Life Cycle of the parasite Microphallus sp. (Trematoda)

Adult worms (0.2 mm) live in the intestines of ducks. These hermaphroditic worms cross-fertilize and produce eggs. The eggs hatch following ingestion by the snail host Potamopyrgus antipodarum, releasing a small larva (miracidium) in the process. Successful infections produce several hundred to one thousand cysts (metacercaria) in about 90 days. The host snail is sterilized in the process. The cysts hatch following ingestion by ducks. The cysts then mature and begin producing eggs in about 24 hours, thus completing the life cycle.

The cysts produced by a single egg are all from the same clone. And they all enter the final duck host at the same time. Hence there is the potential for cross-fertilization among clone mates, which is essentially self-fertilization. However, genetic studies have not indicated the presence of inbreeding (Dybdahl & Lively 1996). These same studies have shown weak population structure for presumably neutral alleles (allozymes), even though reciprocal cross-infection experiments have shown high degrees of local adaptation by the parasite (Lively 1989; Lively & Dybdahl 2000). The snail host, on the other hand, shows high degrees of population structure (Dybdahl & Lively 1996). Interestingly, the genetic distances for both host and parasite are correlated with each other, and with geographic distance among populations.

This trematode is the most common infection in P. antipodarum, with prevalances commonly in excess of 50%. In general, prevalences tend to be higher in the shallow-water margins of lakes, where the final duck hosts seem to concentrate their foraging activities (Jokela & Lively 1995).

The figure on the left shows an experimentally hatched worm. Note that the oral sucker is still inside of the cyst.

The figure below shows an infected snail (top) and an uninfected snail (bottom) after removal from their shells. Notice how the orange cysts completely fill the body, replacing reproductive tissues in the process.

Relevant papers

Lively, C.M. 1989. Adaptation by a parasitic trematode to local populations of its snail host. Evolution 43:1663-1671

Jokela, J., and C.M. Lively. 1995. Spatial variation for infection by digenetic trematodes in a population of freshwater snails (Potamopyrgus antipodarum). Oecologia 103:509-517.

Levri, E. P., and C.M. Lively. 1996. The effects of size, reproductive condition, and parasitism on foraging behaviour in a freshwater snail, Potamopyrgus antipodarum. Animal Behaviour 51:891-901.

Dybdahl, M.F., and C.M. Lively. 1996. The geography of coevolution: comparative population structures for a snail and its trematode parasite. Evolution 50:2264-2275.

Krist, A.C., and C.M. Lively. 1998. Experimental exposure of juvenile snails (Potamopyrgus antipodarum) to infection by trematode larvae (Microphallus sp.): infectivity, fecundity compensation and growth. Oecologia 116:575-582.

Jokela, J., C. M. Lively, J. Taskinen, and A. D. Peters. 1999. Effect of starvation on parasite-induced mortality in a freshwater snail (Potamopyrgus antipodarum). Oecologia 119:320-325.

Krist, A.C., C.M. Lively, E. P. Levri, and J. Jokela. 2000. Spatial variation in susceptibility to infection in a snail-trematode interaction. Parasitology 121:395-401.

Lively, C.M. and M.F. Dybdahl. 2000. Parasite adaptation to locally common host genotypes. Nature 405:679-681.