Salinity tolerance and sodium balance in the prawn Palaemonetes pugio holthuis, in relation to other Palaemonetes spp

1. 1. Determined lethal limits (96-hr LD 50, values) for Palaemonetes pugio adults are 0.5 and 44‰; for larvae (first zoeae), 16 and 46‰ 2. 2. The concentration of blood sodium remains relatively constant over the salinity range 1–40‰ 3. 3. In relation to other species of Palaemonetes, it is suggested that P. pugio represents an intermediate stage in transition from salt to fresh water, the species being tolerant of very low salinity and able to regulate blood sodium over a wide range as adults, but requiring brackish to marine water as recently hatched larvae.