The relationship between the presence of neighbours and rates of sexual and asexual reproduction in a colonial invertebrate

Abstract

The colonial ascidian Didemnum moseleyi (Herdman) is apparently the kind of organism that should conform to the so-called “Strawberry-Coral model”, which predicts that the ratio of sexual to asexual reproduction in organisms like strawberries and corals is greater in crowded than open conditions. It is a thin, white, subtidal colonial ascidian dwelling beneath kelp forests in the shallow coastal waters of New South Wales, Australia. Colonies regularly undergo fission and fusion; the resultant effect is a population of ramets each up to 15 mm diameter. Individuals brood their embryos until a fairly late stage. Rates of sexual and asexual reproduction were examined here for colonies of D. moseleyi that were crowded by a sponge or by conspecific ascidians. Rates of sexual and asexual reproduction were smaller close to sponges, suggesting that exudates from sponges may have had toxic effects on D. moseleyi. Where the natural death of sponges provided free space for D. moseleyi, this was often not colonized, suggesting that, despite appearances, sponges and D. moseleyi were not in immediate competition for space. Rates of fission and production of embryos were also recorded for D. moseleyi ramets at different densities. Again, contrary to expectation, the ratio of sexual to asexual reproduction was larger in colonies at small densities. It was concluded that experimental tests are required to determine causal relationships between crowding and rates of reproduction.