PARASITISM BY THE FORAMINIFERAN, CIBICIDES, ON ITS HOST, THE ANTARCTIC SCALLOP, ADAMUSSIUM COLBECKI: A KEY TO DEPTH AND ENVIRONMENT

The fossil record and paleobiogeography of parasitism is relatively unknown. In Antarctica, a parasitic foraminiferan, Cibicides, commonly encrusts the shells of the Antarctic scallop, Adamussium colbecki. Antarctic Cibicides is known to have 3 trophic modes: filter feeding, grazing the scallop shell and parasitizing the scallop by boring a hole through the shell. Of these modes, parasitism lends itself to evolutionary studies because the boreholes provide a record of this behavior. Yet, the frequency of boring (parasite load) and the distribution of the parasitic boreholes on the valves of A. colbecki are not well documented. Therefore, we examined the parasitic behavior in Cibicides on 30 haphazardly-chosen A. colbecki valves (five valves each depth/site; two depths: 9 m, 24 m) from 3 sites in McMurdo Sound, Ross Sea, Antarctica: Bay of Sails (BOS), with annual sea-ice and open-ocean conditions; Herbertson Glacier (HG), with annual to semiannual sea-ice within the Ferrar embayment; and, Explorers Cove (EC), an embayment with multiannual sea ice with limited flushing by oceanic currents.

Of a total of 4,521 Cibicides, only 12% completely bored through the host’s shell. Parasite loads varied by site: Shallow (9m) BOS had the highest frequency of parasitism (0.48), while the shallow HG9m had the lowest (0.01). Parasite load also varied with depth: shallow BOS had a significantly higher parasite load than deeper BOS (Fisher Exact Test (FT), P < 0.0001); a similar pattern occurred for EC. In contrast, parasite loads were significantly higher at the 24mHG site than 9mHG (FT, P < 0.0001). The siting of parasitic boreholes also varied with depth: boreholes sited at the center of A. colbecki shells were significantly more common at 9mBOS than 24mBOS (FT, P < 0.0001) and at 24mEC than 9mEC (FT, P < 0.02); borehole siting was not significantly different with depth at HG. While parasitism was relatively uncommon for Antarctic Cibicides, it targets the high protein area of the scallop’s muscle region. Center-sited boreholes varied with depth, but the abundance of boreholes was significantly more common at 9m than 24m at two of the three sites. Parasite loads were significantly higher in shallow open-ocean conditions with annual sea-ice cover, suggesting differences in nutrient loads may be driving this unique parasitic behavior.