Chimaeroniscus spheramator, 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 : 238-239

Chimaeroniscus spheramator sp. nov.

( Fig. 9 )

Material examined. Holotype: cryptoniscus larva ( ZRC 2022.0003; 1.4 mm total length) from marsupium of female Akrophryxus pallipalicus sp. nov. (type host, ZRC 2022.0001; 1.8 mm diameter), attached to the host Parapalicus armatus, Sta. CP 4152, 16°2′N, 113°53′E – 16°5′N, 113°55′E, Macclesfield Bank (Zhangsha Island), South China Sea (type locality), 410–412 m, beam trawl, flat rocky bottom, coll. ZhongSha 2015, 27 July 2015.

Etymology. The species name is a compound noun in apposition of the Latin sphaer- (sphere) and amator (lover) and refers to the finding of the species in the marsupium of a spheroid dajid.

Description of cryptoniscus larva. Body cylindrical, elongate ( Fig. 9A ), length 1.4 mm, pereomere 5 widest ( Fig. 9A ), anterior and posterior pereomeres slightly narrower, pleomeres markedly narrower than pereomeres. Cephalon anterior margin round, medial region of posterior margin convex, posterolateral margins not extended posteriorly ( Fig. 9A ); eyes absent. Cuticle striated ( Fig. 9A, G ), body pigmentation lacking. Antennules of three articles each ( Fig. 9B ), basal article triangular with eight large teeth on margin, surface striated, anteromedial corner with two stout simple setae; article 2 quadrate with two anterodistal small simple setae, surface striated; article 3 rounded, inserted into article 2 distoventrally, less than half width of article 2, with brush of numerous long simple setae, lateral “flagellum” basal segment with two long simple setae, second segment with three long simple setae, terminal segment with five long simple setae ( Fig. 9B ). Antennae of nine articles each (four peduncular and five flagellar) ( Fig. 9C ), first two articles subquadrate, distal two articles cylindrical, basal peduncular article broadest, third segment longest, distalmost segment slightly narrower than third segment, minute, distal simple setae on articles 2–4; flagellar articles approximately half width of distal peduncular article, with minute terminal setae, distalmost article with two long terminal simple setae and short simple seta ( Fig. 9C ). Oral cone triangular, anteriorly direct- ed. Pereomeres 1–7 with toothed coxal plates (first with 3 teeth, second–sixth with 4 teeth, seventh with 3 teeth) ( Fig. 9D–F ). Pereopods 1 and 2 ( Fig. 9D ) each with short, slightly curved dactylus, propodus semi-spherical, inner margin smooth; carpus triangular, distal margin with stout setae; merus rounded; ischium and basis cylindrical. Pereopods 3–7 ( Fig. 9E, F ) dactyli slender, elongate, reaching articulation of carpus/merus, distal tips ventrally indented with single simple seta inserted at notch; propodi slender, elongate, each with two small stout setae on ventral margin; carpi and meri subtriangular; ischia and bases elongate, slender, subequal in size. Pleon with five pairs of biramous pleopods ( Fig. 9I ), sympod rounded, bearing two long slender setae with minutely multifid tips; endopods cylindrical, bearing five long simple setae, exopods triangular, bearing four long simple setae. Pleotelson ( Fig. 9G ) subquadrate with distomedial rounded projection. Uropods biramous ( Fig. 9H ), composed of subquadrate sympod bearing two long simple setae on distal corners and cylindrical exopod bearing two distal long simple setae approximately half as long as endopod bearing three distal long simple setae and one short simple seta on proximal dorsal surface.

Remarks. See Remarks under Chimaeroniscus gen. nov. for a review of the unique combination of morphological features of this species. This is the first hyperparasite (=secondary parasite) described from any dajid host (=primary parasite); Pa. armatus is the host of the dajid.

The phenomenon of egg predation in Cryptoniscoidea was reviewed by Buhl-Mortensen et al. (2020); cryptoniscus larvae of some species have been confirmed to actively feed on eggs or young embryos as observed by BocquetVedrine and Bocquet (1972a, b) and Abe and Horiuchi (2000). This feeding stage is relatively short lived (approximately 1 week), with the female transitioning to a mature, non-feeding stage and thus is easily overlooked in preserved samples. Other cryptoniscoid species are suspected to be egg predators based on their morphology, particularly the lack of piercing mouthparts to feed on host hemolymph or a specialized attachment organ for internal penetration of the host marsupium and subsequent loss of host brood ( Holdich 1975). Some females of this group are found unattached within the hosts’ marsupia and include species in Cabiropidae , Crinoniscidae , and Podasconidae , as well as several genera that are incertae sedis within Cryptoniscoidea . Buhl-Mortensen et al. (2020) speculated that species of Onisocryptus Schultz, 1977 (Cyproniscidae) might also be egg predators; this was indeed shown to be the case in the overlooked study by Abe and Horiuchi (2000) who indicat- ed that O. ovalis (Shiino, 1942) could ingest nearly the whole brood of its ostracod host. Chimaeroniscus spheramator gen. et sp. nov. is suspected to be an egg predator by virtue of it being collected in the marsupium of a dajid where, upon the host’s egg production, the hyperparasite would have ample food for its own development.

Figures

Fig. 9. Chimaeroniscus spheramator gen. et sp. nov., holotype cryptoniscus larva, ZRC 2022.0003. A, Dorsal view (specimen broken across pleomere 2); B, right antennule; C, left antenna; D, left pereopod 1; E, left pereopod 3; F, left pereopod 7; G, terminal pleomere, dorsal view; H, left uropod, dorsal view; I, left pleopod 2. Scale bars: 200µm (A); 50µm (B–I).