Thursday, January 22, 2009

snail tentacles

As I mentioned in a recent post, ants that are infected with the trematode, Dicrocoelium, are compelled by the presence of the parasite in their brains to march to the top of a blade of grass, bite down, and hang on. This behavior would seem to make it more likely that the fluke will be eaten by a grazing mammal like a cow or sheep, which needs to happen if the parasite is to complete its life cycle. It looks like a fascinating adaptation on the part of the parasite to modify the behavior of its host allowing the parasite to be transmitted more readily to the next host. Ideally, there would be data to document that the parasitized ants are eaten at higher rates than are uninfected ants. But this is not the ideal system in which to gather that kind of data. In fact, even though the manipulation of host behavior by parasites is not considered to be a controversial phenomenon, rigorous studies that are well supported by behavioral data are not as common as you might think. One of the best is Janice Moore's work on birds, pill bugs, and their acanthocephalan parasites, a system I also mentioned earlier. Moore discusses several other examples in her book, Parasites and the Behavior of Animals.
One of the earliest suggestions that parasites might modify the behavior or appearance of their host came from Wesenberg-Lund in 1931. He was studying another trematode, Leucochloridium. In typical trematode fashion, a snail serves as the initial host and a vertebrate, in this case a bird, serves as a final host. There is no arthropod (no insect or crustacean) anywhere in the life cycle, which is a challenge for the parasite, as the bird prefers insect prey and doesn't typically eat snails. To get itself from the snail into the bird, so the story goes, the fluke moves into the snail's tentacles, making them look like colorful, wiggly, and oh-so-tasty caterpillars, which the bird happily gobbles up, thus infecting itself. Alas, experimental confirmation of this tale is lacking. There haven't even been any reports of natural occurrence of predation by birds of these parasitized snails in the wild. But the behavior is pretty cool anyway, and you can be the judge regarding its adaptive significance and consider what sort of evidence would convince you that the parasite is modifying the snail host so that transmission to the bird is improved. As my father, my initial teacher in skeptical thinking, would say, "interesting if true."