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The platyhelminth class Trematoda, consisting of the subclasses Aspidogastrea and Digenea, contains individuals referred to as digeneans, trematodes, or flukes. Aspidogastreans , not digeneans in the strict sense, number about 80 species in total (Rohde 2005), but adults in the Gulf of Mexico of 4 species have been reported from teleost and chondrichthyan fishes (Hendrix and Overstreet 1977) and one from a turtle (Wharton 1939). Consequently, this chapter deals with trematodes rather than just digeneans, even though all the records but the 5 refer to digeneans. Trematoda is the largest group of platyhelminths, and trematode adults, other than exceptional cases in both Aspidogastrea and Digenea, occur in vertebrates. Rather than being totally restricted to a lumen like members of its sister group Eucestoda (tapeworms), trematode species occur in a variety of sites, exhibiting rather restricted site specificity for each species or higher group. The intriguing situation about trematodes is that they have a molluscan first intermediate host. The larval stage of the aspidogastreans (cotylocidium) does not undergo asexual replication in the mollusk like it does for the digenean counterparts. Digeneans have a series of asexual stages reproducing large numbers of individuals originating as the larva (miracidium) from a single egg that assure transmission and dispersal to a series of one to 3 different, necessary intermediate hosts. With the exception of a few aporocotylids (fish blood flukes) that have larvae developing in polychaetes and maturing in fish, all species in the approximately 175 families require the molluscan first intermediate host. Of the approximately 18,000 nominal trematode species worldwide, including well over 5000 of those from fishes alone (Cribb 2005b), perhaps the largest group of internal metazoan parasites (Cribb et al. 2001), almost all are highly specific for at least the first, second, or final host. On the other hand, at one or more of those 3 stages many digenean species infect several different species of hosts. The number of named or accepted species is low, primarily because of the lack of attention paid to this difficult group of mostly tiny soft-bodied worms. This chapter provides an initial attempt to document records. We do not agree with all, are sure several have been unintentionally omitted, and are sure the lists present a woefully incomplete record because of a paucity of 419 23 Trematoda (Platyhelminthes) of the Gulf of Mexico Robin M. Overstreet, Joshua O. Cook, and Richard W. Heard  Trematoda. After Overstreet 1973. 420 ~ Trematoda (Platyhelminthes) Overstreet 1981) reported the presence of over 6000 heterophyid metacercarial cysts per gram of mullet flesh for an estimated total number of 582,000 cysts in the single specimen of Liza ramado (Risso, 1810). Each of those cysts has the potential to produce one adult in its human or other mammalian final host. Most humans in fishing communities where uncooked or inadequately cooked seafood products are routinely eaten each harbor thousands of individual adults in the intestine, often causing considerable discomfort and even death (Deardorff and Overstreet 1991). Some public health literature implies that most digeneans cause harm or death in their final host, but the fact remains that the great majority of digenean species produce very little pathological response in the final definitive host. Occasionally a metacercaria will infect a fish host normally infected by an adult and produce lesions (e.g., Raptopoulou and Lambertsen 1987). Of those trematodes that produce disease in people, most develop into pathogenic adults in those people; however , the cercariae of several avian and mammalian blood flukes can produce a severe dermatitis in those people (accidental abnormal hosts) penetrated by a challenge of cercariae (e.g., Penner 1953, Leigh 1955). Adults or larvae of several species cause economic or environmental losses resulting from public health aspects (loss of host groups/pods/schools of marine mammals and fishes as well as flocks of bird hosts from infections by both adults and juveniles; aesthetic appearance of the seafood products , usually comprising the first or second intermediate bivalve or fish hosts; castration of first intermediate molluscan hosts; and behavioral modifications in the first or second intermediate host and the accidental final host). As an example of pathological changes, disease, normal and accidental hosts, and potential public health risk, Cryptocotyle lingua (Creplin, 1825), a listed heterophyid from a gull but also commonly observed in terns elsewhere, has been shown experimentally to infect some mammals , and to be refractory in ducks (Willey and Stunkard 1942). A somewhat emaciated but not healthy rat produced mature worms. When the tern—a good host in the northeast...

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