Heavy metals in bullfrog (Rana catesbeiana) tadpoles: Effects of depuration before analysis

Abstract

AbstractAlthough tadpoles may well be excellent organisms to use as bioindicators of heavy metal contamination, the relationship of deposition in the body compared to the tail, and the effect of sediments or other debris in the digestive tract on heavy metal concentrations is unknown. We examined the effect of experimental depuration of bullfrog (Rana catesbeiana) tadpoles on heavy metal and selenium concentrations in intact tadpoles, as well as their bodies and tails. We defined depuration in this experiment as allowing defecation as an elimination process for intestinal contents (=clearing). We maintained wild‐caught tadpoles in clean water for 0, 24, 48, and 72 h to determine the effects of clearing on heavy metal concentrations. We also examined the concentrations of heavy metals in the whole body and digestive tract separately. We test the null hypotheses that no differences occur in metals as a function of time in uncontaminated water, and that no differences occur in metal concentrations in the body compared to the tail and to the digestive tract. We rejected these hypotheses based on regression models. Variance in concentrations of chromium (77%) and lead (70%) were explained by part (body, tail, whole body) and clearing time; for manganese (80%), mercury (64%), selenium (28%), and cadmium (25%) the variation was explained only by body part; for arsenic (53%), the variation was explained by part, clearing time, and weight of the various parts. For those metals in which clearing time explained part of the variation, metal concentrations in both the body and tail decreased after 24 and 48 h, but increased slightly thereafter. Clearing, however, did not greatly decrease metal concentrations in either the body or tail. These data suggest that for some metals (mercury, manganese, cadmium, selenium), clearing has no effect, and for others the effect is slight. For fresh tadpoles, however, the digestive tract contained significantly higher concentrations of all metals than either the body or head, probably reflecting metals absorbed to sediment particles in the gut.