ABSTRACT
Nematodes are common parasites of fishes in both marine and freshwater environments, and several species are also significant as disease-causing agents. At least 17 families of nematodes include species that parasitize fish, five of which appear to be restricted to this group of vertebrates (Anderson, 1984; Dick and Choudhury, 1995). They are most often encountered as adults in the gut of their fish hosts but are also commonly found in/on the viscera as larvae. Some fish nematodes are also found as adults in the skin (e.g., Cystoopsis spp., philometrids such as Philometra spp. and Philometroides spp.), abdominal cavity and viscera (philometrids such as Philonema spp.), swimbladder (Cystidicola spp., Anguillicola spp.), and liver (some capillariids). Larval ascaridoids (anisakids) and Eustrongylides spp. may be particularly common in the viscera and flesh of fishes in environments that support populations of fish-eating birds
Some families of nematodes with fish as definitive hosts (hosts in which a parasite attains sexual maturity) are very widespread. Families of the order Spirurida and Ascarida are particularly well represented in fishes. Spiruridan families such as Cystidicolidae, Rhabdochonidae and Camallanidae as well as the ascaridan families Anisakidae and Cucullanidae are represented in the Americas, the Eurasian landmass and Africa. The superfamily Dracunculoidea (of which Anguillicola crassus and the philometrids are members) is also widely distributed. Some families and species apparently face greater geographical constraints than others; camallanids are much more common and speciose in the Neotropical region than in the nearctic or palaearctic regions (Sood, 1988; Moravec, 1998; Hoffmann, 1999). The reverse is true for rhabdochonids (Vismanis et al, 1987; Moravec, 1998; Hoffmann, 1999). Others are considerably restricted in their occurrence, such as the pinworm family Pharyngodonidae, which is particularly diverse in Neotropical fishes but absent from nearctic (North American) and palaearctic freshwater fishes (in general, pinworms (Oxyurida) are rather rare in fishes). Cosmocercoideans are found in Neotropical freshwater fishes but not in the nearctic or palaearctic regions. Several nematode families have representatives in freshwater and marine environments; this includes all the spiruridan and ascaridan (except for the cosmocercoidean) families mentioned above. Several families, such as cucullanids, anisakids and camallanids seem to more speciose in marine fishes, particularly in teleosts (Ivashkin and Khromova, 1976; Rigby, 1999). At least ten families of nematodes have representatives in chondrichthyans (sharks, skates and rays), of which the family Acanthocheilidae is restricted to this group of fishes (Benz and Bullard, 2004).
