Nearshore marine benthic algal communities along the western Antarctic Peninsula harbour extremely high densities of amphipods that probably play important roles in nutrient and energy flow. This study extends our evaluation of the importance of amphipods in the nearshore Antarctic Peninsular benthic communities and focuses on sponge associations. We found a mean density of 542 amphipods per litre (L) sponge for twelve species of ecologically dominant sponges. The highest mean density (1295 amphipods per L sponge) occurred withDendrilla membranosaPallas. The amphipod community associated with the 12 sponges was diverse (38 species), with mean species richness values ranging from two to eight species. Mean Shannon diversity indices (H’) ranged from 0.52 to 1.49. Amphipods did not appear to have obligate host relationships. Qualitative gut content analyses indicated that 12 of the 38 amphipod species were found with sponge spicules in their guts. However, only one of the amphipods,Echiniphimedia hodgsoniWalker, had considerable amounts of spicules in the gut. Organic lipophilic and hydrophilic extracts of the twelve sponges were presented in alginate food disks to a sympatric omnivorous amphipod in feeding bioassays and extracts of only two sponges deterred feeding.

Plocamium cartilagineum is a common red alga on the benthos of Antarctica and can be a dominant understory species along the western Antarctic Peninsula. Algae from this region have been studied chemically, and like “P. cartilagineum” from other worldwide locations where it is common, it is rich in halogenated monoterpenes, some of which have been implicated as feeding deterrents toward sympatric algal predators. Secondary metabolites are highly variable in this alga, both qualitatively and quantitatively, leading us to probe individual plants to track the possible link of variability to genetic or other factors. Using cox1 and rbcL gene sequencing, we find that the Antarctic alga divides into two closely related phylogroups, but not species, each of which is further divided into one of five chemogroups. The chemogroups themselves, defined on the basis of Bray-Curtis similarity profiling of GC/QqQ chromatographic analyses, are largely site specific within a 10 km² area. Thus, on the limited geographical range of this analysis, P. cartilagineum displays only modest genetic radiation, but its secondary metabolome was found to have experienced more extensive radiation. Such metabogenomic divergence demonstrated on the larger geographical scale of the Antarctic Peninsula, or perhaps even continent-wide, may contribute to the discovery of cryptic speciation.

Gastropods are an important component of subtidal Antarctic communities including in common association with macroalgae. Nonetheless, limited data exist detailing their abundance and distribution on macroalgal species. This study documents the abundance and species composition of gastropod assemblages on the two largest, blade-forming Antarctic macroalgae, Himantothallus grandifolius and Sarcopeltis antarctica, sampled across two depths (9 and 18 m) at four sites for each species off Anvers Island, Antarctica. Gastropods were also enumerated on Desmarestia anceps, Desmarestia antarctica and Plocamium sp. but were not included in the main analyses because of small sample sizes. There were major differences between the gastropod assemblages on deep vs shallow H. grandifolius and S. antarctica with much higher numbers of individuals and also greater numbers of gastropod species at the greater depth. Differences between the gastropod assemblages on H. grandifolius and S. antarctica across sampling sites were apparent in non-parametric, multivariate analyses, although depth contributed more than site to these differences. Within common sites, assemblages on H. grandifolius were significantly different from those on S. antarctica at 18 m depth but not at 9 m depth, indicating that the host species can be but is not always more important than site in influencing the gastropod assemblages.

Biogenic carbonate production in benthic marine ecosystems is dominated by representatives of the Echinodermata. Carbon and other major, minor, and trace elements are exported to the seabed where they accumulate or dissolve. Preserved carbonates (Mg‐calcite) have applications in oceanography and geochemistry and are used to reconstruct various parameters of ancient seawater, such as temperature (from Mg/Ca, Sr/Ca), seawater Mg/Ca (from Mg/Ca), and pH (from B/Ca). In general, the benthos is widely ignored for its role in the global carbon cycle despite the importance of echinoderms as a carbon sink (∼0.1–0.2 Pg C/yr). Echinoderms produce their skeletons from Mg‐calcite, which is more soluble than pure calcite and, therefore, more vulnerable to ocean acidification (OA). Minor and trace elements can also destabilize the calcite lattice, increasing the mineral's solubility. Little is known about the concentration of such elements in echinoderm tests. Expanding our knowledge on echinoderm skeleton composition will improve our understanding of elemental flux in the oceans. Furthermore, establishing relationships between the physical parameters of seawater and minor/trace elemental ratios within echinoderm Mg‐calcite should expand the utility of fossils as geochemical archives. Herein, we present elemental composition data for Asteroidea (n = 108; 9 families, 23 species), Echinoidea (n = 94; 8 families, 12 species), Ophiuroidea (n = 24; 4 families, 5 species), Holothuroidea (n = 7; 3 families, 3 species), and Crinoidea (n = 3; 1 family, 1 species), collected from the Arctic to the Antarctic oceans, from depths ranging from surface waters to 1200 m. The following elements were measured and normalized to [Ca]: Li, Be, Mg, Al, P, S, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Mo, Ag, Cd, Sn, Sb, Te, Ba, La, Ce, Nd, Dy, W, Re, Au, Hg, Tl, Pb, Bi, and U. Data are presented for the whole body, arms (plates), calcareous ossicles, spines, and test plates. Elements were quantified using inductively coupled plasma mass spectrometry. Our study presents the most comprehensive data set to date for a phylum whose skeletons are composed of Mg‐calcite.