Ecology and colonial dynamics of some Pacific reef flat Didemnidae (Ascidiacea)

Abstract

Non-cryptic didemnid ascidians containing the prokaryote Prochloron are abundant on the intertidal flat of many Pacific reefs. Species with sheet-like colonies (e.g. Diplosoma similis, Trididemnum paracyclops) are characteristic of high wave energy situations and clean hard surfaces; didemnids in which the colony divides repeatedly into small cormomeres (e.g. Diplosoma virens, D. multipapillata, Lissoclinum voeltzkowi) are in less exposed situations, often on algae or unstable substrata. The unusual back-reef cascade habitat of D. multipapillata on a Fijian fringing-reef is described. In Fiji, replicative division leads to population build-up between June and October, with a hot season decline in November-December. The effects of torrential rain in April 1980 are described. Studying L. voeltzkowi in permanent quadrats has shown that the populations of cormomeres are dynamic, with much movement occurring. The process of colony division is described in Diplosoma virms from the Great Barrier Reef. Possible advantages of didemnid dynamism discussed include maximization of periphery to area (perhaps increasing the efficiency of feeding and/or growth), the exclusion of competitors, and the mingling of clones to facilitate cross-fertilization.