In the marine environment as well as in the estuarine environment salinity variations are a regular feature and salinity as a factor by itself can influence the free and unrestricted movements of the animals in those environments. Hukuda (1931) and Black (1951) have demonstrated both by biochemical as well as by physical tests the changes which the fish undergoes when transferred to different salinities, rather abruptly. The observations presented in the following pages were restricted to the measurement of physical changes only and were intended to supplement the data obtained on the same species of fish under identical conditions, on the metabolic response (Job, 1959) and are by no means exhaustive. 
The catfish, Plotosus anguillaris (Bloch) is quite common in the Gulf of Mannar and in Palk Bay. As in the metabolism experiments the fish were acclimated to three levels of salinity and their physical changes of weight, volume and density— both absolute and reciprocal, were measured. 

Plotosus anguillaris, volume-density changes, salinities, south-east coast of India