Divergent dispersal strategies in the freshwater bryozoan Plumatella repens: ramet size effects on statoblast numbers.

Evaluation of size-dependent variation in statoblast numbers in Plumatella repens indicates that this freshwater bryozoan employs divergent strategies in dispersing its asexual offspring in space and time. While the numbers of sessoblasts which were cemented to local substrates did not vary per unit ramet size, numbers of floatoblasts which disperse away from the parent to colonize substrates elsewhere varied in a size-dependent manner. Large ramets produced disproportionately more floatoblasts than did small ramets. In general, this result is consistent with the optimal dispersal predictions of three alternative theoretical models. However, the fact that the total number of sessoblasts in a ramet increased with ramet size appears to violate the "constant disperser principle" of one alternative model, and the occurrence of fusion in P. repens may violate the assumption of local competition for sites in another. In order to determine which model represents an appropriate description of dispersal in P. repens, future studies should evaluate the relative importance of local competition, cooperative phenomena like fusion, and mortality among dispersed and nondispersed offspring.