Pronotonyx Ward 1936
Creators: name: Ahyong, Shane T.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Pilumnidae
Pronotonyx

description: Pronotonyx Ward 1936
Pronotonyx Ward, 1936: 2.— Ng 1987: 98.— Davie 2002: 426.—Ng et al. 2008: 144.— Poore & Ahyong 2023: 671, 684, fig. 14.112h.
Type species. Ceratoplax laevis Miers, 1884, by original designation.
Diagnosis. Frontal margin of carapace and pereopods with scattered setae, not obscuring margins or surface; carapace wide, regions not indicated, dorsal surface appearing smooth, polished; anterolateral margin convex, entire or with anterolateral teeth scarcely demarcated; posterior margin of epistome distinctly sinuous with the lateral parts distinctly concave; endostome with low oblique ridge. Maxilliped 3 merus with anteroexternal angle auriculiform. Chelae with pollex almost as long as or subequal to palm, dactylus inclined at an angle not exceeding 45° to vertical. P2–P5 slender, unarmed, merus shorter than three-fourths carapace length; coxa without denticulate plate. Male thoracic sternum broad, anterior sternum (sternites 1–4) width 1.7 × length; sternopleonal cavity reaching anteriorly to level of midlength of cheliped coxae. Male pleon T-shaped, somite 1 very wide, reaching laterally to P5 coxae; somite 2 distinctly narrower than somites 1 and 3; somite 3 wide, almost reaching condyles of P5 coxae, separated by distinct gap; coxosternal groove for penis partially exposed. G1 strongly sinuous, distal part strongly curved, hooked.
Remarks. In describing Ceratoplax laevis from off northern Australia, Miers (1884: 245) commented that it closely resembled Notonyx nitidus A. Milne-Edwards, 1873, but differed in “having the carapace, eyes, and chelipedes entirely glabrous, there is apparently no tooth on the upper margin of the arm, and the dactyli of the ambulatory legs are carinated; the carapace is more quadrate, and the merus of the outer maxillipedes longer, not transverse, with the antero-external angle less prominent”. Tesch (1918: 203, footnote) included the species in his key to Ceratoplax but commented that it was “Probably identical with Notonyx nitidus A. Milne-Edwards ” but did not elaborate. In establishing Pronotonyx based on specimens from Queensland, Australia, Ward (1936: 2) appeared to follow Miers’ (1884) arguments and noted that the genus differed in having: the carapace “broader than long, smooth and shining”; the front “more than one-third the width”; “the anterior margin of the buccal frame...entire”; the merus of maxilliped 3 with “the antero-external angle auriculated”; “a strong tooth on the upper border of the merus of the cheliped”; and the first somite of the male pleon that “almost reaches the coxi of the fifth pair” of legs. Ng (1987: 98), in reviewing the genera, provisionally retained Pronotonyx in the Rhizopinae (Pilumnidae), as the condition of the diagnostic male gonopods was not known. Our detailed examination of the specimens of P. laevis at hand corroborates its placement in the Pilumnidae. We currently retain Pronotonyx in Rhizopinae following Ng (1987), Ng et al. (2008) and Poore & Ahyong (2023), because of its close similarity to other rhizopines, many of which lack (or have minimally developed) anterolateral carapace spines and have a male pleon in which somites 1 and 3 are strongly widened and somite 2 narrow. The monophyly of Rhizopinae, however, has not been demonstrated (Ng 1987; Ng et al. 2008) and as noted by Poore & Ahyong (2023), Rhizopinae and Pilumninae are currently best considered as heuristic groupings that probably reflect a degree of phylogenetic and ecological reality, but which require refinement to be taxonomically effective. The Rhizopinae and Pilumninae remain to be properly diagnosed.
The resemblance between Pronotonyx and Notonyx is superficial. Notonyx A. Milne-Edwards, 1873 (type species Notonyx nitidus A. Milne-Edwards, 1873) is now known to belong to the Goneplacidae, its sternal and gonopodal characters all being characteristic for the family (sensu Castro 2007; Castro et al. 2010; Ng & Manuel-Santos 2007). The G1 of all 12 known species of Notonyx is relatively short and stout, the distal parts are lined with prominent spines, and the G2 is elongate, being always longer than the G1 (see Clark & Ng 2005; Ng & Clark 2008, 2010, 2011; Naruse & Maenosono 2009; Naruse & Takeda 2010; Rahayu & Ng 2010a, b; Rahayu 2011). The males of P. laevis examined in this study show conclusively that Pronotonyx belongs to the Pilumnidae: the G1 is sinuous and slender, and the G2 is very short and sigmoid in form (Fig. 4I, J).
Of the known rhizopine genera (sensu Ng 1987; Ng et al. 2008; Poore & Ahyong 2023), Pronotonyx appears to be morphologically closest to Pseudocryptocoeloma Ward, 1936, which was established for one new species, P. parvus Ward, 1936, also from Queensland. Pseudocryptocoeloma was a poorly known genus, the original description being brief and the figures uninformative. Edmondson (1951) interpreted the genus as including a new species he recognised from Samoa, P. symmetrinudus, a taxon with distinct areolae on the carapace. Ng & Rahayu (2023) redescribed the genus and species, restricting Pseudocryptocoeloma to the type species, Pseudocryptocoeloma parvum Ward, 1936, and transferring Edmondson’s (1951) species to Lophoplax Tesch, 1918. Pseudocryptocoeloma can easily be distinguished from Pronotonyx in possessing a more quadrate carapace with the regions weakly but visibly demarcated (Ng & Rahayu 2023: fig. 13A) (versus proportionately wider with the no regions visible and the surface appearing smooth and polished; anterolateral teeth are low but indicated by distinct notches versus entire or scarcely discernible; Fig. 1A, 2A, B, 3A–D, 4B, C); the posterior margin of the epistome is almost straight with the median lobe low and the lateral parts almost straight (Ng & Rahayu 2023: figs. 14A, 15C) (versus margin distinctly sinuous with the lateral parts distinctly convex; Fig. 1B, 2D, 3E, 4A, F); the chelae are distinctly stouter with the pollex clearly shorter than the palm with the dactylus angled obliquely downwards at an angle of about 60° to the vertical (Ng & Rahayu 2023: figs. 13B, 14C, D) (versus chelae longer, with the pollex as long as the palm and the dactylus inclined at an angle not exceeding 45° to the vertical; Fig. 1D, 2F, 3E); the P2–P5 are clearly shorter and stouter (Ng & Rahayu 2023: fig. 13A, 14E) (versus P2–P5 distinctly longer and more slender; Fig. 1A, B, 2A, 3A, B); and the male pleonal somites 1 and 3 are proportionally narrower (Ng & Rahayu 2023: figs. 13B, 14F, 15E, F) (versus male somites distinctly wider; Fig. 2I, 4H). In addition, the carapace and pereopods of Pseudocryptocoeloma are distinctly more setose than those of Pronotonyx (Ng & Rahayu 2023: fig. 13 versus Fig. 3C, D), with the setae longer and coarser as well as covering the anterior parts of the carapace.
The very wide male pleonal somites 1 and 3 is a feature also shared with rhizopine genera such as Rhizopa Stimpson, 1858, Ser Rathbun, 1931, Cryptolutea Ward, 1936, and Rhizopoides Ng, 1987, as well as some species of the heterogeneous Heteropilumnus De Man, 1895 (cf. Ng 1985: fig. 2F; Ng 1987: fig. 2A; Ng & Davie 1991: figs. 2C, 4H; Ng & Rahayu 2020: figs. 4B), but these genera have very different carapace and maxilliped 3 features.
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 393-394, DOI: 10.11646/zootaxa.5293.2.12, http://zenodo.org/record/7960048