THE RELATIONSHIP BETWEEN EGG SIZE AND EMBRYONIC AND LARVAL DEVELOPMENT IN THE FRESH-WATER SHRIMP PARATYA-AUSTRALIENSIS KEMP (DECAPODA, ATYIDAE)

1. Paratya australiensis egg and clutch sizes vary between lower and u pper altitude sites within headwater streams of the Conondale Range, Q ueensland, Australia. The adaptive significance of this variation is e xamined by comparing the development of large eggs from upper sites wi th small eggs from lower sites at ambient temperatures in the laborato ry. 2. Embryonic duration was not dependent on egg size, but was a fun ction of temperature (28 days at 18 degrees C, but only 22 days at 21 degrees C). However, larvae developing from large eggs were significan tly larger at all stages of development, larval duration was shorter a nd growth rate was faster than that of larvae from small eggs. 3. It i s suggested that the larger, more rapidly developing larvae at upper a ltitude sites have a greater chance of maintaining position within hea dwater sections of the stream. This is important as physical barriers such as waterfalls may severely restrict upstream movement. At lower a ltitude sites, maintaining position may not be as critical as there ar e no major barriers to upstream movement following downstream displace ment. The influence of temperature may also be important as larger egg s may be an adaptation to compensate for slower development of eggs an d larvae at cooler, upper altitude sites. The advantage in completing larval development quickly is that larvae would have less risk of remo val by spates occurring late in the breeding season.