Effects of size and shape of colony on rates of fission, fusion, growth and mortality in a subtidal invertebrate

Patterns of fission, fusion, growth and mortality in a subtidal colonial ascidian, Didemnum moseleyi (Herdman), were investigated. Colonies of D. moseleyi are thin and white, dwelling beneath kelp forests in the shallow coastal waters of New South Wales, Australia. Colonies commonly underwent fission and fusion every few days; the resultant effect was a population of ramets each up to 15 mm diameter. In both natural and experimental colonies, fission was faster in large than small colonies. After splitting, colonies tended to fuse again with daughter colonies over a period of days. Relative and actual increases in area were often very large in small colonies as a result of fusion. Small colonies underwent simple areal growth more commonly than large colonies, and increased their relative area more than large colonies did by this means. Whole-colony mortality was greater, and partial mortality (death of some daughter colonies) was less, for small colonies than large colonies. Partial mortality was also a weak positive function of the degree of elongation of a colony. Whole-colony mortality was a negative function of perimeter convolution. Colony shape may prove to be an important dimension in the demography of clonal organisms and warrants further investigations