The use of allozymes and shell morphology to distinguish among sympatric species of the rock oyster Saccostrea in Thailand

Abstract

Three hundred and fifty-five Saccostrea oysters from 12 sites throughout Thailand were scored for allozyme variation at eight enzyme loci (Aat-1, Ap, Est-2, Lap, Mdh-1, Mpi, Pgi and Pgm). Allelic variation within Lap, Pgi, Pgm and Mpi loci was concordant. Principle component analysis (PCA) of these four loci identified two principal components (PC1 and PC2) that resolved individuals into three discrete clusters corresponding to different species. Oysters were assigned to different species by diagnostic alleles at Lap, Mpi and Pgi loci, and correct identification confirmed by a comparison of observed genotype frequencies in putative single-species samples with those expected under Hardy-Weinberg equilibrium. We identified one of these species as Saccostrea commercialis, based on identical mobility's at all eight loci to a reference sample of that species from Australia. The other two species we identified as S. cucullata and S. manilai by comparison of relative allozyme mobility's and shell morphology traits with data published for these species. S. commercialis and S. manilai were found in sympatry in coastal and estuarine sites throughout the Gulf of Thailand. S. cucullata was restricted to offshore islets. Ten measurements were made on the shells of a subset of oysters from seven sites. Observations were also made of adductor scar colour and chomata spacing. Principal component analysis of shell morphometrics of ninety oysters identified by allozymes revealed three principal components that varied between species. These reflected variation in overall size, degree of 'cupping' of left valve, and relative size of the umbo cavity. The most useful character for species identification was the size of the umbo cavity, which was large in S. cucullata and S. commercialis, but almost non-existent in S. manilai. Mean umbo depths were 1.49 cm for S. cucullata, 0.7 cm for S. commercialis and 0.42 cm for S. manilai. Colour of the adductor scar was also of taxonomic value, with 75% of S. cucullata having black scars, while scars were white or brown in the other two species. (C) 2000 Elsevier Science B.V.