Biased distribution of trematode metacercariae in the nephric system of Rana tadpoles

Echinostoma sp. use larval anurans as intermediate hosts. The cercariae ent er the tadpoles via the cloacal openings and form metacercarial cysts in th e kidneys, pronephroi, and Wolffian ducts. To examine the distribution of E chinostoma metacercariae in Rana sylvatica and Rana clamitans tadpoles, 200 individuals of each species were exposed to free-swimming cercariae. There was a significant left:right bias in the distribution of metacercariae wit hin both R. sylvatica and R. clamitans tadpole hosts, with trematodes prefe rentially encysting in nephric structures on the right side. In R. sylvatic a and R. clamitans, respectively, 56.7% and 96.8% of the metacercariae were on the tadpoles' right side. Asymmetry in the distribution of parasites fo llowed the direction of the asymmetry in tadpole kidney size, but was much greater. Trematodes preferentially encysted in the head kidneys of R. clami tans, which regress at metamorphosis. The right head kidney was the most co mmonly infected structure in R clamitans tadpoles, containing 72.7% of all cysts in that species. Despite the preference of trematodes to encyst in th e head kidney, there was no correlation between the number of cysts in the right kidney and the number in the right head kidney. This suggests that li mited space in the head kidney does not influence metacercarial formation i n the kidney proper. The high frequency of unilateral encystment in both an urans, and in the head kidneys of R clamitans, may be the result of a co-ev olved relationship that ultimately benefits both the host and parasite by e nsuring host survival.