Some effects of DDT on the shore crab (Carcinus maenas) were investigated. Parameters examined included the activity of gill ATPases and their effects on ion and ammonia levels of the haemolymph. Histological alterations were observed by light microscopy. In an in vitro study, an homogenate of Carcinus gills was exposed to concentrations of DDT from 10ˉ¹º M to 10-5M. The activity of Na+ , K+ -ATPase was stimulated at all concentrations. The activity of Mg2+ -ATPase was inhibited at all concentrations. Total -ATPase activity represented the combined activity of both. In vivo studies on the effect of DDT on ATPases showed that the activities of both Na+, K+- and Mg2+ -ATPases were inhibited. There was evidence that osmoregulation was affected over the short term, but not over a longer time course. After injection of a solution of ammonium chloride into the haemolymph of Carcinus, there was no consistent pattern to the rate of loss of ammonia from the haemolymph in DDT - exposed and control animals. Histological examination of gill tissue exposed to DDT revealed that the mucopolysaccharides around the gill lamellae were reduced in thickness. Ca2+ -ATPase activity and the R.N.A. content of the tissue were reduced. Estimations of the quantity of DDT and derivatives which had accumulated in some organs after DDT exposure were made. The greatest amounts were found in gills, followed by digestive gland, stomach and gonad.

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