The population dynamics of some freshwater carideans (Crustacea: Decapoda) in Hong Kong,with special reference to Neocaridina serrata (Atyidae)

During a 15-month investigation of the population dynamics of three caridean shrimp species in the Lam Tsuen River, New Territories, Hong Kong, the numbers of the commonest species, Neocaridina serrata (Stimpson)(Atyidae), were reduced as water temperatures fell. By contrast, Caridina lanceifrons Yu became more numerous during the winter. The relatively scarce Macrobrachium hainanense (Parisi)(Palaemonidae) tended to be more abundant during the summer, and the abundance of this species was directly correlated with that of N. serrata. The population size of these two species was positively correlated with water temperatures prevailing one and two months prior to the collection of samples, but there was no significant relationship between C. lanceifrons abundance and water temperature.All species exhibited similar growth patterns with an inflection at the attainment of sexual maturity, occurring after the (presumed) 14th moult for N. serrata and in the (presumed) 13th and 18th stadium for C. lanceifrons and M. hainanensis respectively. Ovigerous shrimps of all species were only recorded when water temperatures exceeded 20 °C and the % incidence of ovigerous N. serrata was positively correlated with prevailing temperatures. It is suggested that restriction of freshwater caridean breeding periods by low temperatures may be common in the subtropics while perennial breeding is more likely to be typical of tropical regions.The mean carapace length of N. serrata populations was negatively correlated with prevailing temperatures, but more strongly correlated with temperatures recorded one and two months prior to sampling when gametogenesis may have taken place. Smallest mean sizes were recorded in late summer upon the cessation of juvenile recruitment. N. serrata has a maximum longevity of approximately 12 months and reproduction occurs in the 7th month of life or later. The hatchlings are well developed and clutch size is not related to the size of the brooding female. An annual life cycle, as seen in N. serrata, may be typical of many small freshwater carideans.Department of Zoology, The University of Hong Kong