Distribution pattern, population structure, and growth of Septifer bilocularis (Bivalvia: Mytilidae) on reefs of Southern Vietnam

The features of the spatial distribution, size and age structure of populations, and growth of the bivalve mollusk Septifer bilocularis (Linnaeus, 1758) were investigated for the first time in the waters of Vietnam. It is shown that in the subtidal area, on reefs well protected from wave effects, S. bilocularis occupies a corallogenous substrate constructed of dead branchy colonies of Acropora and Porites and forms settlements there with a population density and biomass exceeding those in populations located along the open coast and developing on massive coral colonies of the genera Porites, Favia, and Favites. The extensive accumulation of sediments in the inner part of Vanphong Bay (Khanh Hoa Province) correlated with a decrease in the size and age ranges for populations of S. bilocularis. It is found that in places with regularly active hydrodynamics the mollusks grew somewhat more slowly and formed a more convex shell than on reefs protected from surf. The longevity of S. bilocularis estimated both from the greatest observed age and from the growth equation of Bertalanffy was somewhat greater in the former case compared with the latter one, but in general did not exceed 11 years for both cases. The results of the studies are discussed from the position of morphophysiological adaptations of sessile bivalve mollusks to life in contrasting environmental conditions of the upper subtidal zone.