The influence of the epizoic hydroid Hydractinia angusta on the recruitment of the Antarctic scallop Adamussium colbecki

An experiment and field study were carried out in order to investigate the epibiotic relationship between the hydroid Hydractinia angusta Hartlaub, 19 04 and its host, the Antarctic scallop Adamussium colbecki (Smith, 1902), a s well as the effect of H. angusta on the recruitment of scallop larvae, si nce both use the shells of adult A. colbecki as substratum. Four kinds of s ubstrata, with and without epibionts, were exposed to the natural environme nt at 33 rn depth. After 1 year, the analysis of substrata showed that: (1) the hydroid makes use of two substrate colonization strategies, depending on the presence/absence of other epibionts; (2) the settlement of A. colbec ki larvae is strongly affected by substrate characteristics and by the pres ence of the hydroid. Hydroid colonies limit scallop recruitment on the uppe r valve of adults, where spats usually live for at least 3-5 years attached by means of byssus threads. In this way, the hydroid can affect A. colbeck i life history in two ways: by defending, as is usually believed, adults fr om predators, as well as by limiting the recruitment of its own host.