Culexisoma, Poore, Gary C. B., Guinot, Danièle, Komai, Tomoyuki & Naruse, Tohru, 2016

Poore, Gary C. B., Guinot, Danièle, Komai, Tomoyuki & Naruse, Tohru, 2016, Reappraisal of species attributed to Halicarcinus White, 1846 (Crustacea: Decapoda: Brachyura: Hymenosomatidae) with diagnosis of four new genera and one new species from New Ireland, Papua New Guinea, Zootaxa 4093 (4), pp. 480-514 : 496-497

Culexisoma n. gen.

Type species. Culexisoma niugini n. sp., herein designated.

Diagnosis. Rostrum a narrowly triangular projection, elongate,>0.7 times carapace length in male, equilateral, anteriorly upturned, 0.3 times in female; apex with few long setae. Supraocular eave strongly arching, covering about one third of eyestalk, defined anteriorly by subrostral oblique obtuse ridge, obsolete proximolateral angle (pseudorostral element) on each side of rostrum; postocular angle limited by spine dorsal to oblique anterior margin of subhepatic region. Subhepatic region with obsolete tubercle posterodorsally. Carapace 1.1 times as long as wide; with well-developed gastro-cardiac, thoracic grooves; without longitudinal cardiac grooves; hymenosomian groove completely surrounding dorsum, isolating rostrum. Thoracic sternum of male with pleonal cavity reaching posterior margin of buccal cavern, ill-defined anteriorly, defined laterally by sharp rim ending anteriorly with ventrally-directed tooth on sternite 4, without locking button. Pleomeres 1, 2 of male free, 3–4 fused, 5 and pleotelson free; pleomeres 1, 2 barely half as wide as pleomere 3; pleonal margin tapering from widest point at midpoint of fused pleomeres 3–4. Thoracic sternum of female with sternites 4–8 considerably enlarged, sutures 4/5–7/8 short, markedly restricted laterally with paired vulvae placed anteriorly on membranous medial area; with paired openings to branchiosternal canals ventrally on sternite 8. Pleon of ovigerous female grossly expanded, hemispherical; pleomeres 1, 2 fused, 3–5 fused, pleotelson free. Antennule with broad basal article; antennules adjacent, without intervening proepistome interantennular septum. Epistome almost horizontal, slightly depressed posteriorly, anterior margin ridge-like under eyes, antennae, posterior margin truncate with slight median excavation. Eyestalks compact, with tubercle on anterior margin. Maxilliped 3 endopod, exposed exopod about two-thirds of lateral width of buccal cavern when closed; ischium mesially expanded; axial length of ischiummerus 2.5 times maximum ischium width; merus without expanded anterolateral lobe, carpus articulating terminally. Cheliped in male twice as long as carapace length; propodus swollen, fingers gaping only slightly, with irregularly toothed cutting edges, dactylus without dominant proximal tooth. Ambulatory legs elongate (pereopod 2 3.5 times as long as carapace length); with articulation between propodus, dactylus supported by broad plate on each side; dactyli mostly straight, with row of sharp teeth along proximal three-quarters of flexor margin. Gonopod 1 tapering evenly from base, vaguely sinusoidal, not strongly curved, with acute apex directed anterodorsally. Gonopod 2 with swollen base, tapering distal part ending in bifid apex. Female pleopods 2–4 biramous, pleopod 5 with exopod only.

Etymology. The genus name is derived from a combination of Culex (mosquito in Latin), and soma, the suffix of Hymenosoma , alluding to the mosquito-like appearance of the males of the two included species. Gender: neuter.

Included species. Culexisoma ginowan (Naruse & Komai, 2009) n. comb. (ex Halicarcinus ); C. niugini n. sp.

Distribution. Western Pacific.

Remarks. Naruse & Komai (2009) tentatively placed their new species H. ginowan in Halicarcinus and discussed how the morphologically diverse of this genus had become. The narrow maxilliped 3 and nearly straight gonopod 1 of this species and C. niugini differ from those of Halicarcinus s. s. but are shared with species of Neorhynchoplax and similar genera. Females of Culexisoma niugini n. sp. are pleopodal brooders (embryos attached to the pleopods) whereas species of Neorhynchoplax are ovoviviparous (embryos unattached) (Ng & Chuang 1996: fig. 26). Few species of Neorhynchoplax have an elongate rostrum, those of N. euryrostris Davie & Richer de Forges, 1996, and N. elongata Rahayu & Ng, 2004 , are 0.20 and 0.28 times cl, respectively (calculated from illustrations: Davie & Richer de Forges 1996: fig. 3; Rahayu & Ng 2004: fig. 1A). The rostrum of N. inachoides (Alcock, 1900) is 0.4 times cl and proportionally the longest in the genus (calculated from the drawing of Alcock & McArdle 1902: pl. 65 fig. 1). This species is known only from the male holotype (cl. 8.5 mm). Although the rostrum of the male N. inachoides is almost as long as those of the female C. niugini , it is still clearly shorter than males of Culexisoma species. The rostrum of N. inachoides differs remarkably from that of C. niugini in the presence of long lateral lobes that reach about a distal third of the median lobe.

The narrow maxilliped 3, conspicuous length of the male pleon, and absence of a well-defined sternal cavity (the pleon lies loosely between two erect ridges) differ from the pattern seen in all other genera dealt with here.