Limited life-history variations in a tropical stream caenogastropod, Sulcospira hainanensis, in habitats with contrasting resource availability

Abstract

Inter-population variation in the life histories of stream animals has been demonstrated to be induced by environmental differences between sites, which could be a direct consequence of differential food availability. The caenogastropod Sulcospira hainanensis (Brot, 1872) is an abundant consumer in tropical Hong Kong streams. This gonochoristic, ovoviviparous pachychilid tends to maintain higher standing stocks and productivity in less shaded sites. We hypothesized that differences in algal food availability induce diversification of life-history and demographic traits between snail populations. To test this, we compared S. hainanensis populations in four Hong Kong streams with differing conditions of shading (two shaded, two unshaded) and algal availability. Life-table analyses were also conducted for two of these snail populations in a shaded and an unshaded stream. Snails tended to grow faster and reach sexual maturity more quickly in the unshaded streams we studied, which may reflect higher assimilation of more nutritious algal food. Except for shorter life spans of snails in these unshaded streams, other re- productive and demographic attributes were mostly similar among populations; in particular, brood size and reproductive output did not appear to be strongly affected by the availability of algae. Seasonal disturbances due to wet-season spates in monsoonal Hong Kong, which appear to cause sig- nificant stochastic variability in snail abundance, may override the deterministic role of resource avail- ability and/or other environmental variables, in inducing the observed pattern of phenotypic plasticity of S. hainanensis. The present study is the first to incorporate both life-table and secondary-production analyses for a tropical stream snail. Such combined studies have rarely been undertaken on stream animals, which mainly comprise amphibiotic insects, because of the difficulty of constructing life tables for insects with biphasic life cycles and high mobility. Snails are more readily studied and we advocate the wider use of field-based life tables and production estimates in combination so as to provide greater understanding of the role of snails in energy flow and trophic dynamics.