The effect of pp′-DDT on tadpoles of the common frog ( Rana temporaria)

Abstract

Tadpoles of the common frog ( Rana temporaria L.) were kept for 1 hr in amphibian saline containing nominal concentrations of 0·01, 0·1. 1·0 or 10·0 ppm pp′-DDT. Actual concentrations in the liquid phase were probably lower because of absorption of DDT on the glass surface, deposition at the water surface and removal by the tadpoles. Tadpoles were maintained in fresh saline after treatment. After treatment in the 0·1, 1·0 or 10·0 ppm DDT media, all tadpoles enter an uncoordinated, hyperactive phase, which can begin after less than 1 hr. This is followed by either a moribund phase and death or by a return to normal activity. Body weight decreases during the hyperactive period, and the development of certain tadpoles is suppressed, probably because hyperactivity restricts feeding. Larger tadpoles also become hyperactive during treatment, frequently coming to the surface to breathe. This last action may reduce oral intake of pesticide. After treatment, many of these larger tadpoles develop a hole in the snout between the upper mandible and the nostrils, a condition that also retards feeding and development. This abnormal snout reverts to normal after metamorphosis. Smaller tadpoles are more vulnerable to DDT poisoning of the environment. The absolute amount of DDT taken in seems independent of size, so that tissue content is greater for small tadpoles. Also rate of loss of pesticide is higher for large tadpoles. No pp′-DDE was detected in any sample. Small residues of pp′-DDT tend to persist, presumably stored in lipid reserves. At metamorphosis these reserves are probably utilised and DDT mobilised, so causing increased mortality among the young frogs. This susceptibility is perhaps due to an inability to reduce effectively the concentration of circulating pesticide by either metabolism or excretion. In the field, DDT may reduce a frog population by increasing mortality during the larval stage or, more probably, immediately after metamorphosis, or by modifying behaviour. Precise evaluation is difficult because of high natural mortality among wild populations.