Homalometron pallidum

Homalometron pallidum is found on the eastern seaboard of Canada and the United States. Its range extends from Prince Edward Island and the Gulf of St. Lawrence southwards to the Gulf of Mexico. It is found in the tidal creeks, estuaries and salt water marshes where its host species occur.^[3]

Homalometron pallidum has a complex life cycle involving several host species. The adult worm lives in the gut of a small fish, the mummichog (Fundulus heteroclitus).^[2] Eggs are ejected from the host fish in its faeces and are ingested by a small aquatic snail, Hydrobia minuta. This acts as a secondary host. When the eggs hatch, they develop in to rediae. These undergo asexual reproduction and multiply before becoming motile cercariae. The cercariae are few in number and large in size, in comparison to their small gastropod host. They emerge from the snail into the water at night and become free-living. They swim tail first with their body bent round and their tail lashing. They can also crawl with the use of two ventral suckers. They have penetration glands and make their way into an intermediate transfer host. Such hosts can include the amethyst gem clam Gemma gemma, another mummichog, or certain polychaete worms, but not other species of fish. In their new host, the cercariae become metacercariae by encysting in the tissues. In this inactive state, they wait to be ingested by a mummichog. In its gut, they develop into adult worms ready to undergo sexual reproduction and start the cycle again.^[2]

Other fish have been identified as also acting as a host to the adult worms, including Menticirrhus saxatilis, Morone americana, Pseudopleuronectes americanus, Tautoga onitis, and Bairdiella chrysura. However, in some of these cases, the worm species could have been improperly identified.^[2]