Himasthla quissetensis is a digenean trematode, an obligate parasite, whose life-cycle includes early stages (eggs, rediae, cercariae) inhabiting the snail Ilyanssa obsoleta (Eastern Mudsnail, NW Atlantic) and Tritia reticulata (Netted Dogwhelk, NE Atlantic) , middle stages (metacercariae) in filter-feeding mollusks (10+ species), and adult stages in the digestive tracts of gulls (Larus spp.). Among mollusks which have been observed as the second hosts of quissetensis are Mya arenaria (Softshell Clam), Geukensia demissa (Ribbed Mussel), Mytilus spp. (including M. galloprovincialis, Mediterranean Blue Mussel), and Crepidula fornicata (Common Slipper Shell), all of which have been introduced to the West Coast., As a parasite of birds, one can expect that its adult stages have a potential for wide natural dispersal. Specific impacts on the hosts are not known.