Unusual trophic strategies of Hydractinia angusta (Cnidaria, Hydrozoa) from Terra Nova Bay, Antarctica

Hydractinia angusta Hartlaub, 1904 has been recorded at Terra Nova Bay (Ros s Sea. Antarctica) as epizoic on shells of the Antarctic scallop Adamussium colbecki. The species can exploit different trophic resources: first, poly ps are able to detach and ingest tube feet and pedicellariae from the sea u rchins Sterechinus neumayeri, grazing on the scallop shell, and second, the y also eat masses of benthic diatoms settled among the hydrorhiza of the co lony. The particular relationship observed between the hydroid and one of t he most common Antarctic sea urchins may prevent or reduce the damage to A. colbecki shells, otherwise mused by the grazing of sea urchins on the alga l film of the upper valve of the scallops. H. angusta is the first known sp ecies of hydroid that exploits prey several times its own size and the seco nd that does not ingest entire prey but portions of them. The use of benthi c diatoms as a food resource has previously been documented for the sub-Ant arctic marine hydroid Silicularia rosea.