Chimaeroniscus, Williams & Boyko & Moritaki, 2022

Williams, Jason D., Boyko, Christopher B. & Moritaki, Takeya, 2022, Two New Species of Spheroid Ectoparasitic Isopods (Epicaridea: Dajidae) Attached to the Antennules of Brachyuran Crab Hosts, with Description of a New Genus and Species of Hyperparasite (Epicaridea: Cryptoniscoidea), Species Diversity 27 (2), pp. 227-241 : 235-238

Genus Chimaeroniscus gen. nov.

Type species. Chimaeroniscus spheramator sp. nov., by original designation herein.

Etymology. The genus name is a combination of chimaera (a mythical beast of mixed morphology) and - oniscus (a common suffix for isopod genera) and refers to the unique mixture of characters present in the cryptoniscus larva. The gender is masculine.

Diagnosis. Cryptoniscus larva: body cylindrical, elongate, pleomeres markedly narrower than pereomeres. Cephalon anterior margin round, posterolateral margins not extended posteriorly; eyes absent. Cuticle striated. Antennules of three articles each, basal article triangular with eight large teeth on margin, surface striated; article 3 with brush of numerous long simple setae. Antennae of nine articles each, first two articles subquadrate, distal two articles cylindrical. Pereomeres 1–7 with toothed coxal plates (first with 3 teeth, second–sixth with 4 teeth, seventh with 3 teeth). Pereopods 1 and 2 each with short, slightly curved dactylus, propodus semi-spherical. Pereopods 3–7 dactyli slender, elongate, reaching articulation of carpus/merus, distal tips ventrally indented. Pleon with five pairs of biramous pleopods, sympod bearing two long slender setae with minutely multifid tips. Pleotelson subquadrate with distomedial rounded projection. Uropodal exopod half as long as endopod.

Remarks. The cryptoniscus larva (and only known life history stage) of the hyperparasite described from the marsupium of A. pallipalicus sp. nov. possesses a combination of characters not found in any other cryptoniscoid genus and so a new genus is erected for it. Presently, 60% (42/70) of the genera within Cryptoniscoidea are monotypic ( Boyko et al. 2008b) but this is likely to be an artifact of poor sampling of these organisms, many of which are minute and cryptic as adults as well as larvae.

The combination of characters that justify this new genus includes: dorsal cuticle striated, pleomeres markedly narrower than pereomeres, antennule with large teeth on basal segment and strong setal brush on distal segment, 3 or 4 coxal teeth on all pereomeres, pereopods 1 and 2 similar, pereopods 3–7 isomorphic and dissimilar from 1 and 2 with distoventral indentation on dactylus, telson without marginal teeth and with smooth distomedial projection, pleopodal sympods each with two long, robust setae bearing multifid tips, and uropodal exopods half-length of endopods.

Osicryptus hirsutus Schultz, 1977 ( Cryptoniscoidea , family incertae sedis; monotypic) is perhaps the most similar genus and species to the present material (cuticle striated, pleomeres markedly narrower than pereomeres, teeth on antennule with large teeth on basal segment, strong setal brush on distal segment, pereopods 1 and 2 similar, pereopods 3–7 isomorphic and dissimilar from 1 and 2, telson without marginal teeth and with smooth distomedial projection) but O. hitsutus has numerous small teeth on all coxal plates (vs. 3 or 4 stout teeth in the new species), much more robust setae on the ventral margin of the pereopodal dactyli (pair of minute setae in the new species), and pleopodal sympods bearing long thin simple setae (without multifid tips as in the new species).

Another genus and species that is somewhat similar to the present material is Gorgoniscus incisodactylus Grygier, 1981 ( Cryptoniscoidea , family incertae sedis; monotypic) which shares with the present specimen a striated cuticle, teeth on the basal segment of the antennule (albeit fewer and slenderer), similar structure and form of all the pereopods, including the small size of the setae on the ventral margins of the propodi on pereopods 3–7, and shape of the telson. However, the pleomeres of G. incisodactylus are not markedly narrower than the pereomeres and it does not have a similarly robust setal brush on the antennae, lacks coxal teeth on any of the pereomeres, and has plumose setae on the pleopods. Gorgoniscus incisodactylus is a hyperparasite of ascothoracican barnacles ( Gorgonolaureus muzikae Grygier, 1981 ) living in paramuriceid gorgonians from Hawaii (366 m) ( Grygier 1981a, b). Based on the fact that infested barnacle hosts lacked eggs and the hyperparasite was “free” within the dajid host (not attached by mouthparts), Grygier (1981a) indicated that G. incisodactylus might be an egg predator (see more on egg predation in Remarks below).

As with the two species Chimaeroniscus spheramator gen. et sp. nov. is compared to above, it is also placed incertae sedis within Cryptoniscoidea . Molecular data as well as discovery of the form of the female are necessary to determine to what other cryptoniscoid taxa it is most closely related.