Life cycle of <i>Microphallus similis</i> in eastern North America with <i>Carcinus maenas</i> as second-intermediate host.

<p><i>Microphallus similis</i> infects multiple hosts to complete its life cycle, starting with (A) two species of <i>Littorina</i> snails (<i>L</i>. <i>saxatilis</i> and <i>L</i>. <i>obtusata</i>), where the trematode asexually reproduces, producing numerous cercariae. (B) These cercariae are shed from the snail into the water column, where they seek out and encyst as a metacercariae within a second-intermediate host, primarily the green crab, <i>Carcinus maenas</i>. To sexually reproduce, (C) the trematode’s crab host must be ingested by a definitive host, often a <i>Larus</i> gull species, where (D) the trematode’s eggs, containing miracidia, are then deposited into the marine environment with the birds’ feces. Grazing snails accidentally ingest these eggs, and the cycle continues.