The osmoregulatory ability of three grapsoid crab species in relation to their penetration of an estuarine system
The osmoregulatory abilities of the grapsoid crabs Cyclograpsus punctata, Sesarma catenata and Sesarma eulimene were studied comparatively in an attempt to explain, at least in part, their distribution in estuaries. Both survival and haemorymph osmotic pressure were used as indices of hyperosmotic regulatory ability.
All three species were shown to be capable of regulating their haemorymph osmotic pressure against lowered external salinities. S. eulimene was found to be better able to control its haemorymph concentration in very low salinities than S. catenata,which would account for the former's occurrence in the upper reaches of estuaries. C.punctatus was shown to be capable of roaintaining its haemorymph osmotic pressure above that of the environmental medium in lowered salinities, but without the degree of constancy characteristic of the two Sesarma species. It does, however, show a marked ability to tolerate lowered internal osmotic pressure, an ability which appears to develop in response to partial dilution of the haemorymph. These adaptations, together with its semi-terrestrial habit as a shore crab, suffice to enable C. punctata to penetrate an estuary to almost the same extent as S. catenata.