Stimpsoplax, 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 : 507-509

Stimpsoplax n. gen.

Type species. Rhynchoplax setirostris Stimpson, 1858 , by present designation and monotypy.

Diagnosis. Rostrum narrowly triangular, approximately 0.4 of carapace length in male, 0.25 in female, laterally setose; apex without long setae. Supraocular eave strongly arching, narrow, defined anteriorly by conspicuous, anterolaterally directed pseudorostral spine located at base of rostrum; postocular margin limited by anterior margin of subhepatic region. Subhepatic region with tubercle on ventral margin. Carapace with moderately developed grooves; hymenosomian groove completely surrounding dorsum, isolating rostrum; cardiac region with short median ridge. Thoracic sternum of male broadly curved anteriorly, defining posterior margin of buccal cavern; sternites 1–3 forming a triangular plate, with suture 3/4 well marked; sternites 4–8 enlarged, sutures 4/5–7/ 8 restricted laterally; pleonal cavity about 0.7 of sternal length, defined laterally by sharp even rim, without locking button. Male pleomeres 1, 2 free, 3, 4 fused (suture between somites 3–4 clearly discernible), 5 free, pleomere 6, telson fused. Thoracic sternum of female twice as wide as long, with paired vulvae anteriorly on membranous medial area; with paired branchiosternal canal apertures posterolaterally on sternite 8. Pleon of ovigerous female deeply excavate, grossly swollen, pleomeres 1, 2 free, 3–6, telson functionally fused, though sutures separating somites clearly defined. Antennule with relatively narrow basal article; interantennular proepistome short, narrow, sharply carinate. Epistome oblique, anterior margin produced under eyes, antennae, posterior part nearly perpendicular, posterior margin forming buccal cavern slightly produced medially, sinuous on either side of narrow median notch. Eyestalks with minute tubercle on anterior margin. Maxilliped 3 endopod, exposed exopod fully covering lateral half of buccal cavern; lateral length of ischium-merus twice as long as maximum ischium width; merus with slightly expanded anterolateral lobe. Cheliped in males 2.5 as long as carapace length, with swollen propodus, fingers with narrow proximal setose hiatus when closed, cutting edges each with row of low, blunt teeth, dactylus with prominent tooth remote from other teeth proximally. Ambulatory legs slender, elongate (pereopod 2 less than 3 times as long as carapace length), with articulation between propodus, dactylus not supported by broad plate on each side; dactyli each with row evenly spaced, backwardly directed spines on flexor margin; dactylus of pereopod 2 gently curving, those of pereopods 3–5 more strongly curving. Gonopod 1 with swollen base; narrow distal part strongly twisted, sharply curved ventrally over proximal third, tapering distally to curved apex, distal part directed ventrally, hooking inner side of fused distal pleonal segment. Gonopod 2 strongly flattened, subrectangular in general outline, with slender distomesial projection with truncate apex. Female pleopods 2–5 biramous.

Etymology. Stimpsoplax honours the name of William Stimpson (1832–1872), US carcinologist who described the type species, and many other crustaceans, by combining it with the suffix plax (Greek, plate), used in several crab generic names. Gender: feminine.

Included species. Stimpsoplax longipes (Yang & Sun, 1998) n. comb. (ex Halicarcinus ); S. setirostris (Stimpson, 1858) n. comb. (ex Rhynchoplax ); S. unidentata (Yang & Sun, 1998) n. comb. (ex Halicarcinus ).

Distribution. Japan and China.

Remarks. The unusual gonopod 1 sets Stimpsoplax setirostris apart from other hymenosomatids; it appears more curved ( Halicarcinus -like) than in the other species. The gonopod 2 has not been previously described. Naruse & Komai (2009) placed the species in a group with Rhynchoplax messor and R. hondai .

The two Chinese species, Halicarcinus longipes Yang & Sun, 1998 , and H. unidentatus Yang & Sun, 1998 , are probable members of Stimpsoplax n. gen. but both are known only from females (and look very similar). Both have long setae distally on the medial rostral lobe and more elongate ambulatory legs than members of Rhynchoplax , the other possible generic placement. Ng et al. (1999) considered both similar to R. messor and attributed differences to their juvenile status.