Cymothoidae Leach, 1818

Ravichandran, S., Vigneshwaran, P. & Rameshkumar, G., 2019, A taxonomic review of the fish parasitic isopod family Cymothoidae Leach, 1818 (Crustacea: Isopoda: Cymothooidea) of India, Zootaxa 4622 (1), pp. 1-99 : 7

Family Cymothoidae Leach, 1818

Diagnosis of female. Body often asymmetrical; cephalon and antenna small, peduncle not distinct from flagellum. Eyes present or absent. Pereon segments free with well-developed coxal plates. Mandible lacking molar process and lacinia mobilis, incisor blade-like, palp 3 article; first and second maxillae with robust, recurved, terminal and subterminal spines; maxillipedal palp of 2–3 articles, terminal segment with strong recurved spines apically. All pereopods distinctly prehensile, with dactylus longer than propodus; base of posterior pereopods often with elevated carinae and grooved to receive ischium. Pleon usually six-segmented with a few exceptions. Pleopods rami with appendix masculinua reduced and simplified, lacking spines and denticles, absent in some species. Pleopods and uropods lack marginal setae.

Remarks. Cymothoidae parasites can further be distinguished from other isopod families by the eyes that are laterally positioned (when present); a pleon with free pleonites; reduced antenna and antennula, with peduncle and flagellum not easily distinguishable; mandibular palp present with three articulated segments; maxilla with a lateral lobe and smaller medial lobe, each with a minimum of two robust setae; prehensile pereopods; lamellar pleopods lacking marginal setae, some with fleshy folds; and appendages are mostly or completely lacking any kind of setae, with the exception of the mouthparts. Some Cymothoidae present a twisted body shape, ranging in degree from weakly twisted to strongly twisted ( Bruce et al. 2002). Brusca (1975) mentions that the twisted body can most likely be attributed to the physiological stress on the parasite‘s body due to the available growth space and attachment position on the host. Many adults are recorded with having distorted or curved bodies due to their habit of attaching and fitting into snug spaces on the fish host. The growth of external attaching cymothoids is not limited by space and thus they are frequently symmetrical ( Kensley 1978).

Cymothoidae is defined by the following characters: (1) the absence of a mandibular lacinia mobilis, (2) the maxillipedal endite shorter than palp article 1 and 3 pereopodal meri 1–3 having short, blunt and robust setae (see clade 5, fig. 6C in Brandt & Poore 2003). Brucsa (1981) considers the morphology of the coxal plates, certain characteristics of pereopods, pleopods, uropods and second maxillae as relevant taxonomic characters. The identification of cymothoid isopods is further complicated by changes in morphology during development. As a result, species identifications are generally limited to ovig. females ( Bunkley-Williams & Williams 2003).

The three major groups of parasitic isopods are the epicaridians (Bopyroidea Rafinesque, 1815 and Cryptoniscoidea Kossmann, 1880) that are parasites of crustaceans as juveniles and adults; the gnathiids ( Gnathiidae Leach, 1814 ) that are parasites of fish only during their larval stages (adults are free-living and non-feeding); the adult forms of Aegidae White, 1850 and Corallanidae Hansen, 1890 appeared to be temporary parasites which often left their host after their blood meal, but more recently have been classed as free-living “micropredators”; and the Cymothoidae isopods which are obligate parasites of both marine and freshwater fishes as juveniles as well as adults. The positiom of the parasite on the host is usually species pecific. All species belonging to this family are probably protandrous hermaphrodites ( Hale 1926; Brusca 1981; Bunkley-Williams & Williams 1998 a; Trilles 1969). Sexreversal is a common phenomenon in members of this family. Host-specificity is strong in some species and weak in others. Brusca (l981) has elaborated on the natural history and evolution of this family.

Cymothoidae resemble free-living isopods except for their hook-like legs. The stages normally found are the non-swimming, permanently attached mature females, often with a small male nearby. Though most adult isopods on fish belong to the Cymothoidae , there are parasitic forms in other families. Cymothoidae closely resemble the family Aegidae , which are facultative or temporary ectoparasites of fish (Brusca 1975), but are distinguished from the latter by having smaller eyes, pleopods without setae and often an asymmetrical body shape. The pereopods of the cymothoids are all prehensile with a strongly hooked dactylus longer than propodus, whereas only the first three pereopods of Aegidae are prehensile and the rest are ambulatory, meaning they are adapted as walking appendages ( Kensley 1978; Lester 2005).

Molecular and morphological results show that the Cymothoidae is the sister group of the derived parasitic Bopyridae (W ӓgele 1989; Dreyer & W ӓgele 2001). It was also suggested that the Cymothoidae are derived from the Aegidae (Menzies et al. 1955; Dreyer & W ӓgele 2001; Brandt & Poore 2003). The Aegidae and Cymothoidae share small pars incisive of the mandibles (Dreyer & W ӓgele 2001), a strongly curved pereopod 1 and a maxillipedal palp with terminal articles set obliquely with hooks ( Brandt & Poore 2003).

Brusca (1981) hypothesized that cymothoids are composed of three ecomorphological adaptive lineages: the externally-attaching genera, the flesh-burrowing genera, and the buccal and branchial cavity genera. Brusca (1981) also suggested that the externally-attaching cymothoids had further evolved and adapted into the opercular cavity. Williams & Bunkley-Williams (1994 a) however, suggested overcrowding of the buccal cavity caused isopods to move from the buccal regions to attach to the external surfaces.

Figures

FIGURE 6. Nerocila depressa Milne Edwards, 1840, female (30 mm), (ZSI/MBRC D1-541). a, dorsal view; b, lateral view; c, forns; d, antennula; e, antenna; f, mandible; g, maxillula; h, maxilla; i, maxilliped.