Introduction
Marine soft sediments are among the most widespread habitats in the world’s oceans and play a fundamental role in the functioning of marine ecosystems (Gray Reference Gray2002, Pan & Pratolongo Reference Pan, Pratolongo, Pan and Pratolongo2022). These habitats are primarily composed of fine-grained sediments such as mud, sand and gravel and host many diverse benthic communities that are crucial in processes such as energy transfer in trophic networks, nutrient and carbon cycling, contaminant removal and secondary production (Snelgrove et al. Reference Snelgrove, Blackburn, Hutchings, Alongi, Grassle and Hummel1997, Lohrer & Hancock Reference Lohrer and Hancock2004, Schratzberger & Ingels Reference Schratzberger and Ingels2018). Currently, marine soft sediments face many threats, including climate hazards and anthropogenic pressures (Brierley & Kingsford Reference Brierley and Kingsford2009, Gissi et al. Reference Gissi, Manea, Mazaris, Fraschetti, Almpanidou and Bevilacqua2021, Williamson & Guinder Reference Williamson, Guinder and Letcher2021). For instance, ocean warming and pollution can alter sediment characteristics, leading to shifts in species compositions and ecosystem services (Altafim et al. Reference Altafim, Alves, Trevizani, Figueira, Gallucci and Choueri2023, Vlaminck et al. Reference Vlaminck, Moens, Braeckman and Van Colen2023). Despite their significance, marine soft sediments are still poorly studied, particularly in polar areas, making it crucial to obtain a deeper understanding of these ecosystems given the current global climate trends.
The composition of faunal assemblages inhabiting soft-bottom ecosystems is shaped by a complex interplay of environmental factors, where depth, sediment type and organic matter content are particularly relevant (Snelgrove Reference Snelgrove1999, Vause et al. Reference Vause, Morley, Fonseca, Jażdżewska, Ashton and Barnes2019). Among the invertebrate macrofauna, the most representative phyla in these assemblages are annelid polychaetes, crustaceans, molluscs and echinoderms, although other annelids (e.g. echiurans and sipunculans) and nemerteans are also present in lower abundances (Snelgrove Reference Snelgrove1998, Ellingsen Reference Ellingsen2001). The organisms living in these assemblages are typically characterized by limited mobility, and they may inhabit either into the sediment (infauna), the sediment surface (epifauna) or both (e.g. Pearson Reference Pearson2001). Regarding feeding strategies, suspension-feeders (both active and passive), deposit-feeders (surface or subsurface) and predators usually dominate these marine sediments, although other feeding types may also be present (Snelgrove Reference Snelgrove1999, Macdonald et al. Reference Macdonald, Burd and Van Roodselaar2012). Additionally, due to their three-dimensional body structures extending above the sediment, some of these organisms may act as ecosystem engineers and provide substrates for other organisms to attach to and grow upon (Rabaut et al. Reference Rabaut, Guilini, Van Hoey, Vincx and Degraer2007, Meadows et al. Reference Meadows, Meadows and Murray2012).
In the Southern Ocean, faunal assemblages differ from those found elsewhere in the world due to several environmental characteristics, such as isolation, water temperature and continental shelf depth (Barnes & Clarke Reference Barnes and Clarke2011). Although all phyla can be found in this ocean, two types of soft-bottom assemblages can be differentiated: 1) those dominated by sessile suspension-feeding epifauna, such as sponges or ascidians, and 2) those dominated by infauna and mobile epifauna, such as polychaetes, bivalves and echinoderms (Gutt Reference Gutt2007 and references therein). Furthermore, because durophagy is absent in these environments, the main predators are species such as sea stars and nemerteans, among other vagile taxa (Dayton et al. Reference Dayton, Robilliard, Paine and Dayton1975, Aronson & Blake Reference Aronson and Blake2001, Clarke et al. Reference Clarke, Aonson, Crame, Gili and Blake2004, Ortiz et al. Reference Ortiz, Hermosillo-Nuñez, González, Rodríguez-Zaragoza, Gómez and Jordán2017).
Widely reported regional changes, such as the reduction of the annual duration of sea ice, the melting and retreat of marine-terminating glaciers and the collapse of ice shelves around Antarctica during recent decades, are significant threats to biodiversity in these zones (Gutt et al. Reference Gutt, Barratt, Domack, d'Acoz, Dimmler and Grémare2011, Ducklow et al. Reference Ducklow, Fraser, Meredith, Stammerjohn, Doney and Martinson2013, Cook et al. Reference Cook, Holland, Meredith, Murray, Luckman and Vaughan2016, Henley et al. Reference Henley, Schofield, Hendry, Schloss, Steinberg and Moffat2019, Post et al. Reference Post, Alley, Christensen, Macias-Fauria, Forbes and Gooseff2019, IPCC Reference Lee and and Romero2023). Considering that many of these Antarctic areas remain underexplored, predictions for the future of their soft-bottom marine biodiversity in the face of climate change are uncertain (e.g. Vause et al. Reference Vause, Morley, Fonseca, Jażdżewska, Ashton and Barnes2019). This is especially true in the South Shetland Islands, an archipelago close to the Polar Front, a crucial boundary where cold Antarctic waters meet warmer sub-Antarctic waters (e.g. Orsi et al. Reference Orsi, Whitworth and Nowlin1995).
Annelid polychaetes represent one of the most common and abundant taxa in marine benthic communities worldwide, particularly in sedimentary environments (Knox Reference Knox, Reish and Fauchald1977, Hutchings Reference Hutchings1998, Crespo & Pardal Reference Crespo, Pardal, Leal Filho, Azul, Brandli, Lange Salvia and Wall2020, Giangrande et al. Reference Giangrande, Gambi, Gravina, Rossi and Bramanti2020). Several studies have demonstrated their dominance in terms of species richness, abundance and biomass in soft-bottom habitats (Knox Reference Knox, Reish and Fauchald1977, Hutchings et al. Reference Hutchings, Ward, Waterhouse and Walker1993, Smith Reference Smith2000). This dominance is particularly notable in polar areas (Kendall et al. Reference Kendall, Widdicombe and Weslawski2003, Piepenburg Reference Piepenburg2005, Clarke & Crame Reference Clarke and Crame2010, Pabis et al. Reference Pabis, Sicinski and Krymarys2011), where they may reach more than half of the total benthic invertebrate biomass (Knox Reference Knox, Reish and Fauchald1977). Polychaetes play a critical role in ecosystems by creating new habitats and modifying the redox conditions of the sediment through bioturbation (Bolam & Fernandes Reference Bolam and Fernandes2002, Mermillod-Blondin Reference Mermillod-Blondin2011, Kristensen et al. Reference Kristensen, Penha-Lopes, Delefosse, Valdemarsen, Organo Quintana and Banta2012, Jumars et al. Reference Jumars, Dorgan and Lindsay2015). These processes are essential to allowing oxygenation and the transportation of surface materials to the deeper sediment layers, maintaining and even creating soft-bottom ecosystem structures.
Thus, to enhance our understanding of Antarctic marine soft-bottom assemblages in underexplored ecosystems threatened by global warming, this study aimed to provide a preliminary description of the marine fauna inhabiting False Bay in Livingston Island (South Shetland Islands). These new insights represent a valuable baseline for future studies assessing the broader ecological implications beyond the climate-induced changes in Antarctic ecosystems.
Methods
Study location
The present study was carried out during January 2023 (summer) at False Bay, Livingston Island (South Shetland Islands, Antarctica) in the framework of the CHALLENGE Antarctic campaign by our research group (Universitat de Barcelona). This cove of 6.4 km in length lies between Barnard Point and Hurd Peninsula, on the south side of Livingston Island (62°43′S, 60°22′W; Fig. 1). Within this bay, we selected two different stations to obtain a preliminary description of the marine benthic macrofauna: an exterior site and a more internal one (Fig. 1). These two stations were chosen based on their contrasting distance from Huntress Glacier, located in the innermost part of the bay, and according to the logistical necessities of our cruise. False Bay has some marine-terminating glaciers, with Huntress Glacier being the largest, despite having shown a gradual retreat in recent years (Cook et al. Reference Cook, Fox and Thomson2021).
Figure 1. Sampling locations in a. the West Antarctic Peninsula and the South Shetland Islands, b. Livingston Island and c. within False Bay. Sampling locations are shown as green triangles (E = external station; I = internal station).
Environmental variables
Oceanographic data, including seawater temperature (°C), salinity (PSU), chlorophyll-a (mg m-3) and dissolved oxygen (mg l-1), were obtained from both stations within the bay using a Sea-bird SBE 11plus CTD (conductivity-temperature-depth) device to characterize the environmental conditions from the collection sites. The 10 deepest data points from each vertical profile, and thus closest to the biological sampling location, were pooled to obtain the mean and standard deviation for each station and variable (Table I). Substrate descriptions rely on visual observations, indicating a detritic mud environment.
Table I. Sampling location and characteristics of the stations, including dredge and CTD (conductivity-temperature-depth) device sampling coordinates, depths and environmental data. Data for environmental parameters indicate means ± standard deviation.
Chl-a = chlorophyll-a; DO = dissolved oxygen.
Sample collection and classification
More than 20 Van Veen dredges (0.15 × 0.12 × 0.40 m; 0.018 m2 area) were deployed from the starboard side of the vessel, with a depth range of 174–210 m. Only some of these dredges retrieved sediments: 10 from the external station and 4 from the more internal station, thus rendering a total of 14 samples. All of the dredges’ contents underwent meticulous sieving using seawater through a 1 mm mesh sieve. The organisms and sediment captured were subsequently relocated to the wet laboratory. Separation using tweezers was carefully performed to sort all of the macrofauna from the fine sediment. All of the specimens found were then categorized into broad taxonomic groups, mainly classes (although for crustaceans we used orders), and carefully preserved in pre-labelled bottles containing absolute ethanol. Later, at the laboratories of the Universitat de Barcelona, all individuals were counted, their wet weight was obtained and they were photographed using an Euromex® stereomicroscope. Finally, abundances and biomass data were standardized as individuals per square metre (ind. m-2) and grams of wet weight (WW) per square metre (g WW m-2), respectively. As we could not obtain sufficient replicates from the internal station and dredge samples were pooled at each site, a statistical comparison between both stations was not performed, although we provide here all of the descriptive data regarding the fauna collected.
Taxonomic identification and feeding guilds
Most specimens were preliminarily identified by the authors to the lowest possible taxonomic level using the available literature (Brueggeman Reference Brueggeman1998, Engl Reference Engl2012, Rauschert et al. Reference Rauschert, Arntz and Hempel2015, Xavier et al. Reference Xavier, Cherel, Boxshall, Brandt, Coffer and Forman2020, Drennan et al. Reference Drennan, Dahlgren, Linse and Glover2021). Then, the preliminary identifications, together with their respective photographs, were sent to specialists of the different taxonomic groups to confirm their identification (Tables II & S1). Additionally, all organisms found were classified, based on the abundance data, using relative frequency according to their feeding behaviour (grazers, suspension-feeders, predators and detritivores) or their physical habitat (infauna or epifauna), following the most relevant literature (Jumars et al. Reference Jumars, Dorgan and Lindsay2015, Barnes & Sands Reference Barnes and Sands2017, Mohd Nasir et al. Reference Mohd Nasir, Barnes and Wan Hussin2024, Souster et al. Reference Souster, Barnes, Primicerio and Jørgensen2024). The detritivore functional group also included deposit-feeder specimens. Only for specimens that could alternate between both physical habitats (epifauna and infauna) or could present different feeding types, a frequency > 1 was recorded.
Table II. Summary of abundances and biomasses for the main groups and species found in this study. Data extracted from the full Table S1.
ind. = individuals; WW = wet weight.
Distribution and population features of Maldane sarsi antarctica
Due to the clear dominance of M. sarsi antarctica Arwidsson, Reference Arwidsson1911 in the soft-bottom environment at our study sites, an in-depth analysis of some of the ecological characteristics of this species was carried out. A comprehensive search of the spatial distribution of this species across the Antarctic region was conducted to provide context for its occurrence in False Bay. A preliminary search was conducted of georeferenced occurrence records compiled from the published literature (Siciński Reference Ahn and Kang1986, Ahn & Kang Reference Ahn and Kang1991, Cantone Reference Cantone1995, Parapar et al. Reference Parapar, López, Gambi, Núñez and Ramos2011, Pabis & Siciński Reference Pabis, Sicinski and Krymarys2012) and public databases such as SCAR-MarBin (https://www.vliz.be/projects/scarmarbin/), OBIS (https://www.obis.org/), GBIF (http://www.gbif.org/) and Biodiversity.aq (https://www.biodiversity.aq/), selecting only those records that provided geographical coordinates. Occurrence records were obtained from Biodiversity.aq, given that this repository integrates data from all other relevant databases. Moreover, occurrence records were also extracted from research articles in which this polychaete species was recorded (see Data S1). The dataset was processed and visualised using QGIS v.3.40 software to generate a map representing its known distribution and also to display the sampling depths of each record in order to offer further insights into the species’ bathymetric range. However, some studies reporting the presence of this species were not included in the dataset (e.g. Garraffoni et al. Reference Garraffoni, Moura, Vasconcelos, Araújo and Passos2012), as they did not provide exact geographical coordinates or sampling depths nor specify its occurrence at a precise sampling station, despite confirming its presence in the study areas. In general, these studies do not refer to new distribution sites but rather to already-sampled locations that are represented in the compiled dataset (Data S1).
The size structure of the M. sarsi antarctica population in False Bay was also studied. To obtain a representative sample of the population, we used stratified sampling (Andrew & Mapstone Reference Andrew and Mapstone1987). The total population of 2412 individuals were divided into three strata based on body size, categorized as small, medium and large, as determined by visual observation. Once the population was stratified, the number of individuals in each stratum was counted. Each stratum was then assigned a sample size proportional to its representation in the total population using the calculation in Equation 1:
$$\begin{align}{n}_i = \frac{N_i}{N_{total}}\times {n}_{sample}\end{align}$$
where ni is the theoretical number of individuals from the stratum, Ni is the total number of individuals in stratum i, Ntotal is the total number of individuals in the population and nsample is the subsample selected from the total population. Once the sample proportion for each stratum was determined as small (26.83%), medium (56.10%) or large (17.07%), 410 individuals (~17% of the total population) were randomly selected within each stratum to measure their total length. Using a Vernier calliper, the total length (from the tip of the head to the end of the pygidium) was then measured (in cm). Using the size database, the data were sorted in increasing order to determine the minimum and maximum size so as to obtain the total rank (R = maximum - minimum). Then, the interquartile range (IQR) was calculated by subtracting the size value of the first quartile (Q1) from the data of the third quartile (Q3). Using the IQR and the number of samples, the width of each interval was calculated using the Freedman-Diaconis rule (Freedman & Diaconis Reference Freedman and Diaconis1981), shown in Equation 2:
$$\begin{align}h = 2\times IQR\times {n}^{-1/3}\end{align}$$
where h is the width of each interval and n is the number of observations. Finally, the total rank (R) was divided by the interval width (h) to establish nine different and appropriate categories for the frequency histogram in order to display the population characteristics of the species’ size.
Results
Environmental variables
The collected data from both stations in False Bay showed variations in the environmental conditions between the two sites (Table I). The internal station, situated at a shallower depth, presented slightly warmer temperatures (1.75 ± 0.01°C vs 0.82 ± 0.01°C) and higher chlorophyll-a concentrations (0.18 ± 0.01 vs 0.08 ± 0.01 mg m-3) than the external site, located at a greater depth. Furthermore, the dissolved oxygen concentration was higher at the internal station (6.82 ± 0.01 mg l-1) compared to the external one (5.99 ± 0.01 mg l-1). Both stations shared similar sediment characteristics when visually evaluated, consisting of mud and gravel, indicative of a low-energy depositional environment.
Assemblage characteristics
A total of 159 556 ind. m-2 were identified in this study, classified into 15 major taxonomic groups: Polychaeta, Bivalvia, Amphipoda, Ophiuroidea, Asteroidea, Pycnogonida, Isopoda, Holothuroidea, Ascidiacea, Priapulida, Nemertea, Cnidaria, Bryozoa, Gastropoda and Porifera (Fig. 2 & Tables II & S1). From these, total abundances were 151 778 ind. m-2 at the external station and 7778 ind. m-2 at the internal station, corresponding to 15 and 6 major taxonomic groups, respectively (Tables II & S1). For both stations, Polychaeta was the most abundant taxon (~93%; Fig. 3). Ophiuroidea (2.23%) and Bivalvia (1.65%) were the next more abundant taxa for the external station, while Bivalvia (2.86%) and Amphipoda (2.14%) were the next more abundant taxa for the internal one (Fig. 3).
Figure 2. Images of the most representative specimens collected during the second expedition of the CHALLENGE project in False Bay (Livingston Island): a. Phoxocephalidae (Amphipoda), b. Aphelochaeta sp. (Polychaeta), c. Natatolana sp. (Isopoda), d. Maldane sarsi antarctica (Polychaeta), e. Thyasira debilis (Bivalvia), f. Nymphon charcoti (Pycnogonida), g. Amphiura cf. joubini (Ophiuroidea) and h. Amphiura sp. (Ophiuroidea). All scale bars = 1 cm, except for e. scale bar = 2 mm.
Figure 3. Contribution to relative abundances of taxonomic groups found at the a. external and b. internal stations. The pie charts show the overall composition of assemblages, including polychaetes, with the expanded sections detailing the distribution of other taxa.
The total biomass measured was 6673.25 g WW m-2, of which 6239.25 g WW m-2 corresponded to the external station and 434 g WW m-2 corresponded to the internal station (Table S1). Ascidiacea (27.59%) and Polychaeta (30.10%) had similar percentages, followed by Holothuroidea (20.62%) and Ophiuroidea (14.25%), in terms of being the main contributors to the biomass at the external station (Fig. 4). At the internal station, the biomass was instead dominated by Ascidiacea (66.84%), followed by Polychaeta (16.45%) and Pycnogonida (14.90%; Fig. 4).
Figure 4. Contribution to relative biomasses of taxonomic groups found at the a. external and b. internal stations. The pie charts show the overall composition of assemblages, with the expanded sections detailing the distribution of other taxa.
Regarding Polychaeta, both sampling stations exhibited a high relative abundance, ranging from 84.62% to 90.74% for the family Maldanidae (i.e. M. sarsi antarctica; Fig. 5 & Table II), with a biomass of 1348.02 g WW m-2 at the external station and 56.08 g WW m-2 at the internal station (Fig. 6 & Tables II & S1). After maldanids, Cirratulidae (3.94–9.23%), Lumbrineridae (1.65–2.31%) and Spionidae (0.87–1.54%) were the most abundant polychaete families at both stations. For polychaetes, the main difference between the stations was the total number of families, with 15 being present at the external station compared to 7 being present at the internal station (Table S1).
Figure 5. Contribution to relative abundances of different Polychaeta families found at the a. external and b. internal stations. The pie charts show the overall composition of assemblages, including Maldanidae, with the expanded sections detailing the distribution of other families.
Figure 6. Contribution to relative biomasses of different Polychaeta families found at the a. external and b. internal stations. The pie charts show the overall composition of assemblages, with the expanded sections detailing the distribution of other taxa.
The distributions of functional groups based on their feeding behaviour (grazers, suspension-feeders, predators and detritivores) were similar at both stations, with a predominant contribution of detritivores. However, the external station showed slightly higher proportions of predators (5.07%) and detritivores (82.71%) but a lower proportion of suspension-feeders (7.59%) compared to the internal station (4.07%, 77.91% and 13.37%, respectively; Fig. 7). The contributions of the infauna and the epifauna were also similar at both stations, with much higher values observed for the infauna than the epifauna (Fig. 7).
Figure 7. Relative contribution (%) of all sampled individuals according to their feeding behaviour (left) and physical habitat (right).
Distribution and population features of Maldane sarsi antarctica
A total of 186 previous records were found for M. sarsi antarctica, in a depth range from 40 to 3700 m. Our map indicates a circum-Antarctic distribution for this species in the Antarctic benthos, with a major sampling focus on the South Shetland Islands and the Bellingshausen Sea and a significant lack of records in the southern Indian Ocean (Fig. S1 & Data S1).
The size distribution of M. sarsi antarctica ranged from 0.73 to 7.78 cm, with the highest concentration of individuals in the 2.85–3.55 cm (21.95%) and 3.55–4.26 cm (20.73%) intervals (Fig. S2, Table S2 & Data S2). The distribution follows a unimodal pattern, with a greater abundance of organisms in intermediate size classes and a gradual decrease towards the extremes (Fig. S2, Table S2 & Data S2). Therefore, these results indicate that the population is dominated by individuals in intermediate developmental stages, with fewer very small or large organisms.
Discussion
This study provides a first local-scale characterization of the marine soft-bottom assemblages inhabiting False Bay (Livingston Island). Our results show that benthic assemblages were dominated by Polychaeta in terms of relative abundance and a shared dominance of Ascidiacea, Ophiuroidea and Pycnogonida in terms of relative biomass, suggesting their successful adaptation to efficiently exploit the resources of this habitat. In particular, False Bay is a paradise for the maldanid M. sarsi antarctica, which is the most abundant species by far. Considering that several physical and ecological changes have been recorded widely in Antarctica due to global change and anthropogenic causes (Cook et al. Reference Cook, Holland, Meredith, Murray, Luckman and Vaughan2016, Rogers et al. Reference Rogers, Frinault, Barnes, Bindoff, Downie and Ducklow2020), this study represents a first step towards providing a baseline for future studies addressing marine conservation policies and the effects of these changes on these particular soft-bottom marine ecosystems.
Composition of benthic assemblages
Studies on soft-bottom benthic macrofauna involving both infaunal and epifaunal sources are relatively scarce, especially for Antarctica (Table III; Jażdżeski et al. Reference Jażdżeski, Jurasz, Kittel, Presler, Presler and Siciński1986, Sáiz-Salinas et al. Reference Sáiz-Salinas, Ramos, García, Troncoso, San Martín, Sanz and Palacin1997, Arnaud et al. Reference Arnaud, López, Olaso, Ramil, Ramos-Esplá and Ramos1998, Angulo-Preckler et al. Reference Angulo-Preckler, Leiva, Avila and Taboada2017b, Reference Angulo-Preckler, Figuerola, Núñez-Pons, Moles, Martín-Martín and Rull-Lluch2018). Additional studies are required to determine the current situation and the effects of global change in polar areas, including with regard to glacier retreat. Most recent studies, however, focus mainly on assessing changes in the mega-epifauna around Antarctica through seafloor imagery and on infaunal descriptions using environmental DNA (e.g. Vause et al. Reference Vause, Morley, Fonseca, Jażdżewska, Ashton and Barnes2019, Grimes et al. Reference Grimes, Donnelly, Ka, Noor, Mahon and Halanych2023). Our results reveal that in False Bay the dominant group was Polychaeta, followed by lower abundances of Bivalvia, Amphipoda and Ophiuroidea (only at the external station). The observed dominant taxonomic groups in this study align with previous descriptions from other regions of the planet (Table III). In such reports, with some exceptions, the Annelida, Crustacea, Mollusca and, sometimes, Echinodermata (especially in Antarctica) are the most dominant groups, even though differences exist in the sedimentary environment, the depth sampled (~5–4700 m) or the collecting methods used (e.g. Agassiz trawls, Van Veen dredges, box corers, scuba diving; Table III). These study areas include the tropical deep sea (Cosson et al. Reference Cosson, Sibuet and Galeron1997, Quintanar-Retama et al. Reference Quintanar-Retama, Vázquez-Bader and Gracia2023), temperate areas (Sardá et al. Reference Sardá, Martin, Pinedo, Dueso, Cardell, Eleftheriou, Ansell and Smith1995, Martins et al. Reference Martins, Quintino and Rodrigues2013, Soto et al. Reference Soto, Quiroga, Ganga and Alarcón2017), Arctic zones (Ellingsen Reference Ellingsen2001, Ellingsen & Gray Reference Ellingsen and Gray2002) and the South Shetland Islands in Antarctica (Sáiz-Salinas et al. Reference Sáiz-Salinas, Ramos, García, Troncoso, San Martín, Sanz and Palacin1997, Arnaud et al. Reference Arnaud, López, Olaso, Ramil, Ramos-Esplá and Ramos1998, Angulo-Preckler et al. Reference Angulo-Preckler, Tuya and Avila2017a,Reference Angulo-Preckler, Leiva, Avila and Taboadab, Reference Angulo-Preckler, Figuerola, Núñez-Pons, Moles, Martín-Martín and Rull-Lluch2018; Table III). However, in contrast to our work, in these previous studies there was no such clear dominance of a single group, and the total abundances were distributed among two or more different taxonomic groups. Only one previous study carried out in Admiralty Bay (King George Island, South Shetland Islands) found similar results to those reported here in terms of dominance, with Polychaeta constituting 67.7% of the total fauna at 250 m depth, and Bivalvia reaching 86% of the total fauna at 15 m depth (Jażdżeski et al. Reference Jażdżeski, Jurasz, Kittel, Presler, Presler and Siciński1986). In our study, the substantial differences in abundances between the internal and external sites could be related to the low number of replicates obtained in the study, but it could also be related to the distinct environmental parameters reported, such as the slightly higher temperature, higher chlorophyll-a concentration and higher dissolved oxygen concentration at the internal site. The internal site, being closer to the glacier front, was probably colonized much later than the external one, and this is relevant for both the species composition and the biomass found. In addition, near-shore marine Antarctic biodiversity may also be affected by shifts in the ice sheet caused by climate change, altering essential habitats (Pritchard et al. Reference Pritchard, Ligtenberg, Fricker, Vaughan, van den Broeke and Padman2012, Noble et al. Reference Noble, Rohling, Aitken, Bostock, Chase and Gomez2020). For instance, ocean warming has accelerated the melting of glaciers and the collapse of several ice shelves (Cook et al. Reference Cook, Holland, Meredith, Murray, Luckman and Vaughan2016, Davison et al. Reference Davison, Hogg, Gourmelen, Jakob, Wuite and Nagler2023). Additionally, sea ice now forms later and melts earlier within a calendar year, contributing to the intensification of global warming (e.g. Purich & Doddridge Reference Purich and Doddridge2023). Further studies are needed to carefully consider both of these possibilities.
Table III. Main taxonomic groups, based on the total abundance, found in previous similar studies from soft-bottom habitats around the world, from low to high latitudes.
a These values were calculated from the original data.
b Abundance data not available.
ind. = individuals.
Biomass in False Bay, however, was not dominated by a particular taxon but showed a distribution between Ascidiacea, Polychaeta, Holothuroidea, Ophiuroidea and Pycnogonida. These data are similar to those obtained in previous Antarctic studies, where biomass was also distributed among different groups, such as Ascidiacea, Echinodermata, Porifera, Polychaeta and, occasionally, Bryozoa (Jażdżeski et al. Reference Jażdżeski, Jurasz, Kittel, Presler, Presler and Siciński1986, Sáiz-Salinas et al. Reference Sáiz-Salinas, Ramos, García, Troncoso, San Martín, Sanz and Palacin1997, Arnaud et al. Reference Arnaud, López, Olaso, Ramil, Ramos-Esplá and Ramos1998). Considering the patchiness of Antarctic marine assemblages at small spatial scales and the complexity of the environmental variations, it is usually assumed that assemblage compositions can be highly variable (Gutt Reference Gutt2007, Gutt et al. Reference Gutt, Arndt, Kraan, Dorschel, Schröder, Bracher and Piepenburg2019). According to assembly theory, both stochastic and deterministic factors shape the structure of these marine communities (Vellend Reference Vellend2016). It is well established that marine fauna possessing specialized traits to exploit specific microhabitats can successfully colonize these areas, forming assemblages based on habitat affinity (Sebens Reference Sebens, Bell, McCoy and Mushinsky1991, Convey et al. Reference Convey, Chown, Clarke, Barnes, Bokhorst and Cummings2014). Unfortunately, few studies have addressed these dynamics in Antarctica, rendering the current conclusions only preliminary. Some research suggests that, at large spatial scales, certain assemblages are primarily influenced by random processes (e.g. Valdivia et al. Reference Valdivia, Garcés-Vargas, Garrido, Gómez, Huovinen and Navarro2021), whereas studies at smaller scales indicate that environmental filtering is the dominant driver (e.g. Valdivia et al. Reference Valdivia, Garrido, Bruning, Piñones and Pardo2020). Thus, the spatial scale at which assemblage mechanisms are examined appears to be highly relevant. In this context, we propose that strong small-scale environmental filtering in False Bay has probably produced local assemblages characterized by highly convergent functional traits, such as those observed in terms of the dominance of deposit-feeders (Gutt Reference Gutt2007). This assemblage composition could be related to the unique characteristics of the available organic matter, sediment grain size or seafloor stability of this particular glacier bay (Sañé et al. Reference Sañé, Isla, Gerdes, Montiel and Gili2012, Learman et al. Reference Learman, Henson, Thrash, Temperton, Brannock and Santos2016, Vause et al. Reference Vause, Morley, Fonseca, Jażdżewska, Ashton and Barnes2019). Further studies are needed to clarify all of these relationships, and in particular why M. sarsi antarctica is ‘in paradise’ within False Bay, exhibiting the highest values for abundance and biomass of all of the taxonomic groups found there.
Benthic assemblages in False Bay are clearly dominated by deposit-feeding infauna, suggesting that sedimentation of organic matter from phytodetritus to the seabed plays a fundamental role in shaping the trophic structure of the ecosystem, providing a constant food source for these organisms (Gutt Reference Gutt2007, Angulo-Preckler et al. Reference Angulo-Preckler, Figuerola, Núñez-Pons, Moles, Martín-Martín and Rull-Lluch2018). In this context, previous studies have proposed a ‘food banks’ theory for Antarctica (Glover et al. Reference Glover, Smith, Mincks, Sumida and Thurber2008, Mincks et al. Reference Mincks, Smith, Jeffreys and Sumida2008, Smith et al. Reference Smith, DeMaster, Thomas, Srsen, Grange, Evrard and DeLeo2012). This theory suggests that, due to the cold temperatures of the Southern Ocean, organic matter reaching the seafloor from phytodetritus flux and/or resuspension can remain available year-round for consumption by detritivorous (e.g. deposit-feeder) organisms (Glover et al. Reference Glover, Smith, Mincks, Sumida and Thurber2008). In contrast, the lower abundances of filter-feeding epifauna, such as Bryozoa and Ascidiacea, may indicate a more limited connection between the water column and the benthos, potentially resulting from the stability of the seabed, where the lack of strong currents reduces the availability of suspended particles for filter-feeders (Arnaud et al. Reference Arnaud, López, Olaso, Ramil, Ramos-Esplá and Ramos1998, Gutt Reference Gutt2007, Dunlop et al. Reference Dunlop, Harendza, Plassen and Keeley2020). Another potential cause of the low amount of epifauna in False Bay could be the dominant grain size of the sediment. Because of the lack of strong currents, the type of sediment that would accumulate most in this bay is detritic mud. As reported previously, epifaunal species are more abundant in sediments with medium or large grain sizes, such as sand or gravel, than in muddy sediments (Gray Reference Gray2002, Zhulay et al. Reference Zhulay, Iken, Renaud and Bluhm2019). However, despite the predominance of detritivores in False Bay, there were also some mobile epifaunal taxa characteristic of these infaunal-dominated assemblages that could obtain their food through predation or grazing, such as Ophiuroidea, Asteroidea, Amphipoda or Isopoda, although they appeared in much lower densities. This scenario suggests that the benthic community in this area may be highly specialized, with adaptations that enable organisms such as detritivores to thrive in an environment with specific environmental characteristics and ecological niches that support their development.
Ecological importance of Polychaeta and population features of Maldane sarsi antarctica
It has been widely described that polychaetes can significantly alter sediment conditions, which in turn can modify the structure and composition of benthic communities, emphasizing their role as engineering species in sedimentary marine ecosystems (Sanders et al. Reference Sanders, Goudsmit, Mills and Hampson1962, Levin et al. Reference Levin, Blair, DeMaster, Plaia, Fornes, Martin and Thomas1997, Waldbusser et al. Reference Waldbusser, Marinelli, Whitlatch and Visscher2004, Kongsrud & Rapp Reference Kongsrud and Rapp2012). In our study, polychaetes accounted for ~93% of the total relative abundance obtained, with totals of 15 and 7 different families observed at the external and internal stations, respectively. Those results are within the values of previous studies examining soft-bottom polychaete assemblages in Antarctica, with numbers of families ranging from 13 to 27 (Parapar et al. Reference Parapar, López, Gambi, Núñez and Ramos2011, Pabis & Sobczyk Reference Pabis and Sobczyk2015, Paiva et al. Reference Paiva, Seixas and Echeverría2015, Angulo-Preckler et al. Reference Angulo-Preckler, Leiva, Avila and Taboada2017b). This contrasting composition may be associated with various factors such as depth, sediment grain size and organic matter content.
Remarkably, the family Maldanidae emerged as a key group in our study, representing 90.74% of the total abundance for the external station and 84.62% for the internal station. Maldanids are known for building tubes in soft and mixed sediments in both horizontal and vertical orientations (Day Reference Day1967, Jiménez-Cueto & Salazar-Vallejo Reference Jiménez-Cueto and Salazar-Vallejo1997, De Assis et al. Reference De Assis, Alonso and Christoffersen2007). These tube constructions allow them to inhabit a wide range of habitats, from the intertidal region to the deep sea (Paterson et al. Reference Paterson, Glover, Barrio Froján, Whitaker, Budaeva, Chimonides and Doner2009, De Assis & Christoffersen Reference De Assis and Christoffersen2011). Maldanids feed mainly on organic matter, either in surface or in subsurface deposits, contributing to sediment irrigation and the supply of fresh organic matter for other infaunal organisms (Fauchald & Jumars Reference Fauchald and Jumars1979, Rouse & Pleijel Reference Rouse and Pleijel2001, Dufour et al. Reference Dufour, White, Desrosiers and Juniper2008, Jumars et al. Reference Jumars, Dorgan and Lindsay2015). Previous studies found that one of the most abundant maldanid species in Antarctica was M. sarsi antarctica Arwidsson, Reference Arwidsson1911, with maximum densities of between 139 and 2786 ind. m-2 (e.g. San Martín et al. Reference San Martín, Parapar, García and Redondo2000, Siciński Reference Siciński2000, Siciński et al. Reference Siciński, Jażdżewski, Broyer, Presler, Ligowski and Nonato2011, Pabis & Sobczyk Reference Pabis and Sobczyk2015, Cataldo-Mendez et al. Reference Cataldo-Mendez, Brante and Urzúa2024). This species was also the most dominant in our study, reaching up to a maximum abundance of 12 794 ind. m-2 in False Bay. Our data showed much higher abundances of this polychaete species than in previous studies on soft bottoms around the South Shetland Islands at depths ranging from 46 to 694 m (e.g. Jażdżeski et al. Reference Jażdżeski, Jurasz, Kittel, Presler, Presler and Siciński1986, Sáiz-Salinas et al. Reference Sáiz-Salinas, Ramos, García, Troncoso, San Martín, Sanz and Palacin1997). On the contrary, although the total polychaete assemblage is richer and more abundant at the shallower and volcanic-influenced habitat of Deception Island, this particular species is completely absent (Angulo-Preckler et al. Reference Angulo-Preckler, Leiva, Avila and Taboada2017b, Reference Angulo-Preckler, Figuerola, Núñez-Pons, Moles, Martín-Martín and Rull-Lluch2018). These findings suggest that the exceptionally high densities observed in False Bay reflect a combination of favourable food availability and stable benthic conditions that support deposit-feeding species while limiting the presence of suspension feeders. Additionally, recent research indicates that the energy content of this species is lower in areas near melting glaciers compared to more distant sites (Cataldo-Mendez et al. Reference Cataldo-Mendez, Brante and Urzúa2024). These findings collectively suggest that M. sarsi antarctica is a keystone species in Antarctic soft-bottom ecosystems, and further research on the species is essential for predicting how such marine assemblages might respond to global change.
Maldane sarsi antarctica has a circum-Antarctic distribution, probably influenced by oceanographic, ecological and evolutionary factors (Fig. S1). As a sedentary polychaete adapted to cold waters, its dispersal is driven by Antarctic marine currents that facilitate genetic connectivity and larval transport (Rogers Reference Rogers2007, Thatje Reference Thatje2012, Güller et al. Reference Güller, Puccinelli and Zelaya2020, Muñoz-Ramírez et al. Reference Muñoz-Ramírez, Barnes, Cárdenas, Meredith, Morley and Roman-Gonzalez2020). Their trochophore larva, with its high dispersal potential, plays a key role in this process (De Assis et al. Reference De Assis, Bleidorn, Crhistoffersen, Purschke, Westheide and Böggemann2021). Furthermore, the abundance of soft-bottom habitats in Antarctica provides an ideal environment in which maldanids can build tubes, supporting their ecological success (Pan & Pratolongo Reference Pan, Pratolongo, Pan and Pratolongo2022). Its wide distribution also suggests its persistence in glacial refugia, with evolutionary and plastic adaptations, such as cold and depth tolerance, enabling its survival through environmental changes (Thatje et al. Reference Thatje, Hillenbrand and Larter2005, Hoffmann & Sgrò Reference Hoffmann and Sgrò2011, Munday et al. Reference Munday, Warner, Monro, Pandolfi and Marshall2013). These characteristics, combined with the fact that the size-frequency histogram of this population displays a normal distribution with a central peak, suggest a stable and undisturbed population with balanced recruitment and mortality rates (Fig. S2).
Limitations of the study and future perspectives
The present study was conducted at two stations within a specific Antarctic fjord based on a limited sampling scope that did not allow for statistically representative spatial comparisons to be made within the bay. Despite this clear limitation, our study offers a comprehensive biological and ecological characterization of marine assemblages in a relatively underexplored region, establishing an important baseline for future research. These preliminary findings are significant for understanding the structure and functioning of the ecosystem and may also be useful in the development of conservation strategies for this region. Further studies should analyse the composition of the assemblages along the fjord after species begin to colonize the ice-free seabed following glacier retreat (Grange & Smith Reference Grange and Smith2013, Sahade et al. Reference Sahade, Lagger, Torre, Momo, Monien and Schloss2015, Lagger et al. Reference Lagger, Servetto, Torre and Sahade2017). It is important to note that although Huntress Glacier is not retreating as fast as other glaciers in the region (e.g. Cook et al. Reference Cook, Holland, Meredith, Murray, Luckman and Vaughan2016), studying such changes is key. In addition, it would be interesting to relate these biological variables with environmental data (e.g. chlorophyll-a, sediment grain size, temperature) to obtain a more holistic view of this ecosystem. These kinds of studies are also useful for improving our understanding of how fjords are becoming pivotal ‘blue carbon’ ecosystems in terms of negative feedback for climate change by sequestering and storing organic carbon in new ice-free habitats (Barnes et al. Reference Barnes, Sands, Cook, Howard, Roman Gonzalez and Muñoz–Ramirez2020, Zwerschke et al. Reference Zwerschke, Sands, Roman-Gonzalez, Barnes, Guzzi and Jenkins2022).
Several limitations were encountered when searching for biological and ecological data regarding M. sarsi antarctica in the literature. First, data on abundances and specimen sizes were not always provided (most of the available records do not provide species-specific quantitative information). This is because abundance data for M. sarsi antarctica were mainly reported collectively, grouping this species with other, different species from benthic assemblages of Polychaeta, while specimen size data were often completely absent. This limitation highlights the need for more specific quantitative studies on Antarctic fauna. Second, although 186 records of M. sarsi antarctica were found across the Southern Ocean (Fig. S1 & Data S1), some previous studies have identified this species as the parent species Maldane sarsi Malmgren, Reference Malmgren1865 (e.g. Drennan et al. Reference Drennan, Dahlgren, Linse and Glover2021), without specifically mentioning the subspecies M. sarsi antarctica. This is probably due to the fact that the subspecies’ diagnostic characteristics (colour and gland patterns) are not always considered robust taxonomic traits (Wang & Li Reference Wang and Li2016). Though recent studies, including Brasier et al. (Reference Brasier, Wiklund, Neal, Jeffreys, Linse, Ruhl and Glover2016), have suggested that the genetic differences and long geographical distances between Antarctic populations and the type locality of M. sarsi raise questions about the status of M. sarsi antarctica as a subspecies, being more likely a valid species on its own. Thus, although these records as M. sarsi were not considered in our analysis, they probably correspond to the same Antarctic subspecies here discussed. This issue highlights a gap in the current literature and in accurately identifying Antarctic M. sarsi, emphasizing the urgent need for further taxonomic clarification.
Conclusions
Our findings provide the first description of the benthic marine fauna from False Bay (Livingston Island, South Shetland Islands, Antarctica), revealing an assemblage dominated by infaunal, deposit-feeding polychaetes. In particular, False Bay is a paradise for the circum-Antarctic-distributed polychaete M. sarsi antarctica, which appears to have a stable and undisturbed population. Our data highlight the importance of exploring poorly studied areas to improve our knowledge of Antarctic biodiversity. Moreover, as many factors are currently threatening polar ecosystems, our results are key to establishing a baseline for assessing future changes in benthic assemblages, especially those related to global change and other human impacts.
Supplementary material
To view supplementary material for this article, please visit http://doi.org/10.1017/S0954102025000185.
Acknowledgements
We appreciate the contributions of all members involved in the CHALLENGE project and their assistance in sample collection and sorting during the Antarctic campaign. We thank the R/V BIO-Hespérides crew for their support during the cruise. We also thank the taxonomic specialists Dr David Barnes (Bryozoa), Dr Anna Soler (Pycnogonida), Dr Katrin Linse (Bivalvia), Dr Chester Sands (Ophiuroidea), Dr Camille Moureau (Asteroidea), Dr Anna Jażdżewska (Amphipoda), Dr Stephanie Kaiser (Isopoda), Dr Sebastián Rosenfeld (Gastropoda) and Dr José-Ignacio Sáiz-Salinas (Priapulida) for their contributions to the identification and classification of the marine fauna. This study is a contribution to the ‘Integrated Science to inform Antarctic and Southern Ocean Conservation’ (Ant-ICON) research programme supported by the Scientific Committee on Antarctic Research (SCAR). We also wish to thank the reviewers of the manuscript for their very helpful comments and suggestions.
Competing interests
The authors declare none.
Author contributions
MB and CA conceptualized and designed the study; MB, AP, MG-P, PM-R, TA-G, AA-M, GM-V, YT, EB and CA were responsible for sample collection; MB, AP, MG-P, PM-R, TA-G, AA-M, MdL and JG conducted the laboratory analyses and bioinformatic analyses; MB and AP processed, interpreted and discussed the results; MG-P performed the data visualization; CA supervised the study and secured the funding; MB and AP drafted the initial manuscript with contributions from all of the authors to the final version.
Financial support
This work was supported by the CHALLENGE (PID2019-107979RB-100 funded by MICIU/AEI/10.13039/501100011033) and CHALLENGE-2 (PID2022-141628NB-I00 funded by MICIU/AEI/10.13039/501100011033) research grants from the Spanish Government and by European Regional Development Fund (ERDF-EU). The ‘Marine Biodiversity and Evolution’ SGR (Research Quality Group) funding of the Generalitat de Catalunya (MBE; 2021SGR01271) also supported this work. MB acknowledges financial support from Agencia Nacional de Investigacion y Desarrollo (ANID), Becas Chile para Doctorado en el extranjero 72220140. AP acknowledges financial support from an FPI Grant (PRE2019-091200) funded by MICIU/AEI/10.13039/501100011033 and by ‘ESF Investing in your future’. MG-P acknowledges financial support from a FI-SDUR 2021 fellowship from the Departament de Recerca i Universitats of the Catalan Government (Generalitat de Catalunya). PM-R acknowledges financial support from PREDOCS-UB. TA-G acknowledges financial support from a Severo Ochoa FPI Grant PRE2020-096185 funded by MCIN/AEI/10.13039/501100011033 and by ‘ESF Investing in your future’. AA-M acknowledges funding from the CHALLENGE project during this work. JG was funded by an ongoing collaborative protocol signed with CRÉOCÉAN (France).
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