Genetic population structure of two intertidal nudibranch molluscs with contrasting larval types: temporal variation and transplant experiments

Abstract

Previous studies of the genetic structure of populations of two intertidal nudibranchs, Goniodorisnodosa (Montagu) and Adalariaproxima (Alder and Hancock), using polymorphic allozymes revealed an expected congruence between the larval strategies of the two species and spatial differences in allele frequencies. In the northern British Isles, allele frequencies for the planktotrophic species, G. nodosa, lacked significant spatial heterogeneity over distances of >1000 km, while there was significant spatial heterogeneity in allele frequencies over distances of as little as 100 m between populations of the lecithotrophic species, A. proxima. To further understand the mechanisms causing these differences in population structure, allele frequencies were monitored in populations of both species over up to ten annual generations from 1985–1995 (for as many as seven consecutive generations in some cases). The temporal studies corroborated the spatial studies, showing that the population structure for these species is closely related to their realized larval dispersal. Between 1992 and 1995 the scale of larval dispersal and recruitment of A. proxima was examined in the field in three one-way transplant experiments between pairs of populations that were genetically and/or phenotypically different. Two of the three transplant experiments resulted in significant changes either in allozyme allele frequencies at multiple loci, or in the frequency of a color morph between two consecutive generations. This suggests that small, local populations are not totally open demographically and receive at least a proportion of their recruits from larvae generated within that population. The influence of local extinction and chance recolonization by larvae dispersed from more distant populations is therefore likely to be great, and these stochastic processes may affect the evolution of characters such as the previously observed egg size variation among populations of A. proxima. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00227-002-0942-2. On that page (frame on the left side), a link takes you directly to the supplementary material.