Benthic foraminifera are ubiquitous marine organisms in Antarctica, but little is known about their ecology despite their importance as climate-change proxies. A fundamental question remains unanswered: is foraminiferal diversity highly localized and similar to reports for Antarctic benthic invertebrates? If so, high endemism would result driven by dispersal limitation with little connectivity among sites. To answer this question, the composition, abundance, spatial distribution and alpha (α) and beta (β) diversity of foraminiferal species that encrust living shells of the Antarctic scallop Adamussium colbecki were compared within and between two sites located 30-km apart in McMurdo Sound, Ross Sea, Antarctica: Bay of Sails (BOS), an open coastal region with annual sea-ice cover, and Explorers Cove (EC), a protected embayment with multiannual sea-ice cover.

Foraminiferal community structure differed between the sites, generated by variations in species abundance, turnover of rare species, and heterogeneity in species composition between top and bottom valves. Foraminifera were significantly more common on the scallop’s auricles, perhaps driven by nutrients generated by the scallop. Calcareous taxa were significantly more abundant at EC despite lower seawater pH, while agglutinated taxa were significantly more abundant at BOS. Alpha diversity (species richness, Shannon H, eH) was higher at BOS than at EC and is similar to deep-sea diversity reported for Caribbean encrusting foraminifera. Based on Shannon H, foraminifera at EC may represent a stressed community dominated by Cibicides antarcticus, while those at BOS may represent a transitional community. Within-site β-diversity was high, spurred by the turnover of rare species between top and bottom valves, which acted as separate microhabitats. Higher β-diversity at BOS suggests that foraminiferal dispersal and recruitment were more localized than at EC. Additive partitioning revealed that encrusting diversity was mostly held at the local level, and that BOS and EC have high connectivity, similar to deep-sea communities. The Antarctic endemicity of these protists is questionable, and is in stark contrast to Antarctic invertebrates with high within- and between-site diversity. Therefore, Antarctic encrusting foraminifera appear to be influenced primarily by niche-based processes, rather than dispersal limitation. Indeed, habitat islands, such as carbonate shells, are essential for generating Antarctic diversity, and are integral for the conservation of Antarctica’s unique polar ecosystem.

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