They are parasitic on a variety of molluscs, crustaceans, birds, and mammals, some species having complex life cycles involving more than one host.

Microphallus piriformes causes its host, the rough periwinkle, to move upwards, making it more vulnerable to predation by herring gulls.

[4] Microphallus papillorobustus causes its host, the lagoon sand shrimp (Gammarus insensibilis) to swim upwards, making it more vulnerable to predation.

Multilocus allozyme genotype data show that this Microphallus is a single outbred species with high levels of gene flow among South Island populations.

Embryonated Microphallus eggs are ingested from sediment and hatch in the snail's gut, penetrate the intestine, and migrate to the gonads and digestive gland.