The snail's parasite, a genus of trematode worms called Microphallus, can be found in both Alexandrina and Kaniere lakes, as well as a third lake, Poerua. The worms' eggs are ingested by the snails. Once inside, the eggs hatch and worm larvae reproduce within the snail, causing infertility. Ducks that live on the lakes scavenge for food at the lakes' periphery, where the water is shallow. The ducks consume infected snails, but are not, as far as the scientists can tell, adversely affected by the worms. Inside the ducks' enteric tracts, the worms reach adulthood and mate. The worms' eggs are excreted in the ducks' spoor, which often lands in the lake and sinks in the shallows or at depth, where snails help the worm complete its life cycle by eating the eggs and providing a host for the next generation of worm larvae.
The scientists observed that the snails at Alexandrina and Kaniere were more likely to reproduce sexually in the lakes' shallows, where ducks can feed, rather than at depth, where ducks rarely go and where parasites are less common.
"The parasite can infect both sexual and asexual snails," said IU evolutionary biologist Lynda Delph, who also coauthored the Current Biology report. "What matters here is the fact that the parasite can't complete its life cycle in the deeper parts of the lakes because the ducks only forage in the shallow-water margins of the lake."
Sexual reproduction is clearly favored in the lakes' shallows, but why? To test whether worm parasites play a key role in the maintenance of the snails' selective sexual reproduction, the researchers devised an elegant experiment.
The scientists exposed snails from each of the two lakes to worm parasites from their native lakes, but also cross-exposed the snails from Alexandrina with worms from Kaniere, and vice versa. The researchers exposed snails from both lakes to parasites fr
|Contact: David Bricker|