The Timing of Sexual Maturity in Clonal Animals

The indeterminate growth potential of many clonal organisms sets them apart from solitary organisms on two demographic counts: (1) fecundity may increase indeterminately and (2) the age—specific probability of genet mortality may decline exponentially with size. In contrast to most solitary organisms, the fecundity benefits of postponing sexual reproduction are not necessarily offset by a rapid increasing risk of mortality among clonal organisms with indeterminate growth. We examined patterns and causes of variation in the timing of sexual maturity in clonal organisms through experimental manipulations of two phyletically distinct, sessile, colonial marine invertebrates. In the bryozoan Membranipora membranacea, both crowding by conspecifics and simulated grazing triggered the onset of sexual maturity. Within a population of the colonial ascidian Botryllus schlosseri, there are two genetically determined life history morphs. The semelparous morph grows determinately and reproduces at a fixed size. The iteroparous morph can grow indeterminately and reproduces when extrinsic factors (e.g., substratum limitation) intervene. Analysis of the literature on other sessile, clonal taxa suggests that reproduction is timed differently for annual, indeterminately growing, and perennial, determinately growing, taxa. Many annual clonal species show extreme flexibility in size of first reproduction and reproduce over a wide range of sizes depending on extrinsic factors. Many long—lived clonal species delay reproduction throughout unfavorable conditions until attaining some minimum size. Beyond this size, a combination of age and extrinsic factors appears to modify the actual size at first reproduction.