GENETIC-STRUCTURE OF POPULATIONS OF THE APLYSIID OPISTHOBRANCH STYLOCHEILUS-LONGICAUDUS (QUOY AND GAIMARD) AROUND THE SHORES OF OAHU, HAWAII

Populations of the abundant and geographically widespread (circumtropical) aplysiomorph opisthobranch Stylocheilus longicaudus (Quoy & Gaimard) were sampled at three locations around the island of 0'ahu, Hawaii in April 1990. Individuals were screened electrophoretically for up to four polymorphic enzyme loci (6Pgdh, Idh, Acon, Fum) and the genetic structure of the three populations compared. In view of the high individual fecundity (almost-equal-to 10(6) eggs) and extended planktotrophic larval life (minimally 30 days) of S. longicaudus, we expected gene flow (manifest by larval exchange between populations) to preclude any possibility of genetic drift and differentiation between these populations. This expectation was upheld but closer inspection of the data for the most comprehensively sampled population showed marked size (= age) related reductions in heterozygosity. Such reductions in heterozygosity with age are most unusual and the data are discussed from both the 'neutralist' and 'selectionist' viewpoints. Further data, both from around 0'ahu and - the other Hawaiian islands, and for other Pacific islands, are necessary to resolve these and related geographic patterns of the genetic structure of populations of S. longicaudus.