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Cymathoidae is a large family of isopods that are obligate parasites on marine, brackish and freshwater teleost fish worldwide, including many fish of economic importance. Almost all are found exclusively in shallow tropical and subtropical waters. This family includes some of the largest isopod species, reaching up to 75 mm (3 inches), but others reach only 3mm (0.1 inches) in length as adults. Their mouthparts are highly modified and specialized for blood feeding on fish hosts, and they are often asymmetric as a result of attaching to host surfaces (Brusca 1983; Thatcher 2000).
Most cymathids attach to their host by hooking onto the exterior scales or inhabit the gills or mouth (buccal cavity). Those that live in the buccal cavity can reduce or cause complete atrophy of the host’s tongue, and are called “tongue biters”. A few species (almost exclusively fresh-water species) burrow under the skin and live in a muscle tissue pocket. Some genera are highly specific on particular fish species, others are generalist and found on a variety of fish families, and even some invertebrate hosts, such as squid. While records are sketchy, there seems to be a correlation between isopod species with a broad geographic range and larger number of possible host species. Isopods also may have preferences for fish hosts that inhabit similar ecological niches, such as fish that school, rather than fish that are closely related (Brusca 1983; Thatcher 2000).
Cymathoid isopods are protandrous hermaphrodites, that is, they are born male and convert into females as adults. Young male stages disperse by swimming; attaching to and feeding from multiple hosts temporarily until they choose a permanent host, lose their swimming setae and become immobile. The first isopod to settle on a host will grow rapidly and transform into a female; subsequent colonizers will remain male to fertilize her. While all male stages are parasitic, females in some genera are non-feeding, although they survive only in association with a host (Brusca 1983; Thatcher 2000).
Cymathid isopods can cause anemia and reduced weight gain in their hosts, as well as promoting fungal and bacterial infections of the flesh lesions they create, block the reproductive output of their hosts in various ways (for e.g. see Fogelman et al. 2009), and even larvae can cause the death of temporary hosts they parasitize. Some isopods, however, appear to have limited effect on their hosts health, and Brusca and Gillian (1983) suggest that isopods inhabiting the buccal cavity of their hosts may in fact serve as a tongue replacement that is more efficient for feeding that the fish’s own tongue (cited in Thatcher 2000).
The cymathid family classification is poorly understood; many of the original taxa descriptions were cursory and not adequate to identify and distinguish species or even genera from one another. Furthermore, many species show a large degree of morphological variability. Since it is a large family with 42 genera and possibly 400 species (more than 325 recognized as of 2008; Ketmaier et al. 2008), re-assessing and describing taxa is a large project necessary for revision the family. Studies using molecular methods are augmenting these descriptions to estimate a phylogenetic reconstruction of the group, and improving taxonomic revision of this family (Ketmaier et al. 2008).