results on the population dynamics of Sphyraena sphyraena (Linnaeus, 1758) in the western Mediterranean. Otoliths and scales reading were used for its age and growth investigation in Eastern coast of Algeria. The monthly evolution in marginal increment data of scales and otoliths revealed that only one annulus is formed per year in February. Fish length and radii of the scales or otoliths were closely correlated. Both methods showed four age groups. ANOVA test (P < 0.05) revealed insignificant differences between both estimates of length at age results. Growth parameters estimated using length-at-age data from both methods confirming the age determination results. The total mortality (Z ), natural mortality (M ), and fishing mortality (F ) were estimated at 1.19 year−1, 0.826 year−1, and 0.363 year−1, respectively. The growth performance index (Ø ) was 2.57. The estimated exploitation ratio (E ) was 0.305. The length–weight relationship is EW = 0.0104 TL2.702, showed that the growth rates were negative allometric. The results revealed the compatibility of otoliths and scales reading for assessment studies of Sphyraena sphyraena .

We investigated the epizoic diatoms colonizing the carapace of the spider crab Hyastenus hilgendorfi inhabiting Lake Timsah in the Suez Canal. A total of 63 individuals, free of macro-epibionts, were examined to assess the diversity, abundance, and seasonal dynamics of their associated diatom communities. A total of 61 diatom taxa belonging to 31 genera were recorded, with raphid pennate diatoms being the most dominant, particularly Navicula lanceolata. Diatom density and diversity exhibited clear seasonal variation, where Summer had the highest richness and Spring the highest cell densities. Overall, males hosted more diatom taxa than females, however, sex-specific differences fluctuated seasonally. The cluster analysis revealed distinct diatom assemblages between sexes and seasons, showing subtle but significant variation in epibiont structure. These findings provide the first detailed characterization of epizoic diatom assemblages on H. hilgendorfi. They suggest that seasonal variations in diatom communities may influence the crab’s camouflage effectiveness by altering biofilm characteristics which facilitate macro-epibiont settlement.

A new sponge species (Demospongiae: Poecilosclerida: Podospongiidae) is described offshore from the Cape of Good Hope, on the southern edge of the Table Mountain National Park marine protected area. Podospongia capensis sp. nov. is compared to Podospongia natalensis, described by Kirkpatrick (1903) from the east coast of South Africa, as well as to all other Podospongia species described to date. The new species differs from P. natalensis in having a shorter, thicker stalk, lacking anisostrongyles as megascleres, and possessing a second category of large symmetrical aciculospinorhabds microcleres that are present in P. natalensis. Additionally, P. natalensis has larger oxeas and styles than those found in P. capensis sp. nov. Furthermore, the two species are geographically separated, with P. natalensis described from the Natal ecoregion, while P. capensis sp. nov. is described from the Southern Benguela ecoregion. The new species primarily differs from other congeners in external morphology and size of the oxeas and styles.

A new species of spionid polychaete from the coastal waters of southwest India, Trochochaeta chakara sp. nov., is described and illustrated. Adults are common on Alappuzha mud banks (locally known as Chakara) off the coast of Kerala. They live in silty tubes in soft sediment and are characterized by the presence of two pairs of red eyes, caruncle extending to end of chaetiger 1, heavy falcate spines in neuropodia of chaetigers 2 and 3 (those in chaetiger 3 much stronger and darker), capillary chaetae in notopodia of chaetigers 1, 3–10, frayed heavy spines in neuropodia of chaetigers 4–13, hirsute capillaries in neuropodia from chaetiger 14, lateral interneuropodial membranes from chaetiger 14, one pair of ventral papillae on each chaetiger from chaetigers 14–16, bundles of acicular spines in notopodia from chaetigers 50–52, and small pygidium with up to six pairs of short cirri. This is the third species of Trochochaeta described and found in the Indian Ocean, including T. orissae (Fauvel, 1932) and T. cirrifera (Hartman, 1975).

Portunid crabs of the genus Charybdis De Haan, 1833 are among the most frequently reported marine invaders worldwide. Here, we report the first record of Charybdis (Archias) hoplites (Wood-Mason, 1877) outside its native Indo-West Pacific range, collected from the Test Estuary, Southampton Water, United Kingdom. Morphological and molecular analyses confirm the specimen’s identity and clarify diagnostic features useful for distinguishing C. (A.) hoplites from closely related taxa. This represents the northernmost record of any Charybdis species and suggests a long-distance dispersal event, associated with shipping activities in the Port of Southampton. Environmental data indicate that salinity and turbidity at the collection site are within known tolerances for Charybdis spp., although low winter temperatures may limit survival and establishment. The detection of this warm-water species in a major international port highlights the ongoing need to monitor non-native marine fauna.

The paintpot cuttlefish Ascarosepion tullbergi (Appellöf, 1886) exhibits a distinctly benthic lifestyle, in contrast to the typical ontogenetic shift from benthic to nekto-benthic modes observed in most cuttlefish species. While many cuttlefish initially attach to substrates using arms IV and the ventral mantle, they generally transition to swimming-based locomotion as they mature. A. tullbergi, however, remains benthic throughout its life, relying on arms IV and the lateral edges of the ventral mantle for attachment and using an ambling gait for locomotion from hatching to adulthood. Despite the ecological significance of benthic specialisation, embryonic development in fully benthic cuttlefish remains poorly studied, as previous research has primarily focused on nekto-benthic species. To address this gap, we described the embryonic development of A. tullbergi and compared it with that of other sepiid species. Although the overall developmental pattern of A. tullbergi is broadly similar to that of other nekto-benthic cuttlefish, this species exhibits a pronounced early investment in benthic traits, particularly in the development of arms IV and the lateral edges of the mantle. Specifically, arms IV in A. tullbergi develop earlier and more prominently than in other Sepiidae species that undergo ontogenetic shifts in life mode. This early specialisation underscores the unique ecological strategy of A. tullbergi and reflects its adaptation to a lifelong benthic niche. The present study provides a description of embryonic development in A. tullbergi and contributes to a deeper understanding of cephalopod diversity.