Pronotonyx laevis
Creators: name: Ahyong, Shane T.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Pilumnidae
Pronotonyx
Pronotonyx laevis

description: Pronotonyx laevis (Miers, 1884)
(Figs. 1–4)
Ceratoplax? laevis Miers, 1884: 244, pl. 25 fig. C.— Tesch 1918: 203.
Pronotonyx laevis.— Ward 1936: 2, pl. 1 figs. 4–6.— Ng 1987: 79, 98.— Davie 2002: 426 (list).—Ng et al. 2008: 144 (list).
Type material. HOLOTYPE: NHM 1882.7, female (cl 5.0 mm, cw 6.5 mm), Arafura Sea, 32–36 fathoms, dredged, HMS Alert.
Other material examined. AM P93048, 3 males (cl 9.8 mm, cw 13.1 mm to cl 10.3 mm, cw 13.5 mm), 3 females (cl 8.1 mm, cw 11.3 mm to cl 9.5 mm, cw 13.2 mm), Lindeman Island, Queensland, Australia, coll. M. Ward, 1933; ZRC 2023.0030, 1 male (cl 9.8 mm, cw 13.1 mm), 1 female (cl 8.1 mm, cw 11.3 mm), Lindeman Island, Queensland, Australia, coll. M. Ward, 1933; NHM 1937.7.15.18–19, 2 females (cl 5.5 mm, cw 7.3 mm; cl 3.8 mm, cw 5.0 mm), Lindeman Island, Queensland, Australia, coll. M. Ward, 1933.
Diagnosis. As for genus.
Description. Males. Carapace subquadrate, 1.32–1.39 × wider than long; dorsal surface gently convex in anterior half, regions not defined, surface very smooth, polished, without granules or rugosities, frontal and anterolateral margins almost glabrous, with scattered. coarse setae, never fully obscuring margins or surface (Fig. 2A, B). Frontal margin slightly produced, deflexed downwards, with 2 very low lobes, barely separated by very shallow concavity; lateral lobe not visible; with scattered coarse setae just posterior to margin (Fig. 2A, B). Anterolateral margin gently convex; external orbital tooth very low, almost undiscernible, not demarcated from remainder of smooth to weakly granulated margin, entire or at most with 3 very low barely discernible lobes (Fig. 2A, B). Posterolateral margin almost straight or slightly concave, distinctly converging to almost straight posterior carapace margin (Fig. 2A, B). Suborbital, subhepatic, pterygostomial and sub-branchial regions smooth (Fig. 2D, E). Orbit transverse, large; supraorbital margin almost straight, entire; suborbital margin gently sinuous, entire, without tooth on inner angle; eyes large with short peduncle and large cornea (Fig. 2B, D, E).
Antennular fossa rectangular, antennule folding transversely (Fig. 2D, E).Antenna with basal article subquadrate, immobile with epistome but sutures visible, articles 3 and 4 distinctly shorter than basal article, flagellum short, just entering orbit (Fig. 2D, E). Epistome longitudinally wide; buccal margin wide, sinuous medially obtuse with median fissure, separated from convex lateral lobes by short fissure (Fig. 2D, E, 4F). Endostome with low oblique ridge on each half.
Maxilliped 3 relatively short; merus quadrate, anteroexternal angle auriculiform; ischium subrectangular with shallow oblique median sulcus; exopod relatively broad, tip almost reaching distal edge of merus, with long flagellum (Fig. 2C).
Chelipeds stout, size asymmetrical, similar in structure Basis-ischium almost smooth on ventral margin; merus short, with blunt dorsal tooth in adults. Carpus dorsal surface smooth; inner distal margin with low, acute, tooth; distal margins with fringe of short and long setae (Fig. 2A, F). Chela outer surface smooth, glabrous, dorsal margins distinctly setose; palm high; pollex almost as long as or subequal to palm, with distinct keel on ventral margin of palm; dactylus oriented obliquely, when flexed, inclined at angle not exceeding 45° to the vertical; occlusal margins of fingers with low, wide teeth, gape absent to slight (Fig. 2F).
Ambulatory legs (P2–5) relatively long, slender, smooth; margins covered with short pubescence; P3 longest (merus 0.71–0.74 × cl), P5 shortest. Merus without cristae on dorsal margin, entire. Carpus and propodus smooth. P2–P4 dactylus falciform, gently curved along most of length, tip hooked, almost glabrous. P5 dactylus shorter, slightly upcurved (Fig. 2A).
Thoracic sternum relatively wide, smooth. Sternites 1 and 2 completely fused to form triangular structure with weakly concave lateral margins. Sternites 2 and 3 separated by distinct suture. Sternites 3 and 4 fused but oblique lateral depression demarcating sternites visible. Sternites 1–4 width 1.7 × length in males (Fig. 4G). Sternopleonal cavity reaching level of midlength of cheliped coxae (Fig. 2H, G). Sternite 8 exposed when pleon closed, visible as subtriangular plate (Fig. 2I). Penis long, at condyle of P5 coxa, positioned in short, partially exposed coxosternal groove.
Male pleon triangular, T-shaped, all somites and telson free (Figs. 2G–I, 4H). Somite 1 widest, reaching to P5 coxae. Somite 2 much narrower than somites 1 and 3. Somite 3 wide, almost reaching condyles of P5 coxae. Somites 4–6 transversely narrow; somites 4 and 5 trapezoidal with gently concave margins; somite 6 rectangular. Telson linguiform, longer than wide (Figs. 2G, H, 4H).
G1 strongly sinuous, distal part strongly curved, hooked, elongate (Fig. 4I). G2 short, sigmoid, tip spatuliform (Fig. 4J).
Females. Chelipeds symmetrical, proportionally smaller (Figs. 1A, B, 3A–C, E) and ambulatory legs slightly shorter (P3 merus 0.67–0.70 × cl) than in males, otherwise similar to males in all non-sexual characters. Female pleon longitudinally subovate, not covering entirety of thoracic sternum; telson broadly linguiform, as long as wide; somite 1 narrow, very wide, reaching to P5 coxae (Fig. 3G). Vulvae on anterior half of sternite 6, obliquely positioned, subovate, without operculum (Fig. 3H).
Remarks. The series of Pronotonyx laevis examined has permitted documentation of adults of both sexes. We note two misinterpretations in previous accounts (Miers 1886; Ward 1936) in that a blunt dorsal tooth is present on the dorsal margin of the cheliped merus in adults (weakly developed in the juvenile female holotype), and the male pleonal somite 1 reaches (rather than almost reaches) the P5 coxae. Ward (1936: 4) listed two males and three females from Lindeman Island in his study. There are altogether 10 specimens (AM P93048, ZRC 2023.0030, NHM 1937.7.15.18–19) all from the same locality, collected by Ward; it is unclear which of these specimens were those Ward recorded in his 1936 paper.
The holotype female was dredged from depths of 58.5–65.8 m in the Arafura Sea. Ward (1936: 3) stated that the “species occurs in deep water in the vicinity of Lindeman Island where I have dredged it on mud”; nothing else is known about its ecology.
description: Published as part of Ahyong, Shane T. & Ng, Peter K. L., 2023, On the identity of Pronotonyx laevis (Miers, 1884) (Crustacea: Brachyura: Pilumnidae), pp. 392-400 in Zootaxa 5293 (2) on pages 394-398, DOI: 10.11646/zootaxa.5293.2.12, http://zenodo.org/record/7960048