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).
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:27
Specimens with Barcodes:16
Species With Barcodes:10
The Cymothoidae are a family of isopods in the order Cymothoida and are found in both marine and freshwater environments. Cymoithoids are ectoparasites, usually of fish, and among their number they include the bizarre "tongue-biter" (Cymothoa exigua) which attaches to a fish's tongue causing it to atrophy, and replaces the tongue with its own body. Around 40 genera and more than 380 species of cymothoid are recognised.
Cymothoids exhibit various adaptations to their carnivorous and parasitic lifestyles. As juveniles they are not specific in their requirements and attach themselves temporarily to the skin of any fish. They produce anticoagulants and suck the fish's blood. They detach from their first host and later find another host. When they have found the correct species of fish for their adult development, they attach more permanently. As adults, most species require a particular host species and are also site-specific. Locations for attachment chosen by different species of parasite include the skin, the fins, the gills and the mouth, and some species bore into muscle.
Cymothoids are protandrous hermaphrodites. This means that each juvenile develops first into a male, but if there are no females nearby, the male will later become a female and attach permanently to the host. This female is able to secretes pheromones which prevent male cymothoids in the vicinity from becoming female. These parasites can cause serious damage to their hosts, ranging from slow growth rate, through tissue damage and anaemia, to death. Many host fish have mutualistic arrangements with certain shrimps such as Ancylomenes pedersoni, whereby the fish visits a "cleaning station" and the shrimps remove and feed on the cymothoid parasites.
- Aegathoa Dana, 1853
- Agarna Schioedte & Meinert, 1884
- Amblycephalon Pillai, 1954
- Anilocra Leach, 1818
- Anphira Thatcher, 1993
- Artystone Schioedte, 1866
- Asotana Schioedte & Meinert, 1881
- Braga Schioedte & Meinert, 1881
- Catoessa Schioedte & Meinert, 1884
- Ceratothoa Dana, 1852
- Cinusa Schioedte & Meinert, 1884
- Creniola Bruce, 1987
- Cterissa Schioedte & Meinert, 1884
- Cymothoa Fabricius, 1787
- Elthusa Schioedte & Meinert, 1884
- Emetha Schioedte & Meinert, 1883
- Glossobius Schioedte & Meinert, 1883
- Ichthyoxenus Herklots, 1870
- Idusa Schioedte & Meinert, 1884
- Isonebula Taberner, 1977
- Joryma Bowman & Tareen, 1983
- Kuna Williams & Williams, 1986
- Lathraena Schioedte & Meinert, 1881
- Livoneca Leach, 1818
- Lobothorax Bleeker, 1857
- Mothocya Costa in Hope, 1851
- Nerocila Leach, 1818
- Norileca Bruce, 1990
- Olencira Leach, 1818
- Ourozeuktes H. Milne-Edwards, 1840
- Paracymothoa Lemos de Castro, 1955
- Philostomella Szidat & Schubart, 1960
- Pleopodias Richardson, 1910
- Plotor Schioedte & Meinert, 1881
- Pseudoirona Pillai, 1964
- Renocila Miers, 1880
- Rhiothra Schioedte & Meinert, 1884
- Riggia Szidat, 1948
- Ryukyua Williams & Bunkley-Williams, 1994
- Smenispa Özdikmen, 2009 
- Telotha Schioedte & Meinert, 1884
- Tetragonocephalon Avdeev, 1978
- Niel L. Bruce & Marilyn Schotte (2011). "Cymothoidae". In M. Schotte, C. B. Boyko, N. L. Bruce, G. C. B. Poore, S. Taiti & G. D. F. Wilson. World Marine, Freshwater and Terrestrial Isopod Crustaceans database. World Register of Marine Species. Retrieved December 9, 2011.
- R. C. Brusca & M. R. Gilligan (1983). "Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda)". Copeia 3 (3): 813–816. doi:10.2307/1444352. JSTOR 1444352.
- Srour, Marc (2012-07-13). "Tongue Biters and Deep Sea Giants: The Cymothoida (Crustacea: Isopoda)". Teaching Biology. Retrieved 2014-06-03.
- Bunkley-Williams, Lucy; Williams, Ernest H. Jr. (1998). "Ability of Pederson Cleaner Shrimp to Remove Juveniles of the Parasitic Cymothoid Isopod, Anilocra haemuli, from the Host". Crustaceana 71 (8): 862–869. doi:10.1163/156854098x00888. JSTOR 20106067.
- Hüseyin Özdikmen (2009). "A new name, Smenispa for the preoccupied isopod genus Enispa Schioedte & Meinert, 1884 (Isopoda: Cymothoidae)" (PDF). Mun. Ent. Zool. 4 (2): 611–612.
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