Does sperm supply limit the number of broods ­produced by a polyembryonous bryozoan?

Abstract

Polyembryony, the splitting of a single sexually produced embryo into many clonal copies, seems to involve a disadvantageous combination of sexual and asexual reproduction, but per- sists in a diverse range of organisms. It has been suggested that embryonic cloning in cyclostome bryo zoans (colonial, sessile marine invertebrates that mate by the release, dispersal and uptake of water-borne sperm) may be a response to sperm limitation. The cyclostome Crisia denticulata inhab- its subtidal rock overhangs. Cloned larvae are produced by a colony in a series of independent brood chambers (gonozooids). Offspring from different brood chambers are genetically distinct and are, thus, the outcome of separate fertilisations. We investigated the possibility that sperm limitation reduced female reproductive success at low population density, by assessing the relationship between local colony density, as a proxy for sperm supply, and the number of brood chambers pos- sessed by colonies, as a proxy for fertilisation success. In the patchily distributed population of C. denticulata we studied, the number of brood chambers varied enormously between colonies of the same size, and large colonies entirely lacking brood chambers were frequent, suggesting the occur- rence of low fertilisation success within many colonies. However, in colonies with broods, only 17% of the variation in the number of broods per colony could be explained jointly by colony weight and local population density score, with population density being a non-significant predictor in the model. This suggests that sperm supply, as such, does not strongly influence female reproductive success and may, therefore, not be important for the maintenance of polyembryony, at least in the studied population. The wide variation in allocation to female function still requires explanation.