MODE OF LARVAL DEVELOPMENT AND GENETIC POPULATION-STRUCTURE IN NODILITTORINA-AFRICANA-KNYSNAENSIS (PROSOBRANCHIA, LITTORINIDAE)

We examined electrophoretic variation in the supralittoral gastropod, Nodilittorina africana knysnaensis, along the coast of South Africa in 1989. There was little allozyme variation in nine samples collected over 1 500 km for three highly polymorphic loci, Est, Gpi, and Pgm, and Wahlund's effect was absent in the pooled sample. A gene diversity analysis showed that 98.1% of total genic diversity, on average, was contained within populations, and that only 1.9% was due to allozyme frequency differences among populations. Since the mode of larval development has not yet been described for this species, we suggest that the lack of strong allozyme frequency differences over long distances indicates that N. a. knysnaensis has a planktonic mode of larval development.