OSMOREGULATORY CAPABILITIES IN ISOPODS: LIGIA OCCIDENTALIS AND LIGIA PALLASH

¹ Department of Zoology, San Diego State College, San Diego, California 92115

The influence of salinity, temperature, size, and sex on the osmoregulatory response of the seashore isopods Ligia occidentalis and L. pallasii was determined, and the environmental relationships are discussed.

1. Ligia occidentalis and L. pallasii have similar osmoregulatory responses to changing salinities at 12° and 22° C. In salinities of more than 100% sea water. both were hypoosmotic. The difference between internal and external osmotic pressures increased as the external concentration was raised or lowered from 100% sea water.

2. The difference between internal and external osmotic pressures at each temperature-salinity combination was less at 22° than at 12° C indicating less osmotic regulation at the higher temperature.

3. At both temperatures in salinities of less than 100% sea water, both species were hyperosmotic, but Ligia pallasii maintained a relatively higher internal osmotic pressure than did L. occidentalis. This is evidenced in 50% sea water at 12° C where the freezing point depression of the haemolymph (i) of L. pallasii was 1.67 compared with 1.31 for L. occidentalis.

4. At both temperatures in 50% and 25% sea water, survival of Ligia pallasii was greater than that of L. occidentalis.

5. In neither species was the osmotic concentration of the haemolymph influenced by the size or sex of the animals.

6. When both species were adapted to 150% sea water and then transferred to 50% sea water, animals at 12° C came to a steady internal osmotic pressure later and gained weight more slowly than did the animals at 22° C. In these transferral experiments the per cent body weight gain at both temperatures for Ligia pallasii was 5% for the larger males and 8-9% for the smaller females; for L. occidentalis it was 9-10% for males and females.

7. The osmoregulatory responses shown by both species correlate well with their overall behavior and ecology. The slower moving Ligia pallasii live permanently in cool, moist habitats characterizad by fluctuating, hyposaline conditions. The faster moving L. occidentalis alternate their activity between periods of foraging in drier, more terrestrial situations and periods of exposure to wet conditions of variable salinity where they can replace water lost by evaporation.