Osmotic Regulation in the Tadpoles of the Crab-eating Frog (rana Cancrivora)

The major features of the osmoregulatory mechanisms which form the basis for the unusually great salinity tolerance of the adults of the southeast Asian crab-eating frog (Rana cancrivora) have been described by Gordon, Schmidt-Nielsen & Kelly (1961) and by Schmidt-Nielsen & Lee (1962). Adults of this species respond to high external salinities with an increase in internal osmotic concentration, largely due to accumulation of urea. Gordon et al. also found that the tadpoles of this frog appeared to have an even greater tolerance for high salinities than the adults, confirming the original observations of Pearse (1911). Since studies on other amphibians have led to a presumption that pre-metamorphic anuran larvae in general lack enzymic machinery for the production of significant amounts of urea (Bennett & Frieden, 1962, for review), the question of how R. cancrivora tadpoles adjust to high salinities is raised. The present paper describes the unexpected pattern of osmotic regulation in the tadpoles of the crab-eating frog, and investigates the change from the larval to the adult pattern at metamorphosis. Ecological information on the role of salinity in the life history of the species is also presented. For comparative purposes, and to supplement the very meagre information in the literature on osmoregulation in the larvae of freshwater frogs, data are also presented for the tadpoles of the freshwater frog R. Umnocharis, a species very similar morphologically to R. cancrivora.