ABSTRACT
Digeneans (Platyhelminthes: Trematoda: Digenea) (Plates 14-1-14.25)— sometimes referred to as flukes, trematodes, digenes, or digenetic trematodes-comprise the vast majority of Trematoda, which is one of the four entirely parasitic classes within the phylum Platyhelminthes. The other classes are the monogeneans (Monogenea or Monogenoidea, with the latter name being the earliest available name proposed for the class [see Boeger and Kritsky, 2001]), tapeworms (Cestoda), and 'turbellarians'. Collectively, the members of each class can be distinguished by their ecological attributes and appearance. Ecologically, most digeneans differ from all other platyhelminths by the combination of having (1) an indirect life cycle, which means they require more than one host to produce viable eggs, (2) a mollusc only as the first intermediate host (with exception to Authors' addresses: Parasitology, Department of Coastal Sciences, Gulf Coast Research Laboratory, The University o{ Southern Mississippi, College of Marine Sciences, (703 East Beach Drive), Ocean Springs, Mississippi 39564, U.S.A. E-mail: ash.bullard@usm.edu E-mail: robin.overstreet@usm.edu
platyhelminths by the combination of having a non-segmented body, a mouth, and a gut as well as an anterior muscular sucker (oral sucker) surrounding the mouth and a medioventral sucker (termed a ventral sucker or acetabulum) (Fig. 14.1, 14.6, 14.7). Most adult digeneans are about one to several mm in total length. However, several of them are grossly apparent and a few are very large (Fig. 14.1 A). As an extreme example, the thread-like body of the didymozoid species Nematobihothrioides histoidii (cf. Fig. 14.25F), which infects ocean sunfish (Mola mola), can be longer than 12 m. Additional obvious exceptions to this generalized description are the aporocotylids (Plates 14.16-14.21) and unrelated 'monostomes' that lack a medioventral sucker, bucephalids that have a mouth located well-posterior to the anterior end (Fig. 14.6W; Plate 14.14), and 'amphistomes' that have a posteriorly located ventral sucker. For those species not lodged in vessels or within tissues, attachment is usually accomplished with a strong to weak ventral sucker, often accompanied by the anterior oral sucker (Figs. 14.7M-P). The bucephalids attach by the anterior rhynchus, sometimes aided with an incorporated hood, sucker, glandular material, or tentacles. Some diplostomoids attach by an adhesive pad (Erasmus, 1972) or the anterior portion forms a cup, as do some lepocreadiids. Members of the Aspidogastrea, a small group of about 80 freshwater and marine species that is the sister group to the Digenea, have an attachment organ consisting of numerous suckerlets, rugae, or alveoli (Fig. 14.7M) (Rohde, 2005). The syncytial tegument of at least one digenean, the homalometrid Crassicutis archosargi, can transform into a thickened adhesive surfacedorsally or ventrally-that is histochemically different from its counterpart on the unattached (free) surface (Overstreet, 1976). Monogeneans, sometimes referred to as monogenetic trematodes, monogenes, or monogenoideans, typically infect external surfaces of fishes, e.g., skin, gill, olfactory lamellae, and fins, and they differ from digeneans by having the combination of a posteriorly located attachment organ (haptor) as well as exhibiting a direct life cycle, which means they require only a single host to produce viable eggs. Tapeworms are usually segmented, and the vast majority of aquatic species have indirect life cycles that typically include a crustacean first intermediate host (Caira and Reyda, 2005). The 'turbellarians' (see Rieger et aí., 1991) comprise a large, primarily free-living paraphyletic group of platyhelminths that has several parasitic species (Awerinzew, 1925; Ball and Khan, 1976; Blasiola, 1976; Cannon and Lester, 1988; Benz and Bullard, 2004), at least one of those, i.e., Paravortex sp., can cause diseases characterized by host death,
acute focal dermatitis, and secondary infections of 'Vibrio sp.' (Kent and Olson, 1986). Most turbellarians differ from all other platyhelminths by the combination of lacking an oral sucker, moving in a gliding fashion by using multiciliated epithelial cells, and having a direct life cycle (Cannon, 1986).
