Morphological and mitochondrial-DNA analysis of the Indo-West Pacific rock oysters (Ostreidae: Saccostrea species)

Abstract

Species of Saccostrea are common oysters on Indo-Pacific rocky shores. The taxonomy of this genus is unclear, however, because of morphological plasticity among its constituent species. Oyster samples with distinct morphotypes were collected from shores experiencing different degrees of wave exposure in Japan, Taiwan, China, Vietnam, Singapore and Australia. Intraspecific and interspecific variations in shell characters have been identified. Only S. kegaki could be readily differentiated from S. cucullata -B, S. cucullata -C and S. mordax by multivariate shell allometric analyses. Phylogenetic analyses using partial mitochondrial 16S DNA sequences of approximately 513 base pairs identified two clades that correspond to superspecies, i.e. putative S. cucullata and putative S. mordax , comprising nine and two lineages from the Indo-West Pacific, respectively. These are S. cucullata -A, B, C, D, E, F, G, S. kegaki and S. glomerata in one clade, and S. mordax -A and B in the other. Interspecific sequence divergence (Kimura-2-parameter values) between lineages ranged from 1.02% to 11.74%. Both S. cucullata -C and S. kegaki are restricted to the northern hemisphere, whereas S. glomerata seems to be a temperate species from the southern hemisphere. S. cucullata -D occurs only in Taiwan. Other species, i.e. S. cucullata -A, B, D, E and F and S. mordax -A and B, have a wide range on rocky shores in the tropical and subtropical Western Pacific and in both northern and southern hemispheres.