Lepocreadium setiferoides is a digenean trematode, an obligate parasite, whose early stages (eggs, rediae, cercariae) develop in Eastern Mud Snails (lyanassa obsoleta) introduced from the Northwest Atlantic. Its middle stages (metacercariae) develop in platyhelminth and polychaete worms, and in scyphozoan jellyfish (10+ species, Blakeslee et al. 2012). Its adult stages occur in the digestive tracts of fishes (sculpins- Cottidae and flounders- Pleuronectiformes). The Eastern Mud Snail and its parasites were introduced with plantings of Eastern Oysters from the Atlantic Coast in the 19th century. The snail was first collected in San Francisco Bay in 1907. The parasites were found in a study of snail parasites by April Blakeslee in 2003. Parasites with complex life cycles and multiple hosts, such as L, setiferoides, are able to become established in a new environment. However, the prevalence of species which use fishes as a final host, maintain a smaller prevalence than those which have birds as a final host (Blakeslee et al. 2012).