Effects of reduced salinity on juvenile growth of two co-occurring congeneric grapsid crabs

Abstract

The influence of salinity on the time elapsed between two successive molts and the size reached after each molt were studied at 30, 21, 12 and 3‰S in juveniles of two co-occurring grapsid species, Cyrtograpsus angulatus and C. altimanus, cultured under identical conditions of temperature, photoperiod and food. Juvenile growth patterns were compared between these species (which differ in size-at-maturity and maximum size). C. angulatus grew faster than C. altimanus, reflecting a higher increment per molt and a shorter intermolt period. A significant difference existed between the number of instars preceding the size of maturity in both species: >11 in C. angulatus, 6 in C. altimanus. There was evidence of a differential effect of low salinity on growth. By the end of the experiment, individuals of both species were smaller at the lowest salinity (3‰) tested; the largest crabs developed at 21‰ (C. angulatus) and 30‰ (C. altimanus). The size difference between the “optimal” and the less suitable salinities in the sixth crab instar was 12.4% in C. angulatus and 35% in C. altimanus. During early juvenile development (Crab Instars 1 to 4), there were slight differences in intermolt period among salinities in C. angulatus, but large differences in C. altimanus. The longest intermolt period of C. altimanus was at 3‰S and the shortest at 30‰S. In the following instars (5 to 10 in C. angulatus and 5 to 6 in C. altimanus), the longest intermolt period occurred at 21‰S, the shortest at 3‰S, in both species. Interspecific differences in response to low salinities may explain why C. angulatus occurs throughout a whole temperate coastal lagoon, whereas C. altimanus is restricted to its mouth.