Swimming response to water current in Paratya australiensis Kemp, 1917 (Decapoda, Atyidae) under laboratory conditions

The swimming response to flow of the freshwater shrimp Paratya australiensi s Kemp, 1917, was examined in a small recirculating laboratory stream. On t hree occasions during 1993 larvae, juveniles, small and large adults, and o vigerous females were collected from headwater populations and tested in th e laboratory at current speeds of 10 and 30 cm.s(-1). Juvenile and adult sh rimps showed the same strong positive rheotactic response at velocities of 10 and 30 cm.s(-1). However, large adults moved upstream more rapidly than juveniles and small adults. This response was not influenced by the time of year of collection or time of day tested (day/night). Ovigerous females sh owed a weak response to flow and tended to maintain position rather than mo ve into the current. Early stage larvae (stages III and IV) were not able t o maintain position at the lowest velocities. The positive rheotactic respo nse of shrimps, particularly large adults, is seen as an adaptation to comp ensate for downstream movement of larvae by drift and juvenile and adult di splacement during high discharge events.