Rhynchoplax Stimpson, 1858

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 : 490-491

Rhynchoplax Stimpson, 1858

Rhynchoplax Stimpson, 1858: 109 . — Stimpson 1907: 147. — Kemp 1917: 251. — Tesch 1918: 17 (part). — Sakai 1938: 194, 196. — Sakai 1965: 62. — Takeda & Miyake 1971b: 164. — Sakai 1976: 147.

Type species. Rhynchoplax messor Stimpson, 1858 , subsequent designation by Kemp (1917).

Diagnosis. Rostrum elongate-subovate in dorsal view, or narrow, strongly flattened dorsoventrally, plate-like ( R. messor ), or tapered, rod-like ( R. hondai , R. magnetica , R. minutula ), length third to half as long as carapace in males, less than 0.2 times as long in females; apex with ( R. hondai , R. magnetica , R. minutula ) or without ( R. messor ) long setae. Supraocular eave strongly arching, narrow, defined anteriorly by small pseudorostral spine proximally on side of rostrum ( R. messor ), or ill-defined ( R. hondai , R. magnetica , R. minutula ); postocular margin limited by small spine ( R. messor ) or poorly defined ( R. hondai , R. magnetica , R. minutula ). Subhepatic region slightly to substantially inflated, forming low elevation visible in dorsal view, but without conspicuous teeth or tubercles. Epibranchial tooth present. Carapace slightly wider than long; gastro-cardiac groove present though its development different according to species, thoracic grooves or other grooves absent; hymenosomian groove completely surrounding dorsum, isolating rostrum. Thoracic sternum of male with pleonal cavity sharply defined laterally, anteriorly, but not distinctly rimmed, reaching nearly to boundary between sternite 3/4, about 0.8–0.9 of sternal length; without locking button (not known in R. hondai , R. minutula ). Male pleomeres 1, 2 free, 3–4 fused, pleomere 5, pleotelson free; pleomere 1 wider than pleomere 2; pleonal margin tapering most strongly at pleomeres 3–4 (not known in R. hondai , R. minutula ). Thoracic sternum of female with paired vulvae anteriorly on membranous medial area; with paired branchiosternal canal apertures posterolaterally on sternite 8 (concealed by pleomeres 1, 2). Pleon of female deeply excavate, swollen, pleomeres 1, 2 free, 3–5 functionally fused (but suture between pleomeres clearly demarcated laterally), pleotelson fused to pleomere 5 or free. Antennule with broad first article, not laterally expanded; proepistome (interantennular septum) short, low ( R. messor ) or absent ( R. hondai , R. magnetica , R. minutula ). Epistome an oblique broad plate, anterior margin curved, ridge-like under eyes, antennae; posterior part forming buccal cavern nearly perpendicular, posterior margin produced mesially, with narrow, slit-like median notch, margin sinuous on either side of median notch. Eyestalks compact, with protuberance or small tubercle on anterior surface. Maxilliped 3 endopod, exposed exopod fully covering lateral width of buccal cavern when closed; ischium mesially expanded; axial length of ischium-merus twice maximum ischium width; merus with moderately expanded anterolateral lobe. Male cheliped not particularly elongate, twice carapace length at most; fingers not forming ‘nut-cracker’ structure. Ambulatory legs moderate to short (pereopod 2 at most twice as long as carapace length); articulation between propodus, dactylus supported by short narrow plate on each side, not forming specialised interlocking mechanism; dactyli of pereopods 3–5 falcate (pereopod 2 dactylus not always falcate), each with row of sharp erect recurved teeth along most of flexor margin. Gonopod 1 with swollen base; distal part very slender, not coiled but strongly curved dorsally, terminating in acute apex directed ventrally, not sitting in pocket on lateral ventral face of pleotelson when pleon closed. Gonopod 2 with expanding base, mesiodistal lobe about one-fifth length of base. Female pleopods 2–5 biramous.

Included species. Rhynchoplax hondai Takeda & Miyake, 1971 ; R. magnetica Komai & Poore, 2016 ; R. messor Stimpson, 1858 ; R. minutula Komai & Poore, 2016 .

Distribution. Western Pacific.

Remarks. Rhynchoplax was treated as distinct from Halicarcinus until the two genera were synonymised by Lucas (1980). He argued that the rostrum of the 17 species known at the time ranged continuously from a single process through to various forms of even and uneven trilobed structures and no division on this basis could be made. Stimpson (1858, 1907) and Sakai (1938, 1965, 1976) used this character alone to separate the two genera but as shown here this character is not especially reliable. In fact, the structure of the rostrum in the four species assigned to Rhynchoplax is widely variable. We find real differences in the gonopod 1 between Rhynchoplax messor and R. magnetica on the one hand (the only two species whose males are known) and all other species considered in this study.

Rhynchoplax messor , the type species, is a small species, maximum carapace length about 5 mm, differentiated from all species in the restricted Halicarcinus having the rostrum with a prominent median part and much smaller lateral teeth, a small but distinct postocular spine, pleomeres 3–4 fused rather than free in both sexes, the female pleon swollen (rather than discoid), the male cheliped not as swollen as in Halicarcinus s.s., usually falcate dactyli of ambulatory legs (pereopod 2 dactylus is not falcate in R. messor ) and the gonopod 1 bent only proximally and distally (not strongly curved throughout). Maximum carapace lengths of species of Halicarcinus range from 7 to 23 mm and the rostrum never has the median lobe much longer than the lateral.

Takeda & Miyake (1971b) recognised the similarity between their new species R. hondai and R. messor . Rhynchoplax hondai in Japan is known only from an immature female lacking many critical features of adults. Adult individuals of both sexes from Queensland, Australia, identified as this species by Lucas (1980), and ovigerous females from Japan similar to R. hondai have been described as two new species by Komai & Poore (2016). All share similar pleonal fusion. The gonopod 1 of R. magnetica Komai & Poore, 2016 , is more similar to that of R. messor than to any other species treated in this study.