GENETIC DIFFERENTIATION AMONG POPULATIONS OF CARIDINA-ZEBRA (DECAPODA, ATYIDAE) IN TROPICAL RAIN-FOREST STREAMS, NORTHERN AUSTRALIA

1. Caridina zebra is a common atyid shrimp in some tropical rainforest streams in far north Queensland, Australia. Genetic variation at five allozyme loci was used to estimate the level of dispersal among popul ations of this species, within and between stream systems. Shrimps wer e sampled from nine streams in the Tully River catchment and two headw ater streams in the adjacent Herbert River catchment in an area under consideration for extensive hydroelectric development. 2. High levels of genetic differentiation were recorded among most populations which suggests that, like other fully aquatic species, movement is limited t o a very small spatial scale. 3. In the Tully catchment, populations o f shrimp from streams with confluences at high altitude showed less ge netic differentiation than those from streams which directly entered t he lower river. Dispersal between the latter streams is clearly limite d by the presence of large waterfalls and cascades. 4. Adjacent stream populations were often highly differentiated, despite their close pro ximity, suggesting that overland dispersal is unlikely. However, popul ations of shrimp in the two streams in the Herbert catchment were stri kingly similar in genetic structure to those in adjacent headwater str eams of the Tully. Such similarity may reflect relatively recent chang es in drainage patterns.