The expression of sexuality, overall sex ratio and variation in sex ratio with age of 14 southern Chinese freshwater, brackish and marine bivalves have been investigated. Freshwater species are often hermaphroditic and brooding, e.g. Musculium and Pisidium (Pisidiidae), while Corbicula fluminea (Corbiculidae) demonstrates wide intraspecific variability in sexual expression, but also broods. Anodonta woodiana (Unionidae) and Limnoperna fortunei (Mytilidae) are dioecious, with a low incidence of hermaphrodites in the former, but both have a strong female-biased overall sex ratio which does not change with age. Juvenile age classes of Corbicula cf. fluminalis have a female-biased sex ratio. A pronounced female bias is a menas to optimize reproductive success, by maximizing resource allocation into more energy-demanding oogenesis, in temporally and spatially heterogeneous lacustrine habitats. In such habitats, moreover, there are probably diminishing returns to producing sons. Mangrove and brackish water species are all dioecious (except Saccostrea cucullata) with a slight male-biased sex ratio overall, but are strongly male-biased as juveniles. This strategy similarly optimizes success in a fluctuating intertidal environment, where high rates of juvenile mortality may be expected possibly, for example, through enhanced predation. In such a case, therefore, the greater investment in sons is a prerequisite to an overall balanced sex ratio. The marine intertidal species, Perna viridis (Mytilidae) and Donax semigranosus (Donacidae) have a stable 1:1 sex ratio, not varying with age, and are representative of the 'typical' dioecious condition recorded for most marine bivalves. Thus, although intraspecific variations in life history tactics and sexual strategies equip all species for a variable environment, it is postulated that permutations of the 'dioecious' condition, mediated through variations in the sex ratio either overall or with age, broadly equip the Bivalvia for the total range of aquatic habitats.
