J. Zool., Lond. (2000) 252, 534±538 # 2000 The Zoological Society of London Biased distribution of trematode metacercariae in the nephric system of Rana tadpoles Greg W. Thiemann 1 and Richard J. Wassersug 2 1 Department of Biology, Dalhousie University, Halifax, Nova Scotia, B3H 4J1 Canada. E-mail: greg.thiemann@ns.sympatico.ca 2 Department of Anatomy and Neurobiology, Dalhousie University, Halifax, Nova Scotia, B3H 4H7 Canada. E-mail: tadpole@is.dal.ca Abstract Echinostoma sp. use larval anurans as intermediate hosts. The cercariae enter the tadpoles via the cloacal opening and form metacercarial cysts in the kidneys, pronephroi, and Wolf®an ducts. To examine the distribution of Echinostoma metacercariae in Rana sylvatica and Rana clamitans tadpoles, 200 individuals of each species were exposed to free-swimming cercariae. There was a signi®cant left : right bias in the distribution of metacercariae within both R. sylvatica and R. clamitans tadpole hosts, with trematodes preferentially encysting in nephric structures on the right side. In R. sylvatica and R. clamitans, respectively, 56.7% and 96.8% of the metacercariae were on the tadpoles' right side. Asymmetry in the distribution of parasites followed the direction of the asymmetry in tadpole kidney size, but was much greater. Trematodes preferentially encysted in the head kidneys of R. clamitans, which regress at metamorphosis. The right head kidney was the most commonly infected structure in R. clamitans tadpoles, containing 72.7% of all cysts in that species. Despite the preference of trematodes to encyst in the 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 limited space in the head kidney does not in¯uence metacercarial formation in the kidney proper. The high frequency of unilateral encystment in both anurans, and in the head kidneys of R. clamitans, may be the result of a co-evolved relationship that ultimately bene®ts both the host and parasite by ensuring host survival. Key words: trematodes, metacercariae, Rana, tadpoles, Echinostoma INTRODUCTION Several trematode species need an intermediate tadpole host to complete their life cycle (e.g. Haseeb & Fried, 1997). The metamorphosing tadpole serves as a particu- larly effective link between aquatic and terrestrial ecosystems by transporting larval trematodes from aquatic snails to terrestrial vertebrates. Although tadpoles have commonly been used to study various aspects of trematode physiology and behaviour (e.g. Beaver, 1937; Fried, Pane & Reddy, 1997), the distribu- tion of metacercariae within the bodies of tadpoles has received little attention. Previous studies on the localiza- tion of metacercariae within amphibian hosts have mostly focused on the adult frog (e.g. Martin & Conn, 1990). Echinostomatid cercariae enter tadpoles through their cloaca and encyst within the tissues of the nephric system (e.g. Beaver, 1937; Fried et al., 1997). This organ system undergoes substantial change when the tadpoles metamorphose, and primary renal function is trans- ferred from the larval (head) kidneys to the more caudal adult kidneys. The distribution of echinostomatid *All correspondence to: R. J. Wassersug. metacercariae in the larval nephric system has not previously been examined. The factors that might in¯uence metacercarial locali- zation within tadpoles remain largely undetermined (Sukhdeo & Sukhdeo, 1994), but both the host and the parasite could be greatly affected by cyst distribution. Cercariae, in general, may be expected to encyst in locations where they can maximize their chances of reaching the de®nitive host. However, as large verte- brate predators typically completely consume tadpole bodies, distribution of metacercariae within tadpoles may not be a crucial factor in whether they are ingested by the de®nitive host. Alternatively, since the survival of the host is in the best interest of both the host and the parasite, metacercarial distribution may follow a pattern that reduces the impact of parasite infection on the intermediate host. Furthermore, as the chances of the amphibian encountering the de®nitive (i.e. non- aquatic) vertebrate host may be greater after metamor- phosis, it may be in the best interest of the parasite to avoid lethally injuring the tadpole. In this study, Rana sylvatica and Rana clamitans tadpoles were exposed to echinostomatid cercariae and the distributions of metacercariae within the nephric systems of the hosts observed. We report an unusual Short Communications 534