Experimental studies on physiological and behavioural response mechanisms of Nitocra spinipes (Crustacea: Harpacticoidea) from brackish-water rockpools

Volume regulation and salinity-preference tests have been made on Nitocra spinipesBoeck (Crustacea, Harpacticoidea). This species is often dominant in Baltic brackish-water rockpools. The investigation attempts to evaluate the relative importance of some of the different response mechanisms of this species to salinity changes in relation to the unstable environmental conditions of the rockpools. Volume-regulation experiments have shown that N. spinipes is capable of hypoosmotic and probably hyperosmotic regulation in the tested salinity range of 1 to 20 S. In laboratory cultures, reproduction, hatching and moulting oecurred in salinities ranging from 0.5 to 30 S. Preference experiments showed that N. spinipes has a very weak behavioural response even to very large variations in salinity concentrations between the alternative; a significant choice could only be found under conditions which never occur in the natural biotope. It is therefore concluded that, in a biotope such as the rockpool, where salinity changes will affect the whole biotope rather than produce microclimatic variation, regulation and adaptation must have a higher ecological importance than escape responses.