SALINITY PREFERENCE OF THE SHORE CRAB CARCINUS-MAENAS IN RELATION TO COLORATION DURING INTERMOLT AND TO PRIOR ACCLIMATION

Intermoult Carcinus maenas (Crustacea:Decapoda) exhibit a range of carapace colours from green through orange to red, reflecting increasing lengths of intermoult duration, with red crabs less tolerant than green crabs of low salinity. The salinity preference behaviour of red and green colour forms was investigated. Red crabs vacated low salinities earlier than green crabs and green crabs had a lower tolerance range. Acclimation to either low or high salinities affected red and green crabs in a similar manner; the lower the acclimation salinity the earlier the time of exit from the low salinity and vice versa; the higher the acclimation salinity, the longer the crabs remained in the low salinity. Carcinus of both colour forms were shown to be able to discriminate between salinities separated by a difference as little as 0.5 parts per thousand.