Microphallus piriformes

Microphallus piriformes Galaktionov 1983 is a parasitic trematode (fluke) with two hosts: the herring gull and the rough periwinkle. The animal is notable for modifying the behaviour of one of its hosts; it does not appear to have a common name.

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The lifecycle of M. piriformes requires two hosts: the rough periwinkle (Littorina saxatilis), a littoral mollusc) and the herring gull. In the absence of M. piriformes, these species do not come into contact; but if the parasite is present, it modifies the behaviour of the mollusc to facilitate its own transfer.

Writing in Nature, ecologist Helen McCarthy and colleagues compare the behaviour of infested and healthy periwinkles (Nature 406, 840 (2000); doi:10.1038/35022691) and reported that M. piriformes "seems to bend the periwinkles' behaviour to its own ends". In both the laboratory and field experiments on Muck, Scotland, McCarthy showed that periwinkles showed a greater tendency to crawl upwards if parasitized than if unparasitized. The parasitized periwinkles were thus more likely to be eaten by gulls than their unparasitized counterparts.

When parasitized, the periwinkles reduce the time spent in horizontal and downwards movement; infected animals also alter their responses to the tide, moving upwards as it rises (healthy periwinkles move downwards).

These behavioural changes occur only when the fluke is ready to switch hosts. In the early stages, infected periwinkles behave normally (the fluke is well-adjusted and the modified behaviour renders the periwinkle vulnerable to desiccation or predation). The parasite instigates the behaviour at the most effective time to bring periwinkles and gulls into contact: this would be during the summer, when gulls are gathered at their breeding colonies.