Ammonia is one of the major excretory products of. .aquatic crustaceans. The rate of ammonia excretion may reflect the activity of the animal (Subhashini, 1981). Ammonia excretion in crabs as well as in several isopods show diurnal variation (Kirby & Harbaugh, 1974; Subhashini, 1981). The amount of ammonia excreted by the animal can be determined by placing the animal in ammonia free artificial sea water, following the method as mentioned in 8.0. Crabs are known to take in" ammonia from the medium (Mangum & Towle, 1977 ; Subhashini, 1981).