Microphallus similis is a digenean trematode parasite with a complex life cycle. Eggs in the sediment are ingested by littorinid snails during feeding and develop into sporocysts. The sporocysts reproduce asexually and then hatch into cercaria. The swiiming cercariae infect Green Crabs (Carcinus maenas) and develop into metacercariae. When the crab and metacercariae are ingested by a gull or tern, the metacercariae develop into adults, and the eggs are released in the bird's feces (Stunkard 1957). The parasite's first host is Littorina saxatilis (Rough Periwinkle), native to both sides the North Atlantic. The second host, the Green Crab, is native to the Northeast Atlantic, and was introduced to the Atlantic coast of North America before 1817. Both the Rough Periwinkle and the Green Crab, and presumably M. similis were introduced to San Francisco Bay with the fishbait trade, which included crabs, and baitworms packed in seaweed, with snails and their parasites included. The parasites were found ln Rough Periwinkles along San Francisco Bay, and genetically identified by April Blakeslee in 2003.