The population dynamics and life history tactics of Pisidium clarkeanum and P. annandalei (Bivalvia: Pisidiidae) sympatric in Hong Kong

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Abstract

This study is of sympatric species of Pisidium inhabiting an artificially managed flooded vegetable furrow in the New Territories of Hong Kong. It extended over 16 months during 1983–84.

Because of Hong Kong's subtropical climate, the habitat is strongly seasonally influenced with regard to temperature, and is flushed by summer rains. Normally, such a habitat would be ephemeral, drying in winter; flushed in summer. The water is acidic, soft and would typically be slow-moving and nutrient deficient, but because of agricultural practices it is periodically culturally eutrophicated.

The pisidiids inhabiting the furrow show life history tactics which favour the habitation of a normally ephemeral subtropical habitat. Three birth periods occur each year, typically in winter, spring and in either summer (P. clarkeanum) or autumn (P. annandalei). Each generation is largely the product of the previous one. Pisidium clarkeanum attains a maximum shell length of 7.0 mm, sexual maturity at 2.5 mm and shelled larvae are present in the ctenidia at 3.5 mm. Average larval brood size is 9.49. Pisidium clarkeanum is generally univoltine and semelparous, though there is the probability that some individuals may produce one litter, continue to grow and contibute to the succeeding generation. These individuals are thus iteroparous and bivoltine. Pisidium annandalei, on the other hand, is a small species, with a recorded maximum shell length of 4.0 mm. It is born at 0.8 mm shell length and attains sexual maturity at 1.5 mm. Larvae are brooded from a parental length of 2.0 mm, average brood size being 7.17. Pisidium annandalei is typically univoltine and strongly semelparous, with the possibility only (as compared with the probability of this occurring in P. clarkeanum) of one or two individuals of the winter and spring recruits only surviving to be iteroparous and bivoltine. Autumn recruits are always univoltine as the following winter results in total adult mortality.

The life history tactics of these two pisidiids are therefore similar and represent adaptations to ephemeral habitats in subtropical and tropical Asia. Hong Kong's other freshwater bivalves are discussed: all are occupants either of other ephemeral lotie or more predictable lentic and lotie systems and show adaptations appropriate to these habitats.

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