GENETIC POPULATION-STRUCTURE OF THE SOUTHERN OYSTER DRILL STRAMONITA (= THAIS) HAEMOSTOMA

Nine collections in the southeastern USA of the thaidid mollusc Stramonita haemastoma, comprising 963 individuals, were made between June 1988 and November 1989, and examined for allozyme variation at 18 electrophoretically-detected loci. Two genetically-differentiated groups were identified and are referred to as canaliculata-like and floridana-like snails, based on previously-described subspecies of S. haemastoma. The two groups were differentiated genetically at a level that is characteristic of congeneric species in other molluscan taxa. The two groups maintained their genetic differences in areas of sympatry, although rare hybrid individuals occurred (about 1.2% of the total set). Shell and radular characters showed little variation within or between the two groups. Within each group, allozyme allele frequencies showed little geographic variation across distances as great as 1500 km. This result is consistent with the suggestion that species with planktonic larvae (such as S. haemastoma) should have higher rates of gene flow than related brooding species.