Microphallus


 * For the medical condition of having an unusually small penis, see micropenis.

Microphallus is a genus of parasitic trematodes (flukes) in the family Microphallidae. The Greek name means "tiny penis".

They are parasitic on a variety of molluscs, crustaceans, birds, and mammals, some species having complex life cycles involving more than one host. For example, M. piriformes parasitizes the rough periwinkle (Littorina saxatilis); when these are eaten by herring gulls it infects the bird and lays its eggs in the bird's feces to infect new periwinkles.

Several species are notable for manipulating or influencing their hosts. M. piriformes causes its host, the rough periwinkle, to move upwards, making it more vulnerable to predation by herring gulls. M. pseudopygmaeus chemically castrates its host, the snail Onoba aculeus, and causes it to grow larger than normal (it is not clear if this gigantism benefits the host or parasite or if it is a non-adaptive side-effect). M. papillorobustus causes its host, the 'lagoon sand shrimp' (Gammarus insensibilis) to swim upwards, making it more vulnerable to predation. Some species of this genus "hitch-hike" on the manipulations of other species; for example, M. hoffmanni parasitizes the same sand shrimps as M. papillorobustus but does not manipulate the shrimps itself, instead benefiting from the latter's manipulation of the host.