GENETIC DIFFERENTIATION AND DISPERSAL AMONG POPULATIONS OF PARATYA-AUSTRALIENSIS (ATYIDAE) IN RAIN-FOREST STREAMS IN SOUTHEAST QUEENSLAND, AUSTRALIA

We used measures of genetic differentiation at seven allozyme loci to estimate the degree of movement of a stream invertebrate within and be tween catchments. Populations of Paratya australiensis, a common atyid shrimp, were sampled from three rainforest streams from each of two s ubcatchments in two adjacent drainage systems in southeast Queensland. Marked genetic differentiation was observed at all loci, suggesting e xtremely limited movement on a small spatial scale. This result is sur prising given the widespread geographic distribution of P. australiens is in Australia, its great abundance in headwater streams in southeast Queensland, and the presence of a planktonic larval stage. Large diff erences in genetic structure occurred between streams, in some instanc es between those in the same subcatchment. Even larger genetic differe nces were seen between samples from streams in different subcatchments , with alternative alleles being fixed at different sites. The greates t differences were not, however, between the two drainage systems as p redicted by the stream hierarchy model. This extreme spatial structuri ng was remarkably stable over a two-year period.