Description: Glass sponges (Porifera: Hexactinellida) are important structuring elements in many Antarctic shelf communities. Despite many years of research on their ecology, species identification in the common genus Rossella and distinction against the well-studied species Anoxycalyx (Scolymastra) joubini remain problematic. The two main problems are: (1) the unresolved taxonomy of the genus Rossella which is in dire need of revision, and (2) the high morphological variability of some species. Some of the confusion is due to the fact that early species descriptions are partly based on very small individuals or just fragments and are often not comprehensive or clear enough. They usually focus on the spicules and include little information on the general external morphology of the species. However, with the increasing use of non-invasive techniques, such as Remotely Operated Vehicles (ROVs) and other camera systems, researchers have to rely on macroscopic features to identify species, and a working concept to this effect is required right away. We found that it is possible to reliably identify some common Antarctic sponge species based on macroscopic characteristics. This Data Collection contains a wide range of images of the currently well-established species of Antarctic Rossellidae, showing the macroscopic features typical for each species and morphological differences between similar species according to our current understanding. It includes photographs of freshly collected and dried specimens, as well as in situ images recorded by ROVs. We provide a large number of images to showcase the morphological variability and show with detailed images that the same macroscopic characteristics can be observed in collected specimens and in situ. For identification and description of Antarctic glass sponges, it is important to take into consideration not only the spicules and molecular data, but the outer morphology, as well. Therefore, we encourage everybody to use the images provided in this Data Collection as reference material.

Description: Over 30% of the Antarctic continental shelf is permanently covered by floating ice shelves, providing aphotic conditions for a depauperate fauna sustained by laterally advected food. In much of the remaining Antarctic shallows (〈300 m depth), seasonal sea-ice melting allows a patchy primary production supporting rich megabenthic communities dominated by glass sponges (Porifera, Hexactinellida). The catastrophic collapse of ice shelves due to rapid regional warming along the Antarctic Peninsula in recent decades has exposed over 23,000 km**2 of seafloor to local primary production. The response of the benthos to this unprecedented flux of food is, however, still unknown. In 2007, 12 years after disintegration of the Larsen A ice shelf, a first biological survey interpreted the presence of hexactinellids as remnants of a former under-ice fauna with deep-sea characteristics. Four years later, we revisited the original transect, finding 2- and 3-fold increases in glass sponge biomass and abundance, respectively, after only two favorable growth periods. Our findings, along with other long-term studies, suggest that Antarctic hexactinellids, locked in arrested growth for decades, may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats. The cues triggering growth and reproduction in Antarctic glass sponges remain enigmatic.

Description: Sponges (Porifera) host diverse and species-specific communities of microbial symbionts with which they maintain tight interactions. However, knowledge on the microbiomes of sponges from deep waters and remote polar areas is still scarce, especially for the sponge class Hexactinellida. Therefore, our aim was to describe the community composition, richness and density of microbial symbionts of Antarctic deep-water sponges, including several species of hexactinellids, and relate the findings to host ultrastructure and histology. During the Antarctic expeditions PS96 (RV Polarstern, 2015/16, eastern Weddell Sea) and JR17003a (RRS James Clark Ross, 2018, western Weddell Sea), 28 sponge specimens, bottom water and sediment were sampled for molecular analysis of microbial communities. The sponges were collected from deep habitats of 290-845 m by Bottom Trawl or Agassiz Trawl and comprised 19 hexactinellids and 9 demosponges. Bottom water was collected with a CTD rosette sampler which also measured environmental data (temperature, salinity, oxygen) close to the start or end point of the trawls and at four additional stations. Sediment was collected from the Agassiz trawl together with sponge samples during JR17003a. The molecular microbiome analysis targeted bacteria and was based on 16S rRNA gene sequencing of the V3-V4 variable regions. Sequences were processed using the QIIME2 environment. Amplicon sequence variants (ASVs) were generated with the DADA2 algorithm and classified based on the Silva 132 99% OTUs 16S database. Eight sponge specimens collected during JR17003a were further investigated microscopically for microbial symbionts, sponge histology and ultrastructure. Histological sections of 7-30 µm were stained in either Masson's trichrome or Hematoxylin/Eosin and images captured on a Zeiss Axioskop 2 plus with a QiCam camera using Northern Eclipse software. Ultrastructural sections of 60 nm were stained in uranyl acetate and lead citrate and then viewed and photographed with a Philips Morgagni transmission electron microscope equipped with a Gatan CCD camera. This Data Collection includes sampling information, environmental data, NCBI accession numbers and photographs of the analyzed sponges, data on the microbial symbiont communities (amplicon sequence variants and microbial phyla) of sponges, seawater and sediment, as well as micrographs of sponge histology and ultrastructure.