Description: The colonization of a new habitat affords an important advantage to the successful pioneer species because competitors, predators and parasites are likely to be out of step. Here, we report the first retrieval and molecular characterization of a cryo-benthic community of isopod crustaceans that live at depths of 80-150m on the underside of a floating shelf-ice tongue at the Drescher Inlet (Riiser-Larsen Ice Shelf), Eastern Weddell Sea. The specimens were retrieved with a sampler mounted on a remotely operated vehicle (ROV) when video transsects were carried out. The molecular analysis of mitochondrial and nuclear genes of four specimens spanning the size range of individuals collected confirmed that (1) all belonged to a single species and that (2) this species has previously been identified in benthic communities in the Eastern Weddell Sea as Antarcturus cf. spinacoronatus. The molecular phylogeny shows that the cryo-benthic A. cf. spinacoronatus are deeply nested in a family of isopods characterized by increasing complexity of morphological and behavioural adaptations to the acquisition of detrital and planktonic food particles. This demonstrates that the floating shelf-ice was likely colonized from the seafloor and not vice versa and that the filter-feeding life style of A. cf. spinacoronatus formed a predisposition playing a key role in the colonization of the new habitat. Density estimates of A. spinacoronatus under the floating shelf ice (25 adults and 190 juveniles per square meter) are significantly higher than on the seafloor, suggesting that the transition to the new habitat devoid of any macrofaunal competition or predation provides a major advantage to the species and thus may be a geographically more widespread phenomenon.

Description: The colonization of a new habitat affords an important advantage to the successful pioneer species because competitors, predators and parasites are likely to be out of step. Here, we report the first retrieval and molecular characterization of a cryo-benthic community of isopod crustaceans that live at depths of 80-150m on the underside of a floating shelf-ice tongue at the Drescher Inlet (Riiser-Larsen Ice Shelf), Eastern Weddell Sea. The specimens were retrieved with a sampler mounted on a remotely operated vehicle (ROV) when video transsects were carried out. The molecular analysis of mitochondrial and nuclear genes of four specimens spanning the size range of individuals collected confirmed that (1) all belonged to a single species and that (2) this species has previously been identified in benthic communities in the Eastern Weddell Sea as Antarcturus cf. spinacoronatus (Baltzer et al 2000). The molecular phylogeny shows that the cryo-benthic A. cf. spinacoronatus are deeply nested in a family of isopods characterized by increasing complexity of morphological and behavioural adaptations to the acquisition of detrital and planktonic food particles. This demonstrates that the floating shelf-ice was likely colonized from the seafloor and not vice versa and that the filter-feeding life style of A. cf. spinacoronatus formed a predisposition playing a key role in the colonization of the new habitat. Density estimates of A. spinacoronatus under the floating shelf-ice (25 adults and 190 juveniles per square meter) are significantly higher than on the seafloor, suggesting that the transition to the new habitat devoid of any macrofaunal competition or predation provides a major advantage to the species and thus may be a geographically more widespread phenomenon. Our results are an indication that earlier reports of increased mid-water foraging of Weddell Seals (Liebsch et al. 2007) and the identification of filter-feeding isopods on seal-mounted still images from the underside of the shelf ice (Watanabe et al. 2006) are causally linked. References Baltzer, C., Held, C., & Wägele, J. W. 2000. Furcarcturus polarsterni gen. nov., sp nov., a large deep-sea arcturid isopod from the Drake Passage, with a preliminary molecular characterization. Polar Biology, 23(12), 833-839. Liebsch N., Wilson R.P., Bornemann H., Adelung D. & Plötz J. 2007. Mouthing off about fish capture: jaw movements in pinnipeds reveal the real secrets of ingestion. Deep Sea Research II 54:256-269. Watanabe, Y., Bornemann, H., Liebsch, N., Plötz, J., Sato, K., Naito, Y., & Miyazaki, N. 2006. Seal-mounted cameras detect invertebrate fauna on the underside of an Antarctic ice shelf. Marine Ecology Progress Series, 309, 297-300.

Description: The Weddell seal (Leptonychotes weddellii) inhabits the Antarctic coastal ecosystem and aggregates in areas characterized by a stable fast ice layer. Due to their extreme diving capabilities, they are able to exploit both pelagic and benthic prey resources. They mainly feed on fishes but occasionally also take cephalopods and crustaceans. Weddell seals instrumented with still-picture camera loggers in the Drescher Inlet, eastern Weddell Sea, detected an unknown cryo-benthic community underneath the floating ice shelf. Images show dense aggregations of invertebrates that likely represent an attractive food horizon for Weddell seals. In this context, we conducted a retrospective analysis of dive profiles collected in the Drescher Inlet to identify favoured hunting depths of Weddell seals and correlate those to the local physical and biological environment. A total of 34 adult Weddell seals were instrumented with dive loggers in the course of six summer field campaigns between 1990 and 2016. An automated broken stick algorithm was used to separate each dive profile into different segments. Segments with a high sinuosity were considered to indicate hunting. Segments characterized by a straight dive trajectories (low sinuosity), were assumed to be transit phases with no hunting activities. A tri-modal distribution of mean hunting depths suggests that Weddell seals concentrated their foraging activities in three depth strata. A peak in hunting depths below 370 m corresponds to the sea floor of the Drescher Inlet, indicating demersal foraging. A second peak between 110 and 160 m matches with the depth of the underside of the floating ice shelf, which suggests shelf ice associated foraging. The third peak probably represents hunting in the pelagic realm. Our investigation highlights the importance of the shelf ice underside as an attractive food horizon for Weddell seals suggesting a re-evaluation of trophic interactions and bentho-pelagic processes in the coastal Antarctic ecosystem.

Description: The Drescher Inlet is a 25km long crack in the Riiser-Larsen Ice Shelf, eastern Weddell Sea, and is characterized by a stable fast ice layer. The fast ice represents an attractive habitat for Weddell seals, aggregating along the tidal cracks of the inlet in numbers of ca. 200 – 300 individuals. Multiple field campaigns aimed to investigate the seals’ diving behaviour and foraging ecology in relation to their environment. A recent joint seal and ROV project was focussed on the investigation of the cryo-benthic community underneath the floating ice shelf of the Drescher Inlet, which were previously detected by seal-borne cameras, and 3D-multi-channel dive loggers (Watanabe et al. 2006; Liebsch et al. 2007). Images show dense aggregations of isopods that likely represent an attractive food horizon, where seals could benefit from a local hotspot of high biological productivity. This context triggered a retrospective analysis of available high-resolution dive profile data to identify within-dive hunting phases and correlate those to the local physical environment. A total of 34 adult Weddell seals were instrumented with different types of dive loggers (time-depth recorders, digital still image loggers) in the course of six summer field campaigns between 1990 and 2016. Dive profiles were zero-offset corrected to reduce noise in the dataset and subsequently only dives deeper than 20m were regarded as ‘true’ dives, accounting for the thickness of the local fast ice and platelet ice layer. An automated broken stick algorithm (Heerah et al. 2014) was used to separate the dive into different segments. Segments with a high sinuosity were considered to indicate hunting. Segments characterized by a straight dive trajectories (low sinuosity), were assumed to be transit phases with no hunting behaviour. A tri-modal distribution of mean hunting depths suggests that Weddell seals concentrated their foraging activities in three depth strata. A peak in hunting depths below 370m corresponds to the sea floor of the Drescher Inlet, indicating demersal foraging. A second peak between 110 and 160m matches with the depth of the underside of the floating ice shelf, which suggests shelf ice associated foraging. The third peak probably represents hunting in the pelagic realm. Our investigation highlights the importance of the shelf ice underside as an attractive food horizon for Weddell seals suggesting a re-evaluation of trophic interactions and bentho-pelagic processes in the coastal Antarctic ecosystem.

Description: Images taken by seal-borne cameras at the Drescher Inlet (Riiser-Larsen Ice Shelf) in 2004 led to the discovery of an unknown cryo-benthic community being attached head-down to the underside of the floating ice shelf at depths of around 130-150m (Watanabe et al. 2006). Resolution and exposure of the images did not allow distinct identification of species considered as likely isopods or cnidarians, and no information could be gained on the composition, size, dimension and density of the faunal aggregation. In order to re-assess the earlier findings, an Ocean Modules V8Sii Remotely Operated Vehicle (ROV) was launched through an artificial ice hole in the vicinity of the shelf ice cliff. The ROV provided high resolution and scalable video footage of the shelf-ice associated fauna. A custom-made ROV-mounted dredge was used to collect samples for further identification and DNA-analysis. A total of thirty specimens (15 adults, 15 juveniles) were sampled between 60 and 80m water depth. Additionally, 2 hours of high resolution digital video footage could be taken during two ROV transects at depths between 80m and 100m (along the vertical shelf ice wall and beneath the floating shelf ice). According to our standard ROV procedure, concurrent hydrographic (depth, temperature, salinity, pH, fluorescence, and oxygen) and three-dimensional dive data (depth, roll, pitch, orientation, and position) were recorded. Acoustic distance measurements and lens geometry of parallax free optics allowed to determine the strip width (0.5 – 2.0m) and cumulative length (122m) of the transect covering a total area of 133 square meters within the footage. The continuous scenes of video recordings were converted into single frame sequences and the number of individuals on the images was determined by visual counts. Tagging each individual with a marker and automatic registration of number and coordinates, prevented double counts and allowed for further distribution analyses. In total, 3,280 adults as well as 10,368 juveniles could be identified, leading to initial abundance estimates of 25 adults and 190 juveniles per square meter. The video footage shows large numbers of probably one single species of isopods. Significant size differences and clustering of the dense aggregations imply a specific association of adults and juvenile life stages, and the successful retrieval of samples provided material for further taxonomic and genetic investigations.

Description: Incidences of cryo-benthic communities beneath ice shelves are rare and recent discoveries. Images taken by seal-borne cameras at Drescher Inlet (Riiser-Larsen Ice Shelf, eastern Weddell Sea) in 2004 led to the discovery of a hitherto unknown cryo-benthic community of crustaceans being attached head-down to the underside of the floating ice shelf at depths between 130-150m (Watanabe et al. 2006). Resolution and exposure of these images did not allow distinct identification on species level, being considered as likely isopods or cnidarians, and no information could be gained on the composition, size, dimension and density of the faunal aggregation at that time. Recently, however, a re-assessment and augmentation of the earlier findings (Bornemann et al. 2016) has become feasible due to the use of combined seal- and ROV-borne imagery and novel sampling technologies. The Drescher Inlet is a 25km long and between 2 and 4km wide crack in the Riiser-Larsen Ice Shelf and characterized by perennial fast ice that disintegrates at irregular intervals. The fast ice of the inlet provides habitat for Weddell seals, hauling-out along tidal cracks in numbers of 200 – 300 individuals. In December 2016 German logistics and the research platforms Neumayer Station III and RV Polarstern coordinated a field camp on the ice shelf close to the inlet that had been maintained for four weeks. Weddell seals were instrumented with infrared still picture camera loggers (IR-DSL; Little Leonardo, JP) or CTD-combined satellite relay data loggers (CTD-SRDL; Sea Mammal Research Unit, UK). A Remotely Operated Vehicle (ROV; Ocean Modules, SE) was launched through a hole dug in the fast ice near the shelf ice cliffs. Seal-borne IR-DSLs took close-ups of aggregations of isopods underneath the floating ice shelf at 100m water depth. ROV-borne high resolution video footage showed dense aggregations of a single morphospecies of filter-feeding isopods. Significant size differences and clustering of the isopod aggregations imply a specific association of adults and juvenile life stages along the scallop structure of the shelf ice underside. A custom-made ROV-mounted dredge collected several specimens at depths between 60 and 80m. Molecular barcodes from a dozen specimens from all size classes and both sexes revealed all individuals as members of a single species. Furthermore, an exact molecular match of these newly sampled cryobenthic isopods with Antarcturus cf. spinacoronatus from nearby benthic communities (Baltzer et al. 2000) confirms that this species inhabits the seabed, too, albeit at significantly lower abundances. The aggregations of isopods are likely to represent an attractive food horizon, where seals could benefit from a local hotspot of highly biological productivity. The factors contributing to this hotspot and its relation to physical processes are not yet understood. We therefore compiled available local physical and biological data and will discuss their relevance in the wider regional context for this faunal hotspot. These include data on shelf, sea and platelet ice, seafloor topography, and hydrography, as well as associated pelagic and benthic marine life. Diving profile data from over thirty adult Weddell seals that had been instrumented with time-depth recorders during the course of six field campaigns between 1990 and 2016 were retrospectively analyzed using an automated broken stick algorithm (Heerah et al. 2014), to identify within-dive hunting phases and to correlate those to the local physical environment of the Drescher Inlet. A tri-modal frequency distribution of mean hunting depths suggests that Weddell seals concentrated their foraging in three depth strata corresponding with pelagic (20-50m), shelf ice associated (110-160m), and demersal (below 370m) foraging depths. Midwater and demersal foraging depths are concordant with data from air-borne and bathymetric measurements of the shelf ice contour and seafloor topography. Our results indicate that earlier reports of increased mid-water foraging of Weddell Seals (Plötz et al. 2001; Liebsch et al. 2007) and the identification of filter-feeding isopods on seal-borne still images from the underside of the shelf ice (Watanabe et al. 2006) are causally linked.