The population dynamics, reproductive strategy and life history tactics of Musculium lacusfre (Bivalvia: Pisidiidae) in Hong Kong



Hitherto uninvestigated in Hong Kong are the population dynamics, reproductive strategies and life history tactics of the Pisidiidae of which three species are known to occur locally. This study is of Musculium lacustre inhabiting a drainage ditch of the Lam Tsuen River in the New Territories of Hong Kong and extended over 15 months during 1983–84.

In Hong Kong, M. lacustre is a protandric, simultaneous hermaphrodite that matures at a length of 2 mm; the majority of each generation is, however, not mature until a length of between 4–6 mm is attained. Fertilized eggs are brooded in the inner demibranchs until they reach a length of 1.5 mm when they are released as post-foetal larvae. Musculium lacustre is univoltine. Reproduction occurs twice a year but, since M. lacustve is semelparous, the population contains, at any one time, two overstepping generations. Thus, a spring generation is recruited and grows rapidly to generate a second, smaller, autumn generation. This in turn grows rapidly and matures to produce the spring generation of the succeeding year. Adult mortality occurs in late summer and winter. A few of the late-born spring generation may overwinter to contribute to the spring generation of the succeeding year. This is not so with the autumn generation.

Such sexual and life history tactics can be related to the climate and hydrology of Hong Kong and its freshwater habitats. Reproduction takes place in spring and autumn when temperatures are moderate. Mortality is high in summer and winter when the habitat is either, respectively, hot and flushed out by summer rains or cold and dry. As M. larustre is able to overwinter but not aestivate at these latitudes (22° N), it is postulated that this holarctic species is approaching the southern limit of its asiatic range. Musculium larustre is compared with the other bivalves inhabiting fresh and brackish-water habitats in southern China.