Cymothooidea Leach 1814

Superfamily CYMOTHOOIDEA Leach, 1814

Remarks. Members of this superfamily differ from other isopods in several respects. Cymothooidea have a pleon; pleonite 1 has suture lines indicating fusion of other somites. Mouthparts are of a reduced form; mandibles stout, lacinia mobilis and molar process well developed, palp has three articles; maxilliped palp has five articles. Uropods lateral, exopod free and movable, endopod firmly attached to the peduncle, immovable. Sexual dimorphism not pronounced, except in Gnathiidae and Cymothoidae. Unlike in other isopods, the fertilised eggs are deposited in an internal brood pouch. Another difference is the presence of a tubular channel on the ventral side of the pleotelson, probably for the passage of respiratory current from the pleonal vault.

Cymothooids are best defined by the absence of the extreme mouthpart modifications and reductions seen in the anthuroids, bopyroids and cryptoniscoids; rather, they are most similar to the Cirolanoidea (Poore 2002). The most plesiomorphic members of most families possess an elongate maxilliped endite, not reduced or absent as seen in Cirolanoidea. The cirolanoids are immediately separated from cymothooids by their broad tridentate mandibular incisor, generally unreduced mouthparts, and the maxillule being multi-articulate and not possessing spines. The parasitic cymothooideans include the cymothoids, tridentellids and gnathiids (Smit et al. 2014; Wetzer 2015). As the level of parasitism increases, setae are fewer (particularly the pereopods and mouthparts), mouthparts develop abrading serrate scales and body segments are smoother (Poore and Bruce 20121).

The Cymothooidea is the largest superfamily of the suborder Cymothoida and includes members that are primarily littoral or shallow benthic. However, many species from deeper waters are also recorded. Freshwater, cave-dwelling and hotspring species are also known. The 1047 species of cymothooidan isopods are grouped in about 139 genera and eight families, of which six families are represented in Indian waters.

The seven families included in Cymothooidea according to the classification of Brandt and Poore (2003) are Aegidae White 1850; Anuropidae Stebbing 1893; Corallanidae, Hansen 1890; Cymothoidae Leach, 1818; Gnathiidae Leach, 1814; Protognathiidae Wägele and Brandt; 1988 and Tridentellidae Bruce, 1984. The additional family Barybrotidae Hansen 1890 were re-validated by Bruce (2009). The two nonparasitic families are the Protognathiidae and Anuropidae; the other six families show progressive development towards parasitism. The protognathiids (two species in the genus Protognathia) are mesopelagic species found in the Antarctic and Southern Oceans (W ӓ gele and Brandt 1988). The anuropids (10 species in the single genus Anuropus) are bathypelagic species found in the Caribbean and the Pacific Ocean (Beddard 1886).

Key to the families of the superfamily Cymothooidea known from India

The families Anuropidae and Protognathiidae have not been reported in Indian waters. Thus, these families are not included in the key.

1. Mandibles of males grossly enlarged and extended beyond the front of cephalon; adult females without mandibles; pereonite 1 fused to the cephalon........................................................................................................................................................... Gnathiidae Leach, 1814

– Mandibles not as above; in both sexes the mandibles well developed; cephalon deeply immersed into pereonite 1................................................................................................................................2

2. Maxilliped lacking, or with very reduced, endite; mandible with lacinia and molar process greatly reduced, vestigial or absent; mandibular incisor narrow; maxillule lateral (outer) lobe simple and falcate; maxilla reduced......................................................... 3

– Maxilliped with elongate endite; mandible with distinct lacinia and large, bladelike molar process; mandibular incisor generally broad, three-dentate; maxillule lateral (outer) lobe often with several (10–14) stout pines, never stylet-like or falcate; maxilla well developed....................................................................................... Tridentellidae Bruce, 1984

3. Maxilliped lacking distal recurved hooks; mouthparts not forming a ventrally directed cone; maxilliped without stout, curved setae; mandible with or without lacinia and molar process; maxilla not a slender stylet; pereopods 1–3 ambulatory or at most weakly prehensile................................................................................. Corallanidae Hansen 1890

– Maxilliped bearing distal recurved hooks; mouthparts forming a ventrally directed cone; maxilliped and maxillule and maxillae with stout, curved, apical setae; lacinia and molar process of mandible reduced or absent; maxilla reduced to a single slender stylet; pereopods 1–3 strongly prehensile.................................................................................... 4

4. Pereopods 4–7 prehensile, with dactylus longer than propodus; antennae reduced, without a clear distinction between peduncle and flagellum; maxilliped palp twoarticulate; obligate parasites of fishes............................................................................................ 5

– Pereopods 4–7 ambulatory, with dactylus shorter than propodus; antennae not as above, with clear distinction between peduncle and flagellum; maxilliped palp of 2–5 articles; micro-predators of fishes.............................................................. Aegidae White 1850

5. Body mostly asymmetrical; coxal plates well developed, produced; pleopods, telson and uropods lack marginal setae................................................... Cymothoidae Leach, 1818

– Body symmetrical; coxal plates developed, though not produced; pleopods, pleotelson and uropods marginally setose............................................ Barybrotidae Hansen 1890