1990 Volume 1990 Issue 38 Pages 1-6
In the prawn Palaemon (Palaemon) paucidens DE HAAN which is widely distributed in inland fresh waters in Japan, affinity with environmental fresh and saline water was investigated for zoeal larvae. The larval survivorship was much better in diluted sea water (20∼60 % or 80 % concentrations of ordinary artificial sea water) than in fresh water under both conditions of starvation and feeding. Oxygen-consumption rate of newly hatched larvae (first-stage zoeae) was considerably greater in fresh water than in diluted or not diluted sea water, which is regarded to reflect the increment of energy cost for osmoregulation in freshwater medium. The cause of declined survivorship of P. paucidens larvae in fresh water was discussed in connection with their energy consumption and other physiological factors. Judging from larval physiological affinity with saline water, it is very likely that the infered evolutionary change of habitats from saline to fresh waters in this species has been accomplished without acquiring complete adaptation to external freshwater medium.