Chronic effects of environmentally-relevant concentrations of lead in Pelophylax nigromaculata tadpoles: Threshold dose and adverse effects.

Lead (Pb) is a common heavy metal in the natural environment, but its concentration has been increasing alongside widespread industrial and agricultural development in China. The dark-spotted frog Pelophylax (formerly Rana) nigromaculata (Anura: Ranidae) is distributed across East Asia and inhabits anthropogenic habitats such as farmland. Here, P. nigromaculata tadpoles (Gosner stage 19-46) were exposed to Pb at different concentrations (0, 40, 80, 160, 320, 640 and 1280µg/L) and Pb-induced survival, metamorphosis time, development, malformations, mobility and gonad structure were monitored. The results showed that above the threshold concentration of Pb, adverse effects were obvious. As the concentration of Pb increased, the adverse effects on different traits followed different patterns: the effects on hindlimb length, survival rate, metamorphosis rate, total malformation rate, swimming speed and jumping speed largely exhibited a linear pattern; the effects on snout-vent length, body mass and forelimb length largely exhibited a bimodal pattern. Sex ratio and gonadal histology were not affected by Pb, suggesting that Pb is not strongly estrogenic in P. nigromaculata.