Nasutoplax, 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 : 506-507

Nasutoplax n. gen.

Type species. Hymenosoma rostratum Haswell, 1881 , herein designated and monotypy.

Diagnosis. Rostrum a long, laterally compressed median spine with rounded apex, apically expanded, erect in lateral view, 0.35 times carapace length in male, 0.2 times in female; lower apex tubercle-like in male, angular in female; upper apex without long setae. Supraocular eave strongly arching, narrow, defined anteriorly by angle on side of rostrum (=pseudorostral element) with well-defined subrostral oblique ridge; postocular margin limited by squarish lobe of anterior margin of subhepatic region, without distinct spine. Subhepatic region with strong anteriorly directed spine, lateral tubercle. Carapace as long as wide; with well-developed gastro-cardiac, thoracic grooves; hymenosomian groove completely surrounding dorsum, isolating rostrum. Thoracic sternum of male, female with sternites 1–3 forming a separate plate, suture 3/4 being well marked, sternites 4–8 considerably enlarged, sutures 4/5–7/8 restricted laterally. Thoracic sternum of male with pleonal cavity defined laterally, anteriorly by sharp erect rim, about half of sternal length; with locking button on sternite 6. Male pleomeres 1–5, pleotelson free; pleomere 1 widest, pleon almost parallel-sided to pleomere 3, tapering evenly posteriorly. Thoracic sternum of female with paired vulvae anteriorly on membranous medial area; with paired branchiosternal canal apertures ventrally on sternite 8. Pleon of ovigerous female discoid, pleomeres 1, 2 free, 3–5 fused, pleotelson free. Antennule with broad basal article, produced laterally, acute distolaterally; antennules separated by short sharp subrostral proepistome. Epistome a broad horizontal plate, anterior margin projecting forward under eyes, antennae, posterior margin deflexed, sinuous. Eyestalks elongate, without tubercle on anterior margin; cornea erect. Maxilliped 3 endopod and exposed exopod covering most of lateral width of buccal cavern when closed; ischium mesial margin expanded; axial length of ischium-merus 2.2 times maximum ischium width; merus with moderately expanded anterolateral lobe. Cheliped in male twice as long as carapace length, with swollen propodus with keel on upper margin, fixed finger with basal tooth, oblique distal cutting edge; dactylus with simple proximal tooth, distal cutting edge. Ambulatory legs elongate (pereopod 2 3 times as long as carapace length); with articulation between propodus, dactylus supported by broad plate on each side; dactyli evenly curved, with row of blunt teeth along most of flexor margin. Gonopod 1 with swollen base, almost straight distal part directed anteriorly, apex truncate, with subterminal blunt projection with spinulose tubercle on anterior (dorsal) face. Gonopod 2 comprising almost semicircular base plus long mesiodistal section about as long as base, with truncate apex. Female pleopods 2–4 biramous, pleopod 5 with exopod only.

Etymology. Nasutoplax combines nasuto (Latin, long-nosed), alluding to its unique rostrum, and the suffix plax (Greek, plate), used in several crab generic names. Gender: feminine.

Included species. Nasutoplax rostrata (Haswell, 1881) (ex Hymenosoma ).

Distribution. Southern Australia, subtidal to 130 m depth.

Remarks. With maxillipeds 3 completely covering the buccal cavern Nasutoplax rostrata belongs to the Halicarcinus -group of genera. Nasutoplax n. gen is superficially similar to those genera with a long rostrum and smaller lateral lobes, such as Culexisoma n. gen., Rhynchoplax and Stimpsoplax n. gen. but differs from all of these in the unusual lateral profile of the rostrum and in the unique gonopod 1 (with an unusual subterminal spinulose projection on the anterior (upper) face). While N. rostrata has no postocular spine it possesses instead a strong acute spine away from the margin on the subhepatic region. The strongly carinate cheliped of the male is found only in this genus.