POPULATION AND SIZE DISTRIBUTION OF CIBICIDES ANTARCTICUS ON ADAMUSSIUM COLBECKI WITHIN EXPLORERS COVE, ANTARCTICA

We studied the population and size distribution of the parasitic foraminifer Cibicides antarcticus living on the shell of the Antarctic scallop Adamussium colbecki within Explorers Cove, western McMurdo Sound, Antarctica. Previous work examined populations and parasite load between two distinct geographic locations, but our study focuses on the population and size distribution of C. antarcticus within one embayment, Explorers Cove. We hypothesize that if A. colbecki are living in the same embayment and has one recruitment event, then C. antarcticus populations and their size distributions should be similar; but, if they have differing populations and sizes, they likely are recruiting from very localized microhabitats with varying recruitment events.

Live A. colbecki were collected from three sites in Explorers Cove: Jamesway (water depth 24.4 m), Smallberg (9.1 m), and Anoxic Pit (9.1 m). Five top valves from each site were examined for C. antarcticus under 75x magnification. The foraminifera were counted, their spatial distribution noted, and their largest diameter was measured using ImageJ. All data from each site was pooled to compare the sites.

Results indicate that all the sites had different populations of parasitic C. antarcticus. Smallberg had the most parasitic foraminifera (n = 663), followed by Jamesway (n = 319) and the Anoxic Pit site had the fewest (n = 55). The largest size classes (0.70–1.30 mm) occurred at Anoxic Pit and Smallberg, while the smallest size classes (0.18–0.70 mm) were found at Jamesway, the deepest site. The average size of Cibicides was also smaller at Jamesway (0.73 mm) compared to Smallberg (0.89 mm) and Anoxic Pit (0.91 mm). In general, C. antarcticus recruits to the youngest part of the scallop shell while larger adults are found on the oldest part of the shell. The skewed size frequency distributions and differing population sizes suggest that C. antarcticus has localized microhabitat recruitment in Explorers Cove, rather than one synchronous recruitment event.