Osmoregulation in some palaemonid prawns

Abstract

1. The brackish-water prawn Palaemonetes varians and the marine prawns Leander serratus and L. squilla are hypotonic in normal sea water, the blood of these species showing osmotic pressures equivalent to 2·3, 2·8 and 2·6 % NaCl respectively, in an external medium of 3·5 % NaCl. 2. Palaemonetes varians is isotonic in water of about 2·0 % NaCl and the species is practically homoiosmotic, the difference in its osmotic pressure over a range of 5·0 % NaCl in the external medium being only 0·8-1·0 %. The species has a very wide range of tolerance from water that is nearly fresh to concentrated sea water equivalent to 5·2 % NaCl. 3. Leander serratus is much less homoiosmotic than Palaemonetes, and has a limited tolerance to dilution and concentration of the environment. Homoiosmoticity is maintained up to a dilution of 2·5 % in the external medium when isotonicity is reached; but in lower dilutions there is a steady decline in osmotic pressure and the regulatory mechanism evidently breaks down. 4. The osmotic behaviour of Leander squilla is very similar to that of L. serratus, but the homoiosmotic behaviour is more marked and it has greater tolerance to dilution of the environment. 5. When Leander and Palaemonetes are transferred to very dilute sea water, the internal osmotic pressure falls gradually for about 14-24 hr., varying according to the size of the individual. After the lowest value has been registered there is a slight rise, and a steady state is thereafter maintained. 6. Studies on the changes of weight of prawns when transferred to diluted media indicate that the integument (gills) is permeable to water and that, at least in Leander serratus, the amount of water entering is mainly responsible for the dilution of the blood. There is a similar fall in weight when prawns are transferred to concentrated media, due to loss of water.