ECOLOGICAL, MORPHOLOGICAL, AND GENETIC-DIFFERENCES BETWEEN THE SYMPATRIC BIVALVES DONAX-VARIABILIS SAY, 1822, AND DONAX-PARVULA PHILIPPI, 1849

To clarify the status of Donax variabilis Say, 1822, and D. parvula Ph ilippi, 1849, as separate species, comparisons of measurements of shel l characteristics, spatial distribution patterns, and allozyme frequen cies were made on sympatric populations from Melbourne Beach, Florida, USA. A significant difference in the regression slopes of shell width versus shell height was found, although there was overlap in the data for most of the size range of the specimens examined. The angle of th e dorsal margins of the shell immediately anterior and posterior to th e umbo provided a clear separation of the species. Marked differences in frequencies of alleles, but no unique alleles, were found for the t wo species. While D. parvula tended to be distributed more subtidally than D. variabilis, this pattern varied with season and there was litt le spatial separation of the species particularly during spring and fa ll. The balance of evidence indicates that D. variabilis and D. parvul a are separate, but highly similar species.