Parvonotus, Davie & Ng, 2024

Davie, Peter J. F. & Ng, Ngan Kee, 2024, A new genus for the small shore crab previously known as Brachynotus spinosus (H. Milne Edwards, 1853) (Crustacea: Brachyura: Varunidae), with two new species from southern Australia and Samoa, Zootaxa 5476 (1), pp. 66-81 : 67-68

Parvonotus gen. nov.

Heterograpsus . —H. Milne Edwards 1853: 192 (p. 158 in reprint) [in part].—A. Milne-Edwards 1873b: 317–318.— Kingsley 1880: 210.— Ortmann 1894: 715. [Not Heterograpsus Lucas, 1846 = Brachynotus De Haan, 1833 ].

Eriocheir . — Hale 1927a: 184; 1927b: 312. [not Eriocheir De Haan, 1835 ].

Brachynotus .— Tweedie 1942: 57.— Griffin 1969a: 88; 1969b: 329.— Davie 2002: 228.— Poore 2004: 510. [not Brachynotus De Haan, 1833].

Type species. Parvonotus meridionalis sp. nov., by present designation. Gender is masculine.

Diagnosis. Carapace quadrangular, slightly broader than long, surface flat, lateral margins slightly upturned; protogastric, branchial and mesogastric regions moderately well defined. Frontal margin produced, straight or sinuously bilobed. Anterolateral margin with four teeth including exorbital tooth, three teeth behind orbital tooth closer to each other, second tooth smallest, widest breath of carapace between last anterolateral teeth. Infra-orbital ridge finely granulate. Cornea large. Antennules large; antenna filling entire orbital hiatus. Epistome entire, with lobulations laterally. Third maxilliped grapsoid in shape, merus and ischium broad; rhomboidal gape when closed. Male chelipeds smooth and enlarged; very small gape when closed; setose at base of fingers; female chelae smaller, lacking setal patch; fingers lined with row of small teeth, but straight corneous crest distally. Pereiopods 2–5 glabrous, relatively long, slender; males with tuft of short setae on distal half of posterior edge of propodus of P2 and P3 (absent in females); merus with anterior subdistal spine or blunt prominence. Male pleon narrow, slender, tapering, with all segments freely moveable (six somites plus telson). G1 relatively slender. Female gonopore oval, distinctly protruding.

Etymology. “Parvos” is Latin (accusative masculine plural adjective of parvus) for “small or little” and is used here because the three included species are all relatively diminutive for a grapsoid crab, and in fact, the popular name of “Little Shore Crab” is commonly used in Australia; this is joined with “notus” to indicate its phylogenetic affiliation and confusion with Brachynotus in the Varunidae . Gender is masculine.

Remarks. The correct generic allocation of Heterograpsus spinosus H. Milne Edwards, 1853 (sensu lato) has long been in conjecture. It was transferred to Brachynotus by Fulton & Grant (1906) without explanation (but presumably because Heterograpsus Lucas, 1846 is a junior synonym of Brachynotus anyway), and then later moved to Eriocheir by Hale (1927a, 1927b). Finally, Tweedie (1942) transferred it back to Brachynotus based probably on the form of the anterolateral teeth and ambulatory legs. There has long been confusion between the genera Brachynotus De Haan, 1833 , Heterograpsus Lucas, 1847 , and Hemigrapsus Dana, 1852 , and this has been fully discussed by McLay & Schubart (2004). Suffice to say Hemigrapsus is currently considered the genus most closely related to Brachynotus .

Until now, “ Heterograpsus spinosus ” has been the only Indo-West Pacific representative of the genus Brachynotus which is otherwise restricted to the Mediterranean Sea, the Atlantic coast of northern Africa and southern Europe. So, biogeographic considerations also support our reasoning that B. spinosus and the other two new species described herein are distinctly and generically different from all other species of Brachynotus . With regard to Hemigrapsus , the constitution of this genus remains problematic and in need of revision (N.K. Ng 2006), but after careful consideration of the type species of Hemigrapsus ( H. crassimanus Dana, 1851 ) and the other species currently attributed to it (N.K. Ng 2006), we are convinced that a new genus, Parvonotus , must be erected to accept H. spinosus and the other two new western Pacific species described herein. This genus has been previously recognised as novel under the proposed name of “ Austragrapsus ” in the unpublished doctoral thesis of the co-author (N.K. Ng 2006), however this name was formally disclaimed as having taxonomic status and therefore does not constitute a valid name under the ICZN Code (Article 8.3). Thus a new name is here provided.

Parvonotus gen. nov. is unique in the following characters: a) anterolateral margin weakly concave, and possessing four anterolateral teeth (including the exorbital tooth) versus anterolateral margin convex, and bearing only three teeth in Brachynotus and Hemigrapsus ; b) the length of the margin of the exorbital tooth is subequal to the total length of the following three smaller teeth; c) the posterior anterolateral teeth are well-separated from the exorbital tooth, and while distinct are small and much less prominent (vs. almost equally spaced and prominent in Brachynotus , and Hemigrapsus ); d) the greatest carapace width is at the level of the fourth anterolateral tooth (vs. second anterolateral tooth in Brachynotus and Hemigrapsus ); e) the infra-orbital ridge is composed of numerous isomorphic granules ( Figs. 1F , 2B ) vs. heteromorphic granulation in Brachynotus and Hemigrapsus (cf. Asakura & Watanabe 2005: fig. 2B); f) Parvonotus lacks a pulvinus at the base of the cheliped fingers ( Brachynotus species possess a pulvinus); g) the propodus of the ambulatory legs is proportionately longer and more slender (vs. short and stout in Brachynotus and Hemigrapsus ); h) the merus of the last ambulatory leg has the posterior margin distinctly convex near the proximal end (vs. a straight margin in Brachynotus and Hemigrapsus ); i) the male pleon ( Fig. 6H– K ) is relatively narrow, vs. relatively broader in Brachynotus ( Froglia & Manning 1978: fig. 2) and Hemigrapsus ( Asakura & Watanabe 2005: fig. 2A, C; Lee et al. 2013: fig. 2E); j) the G1 is relatively slender ( Fig. 2H ) vs. relatively stout in Brachynotus ( Froglia & Manning 1978: fig. 2) and Hemigrapsus ( Asakura & Watanabe 2005: fig. 6; Lee et al. 2013: fig. 2F); and k) the female gonopore is oval in shape and distinctly protruding ( Fig. 2J, K ) vs. rounder and much less protruding in Brachynotus and Hemigrapsus (cf. H. penicillatus (De Haan, 1835) and H. sanguineus (De Haan, 1835) figured in Ng (2006: figs. 19J and 22J, respectively).

The carapace shape and four anterolateral teeth of Parvonotus is reminiscent of Ilyograpsus Barnard, 1955 , in Macrophthalmidae Dana, 1851 , but the broad front, the short ocular peduncles, and the shape of the infraorbital ridge (amongst other characters) places Parvonotus firmly in Varunidae . Within Varunidae , Parvonotus somewhat also resembles the South American genus Cyrtograpsus Dana, 1851 , in that both genera have the fourth anterolateral tooth as the most laterally prominent and forming the widest point of the carapace. However, the two genera differ markedly because: a) the carapace shape of Parvonotus is subquadrilateral, while Cyrtograpsus is more hexagonal; b) the front is proportionately much wider in Parvonotus ; and c) the male pleon is relatively narrower (vs. broader in Cyrtograpsus ).

Figures

FIGURE 1. Parvonotus meridionalis gen. & sp. nov., A, B, dorsal and ventral views of holotype male (QM-W29654), (12.6×10.9 mm), Carlton River estuary, Tasmania; C, D, dorsal and ventral views of paratype female (QM-W27951); E, frontal view of chelae of holotype male; F, frontal view showing epistome, infraorbital borders and postfrontal lobes of holotype.

FIGURE 2. Parvonotus meridionalis gen. & sp. nov., reproduction of figures from published descriptions, A, from Hale (1927a), South Australia; B–E, H–J from Griffin (1969b), male (17 mm c.b.), Pittwater, Tasmania; F, G, from Hale (1927b), male (17 mm c.b.), Kangaroo Island, South Australia. A, dorsal view of male; B, suborbital margin and lower orbit; C, third maxilliped; D, propodus and dactylus of right P2 in anterior view; E, propodus and dactylus of right P5 in posterior view; F, G, internal and external views of right male chela; H, G1 pleonal view; I, G1 tip, sternal view; J, G1 tip, pleonal view. Paratype female (QM-W27951): J, K, female left gonopore showing shape, and position on sternite (J); and in anterior lateral view showing degree of prominence (K).

FIGURE 6. Comparison of males of Parvonotus samoensis gen. & sp. nov. and P. meridionalis sp. nov., and comparison of P5 between all three species. A, C, F, H, J = P. samoensis sp. nov., holotype (11.5×10.3 mm), RMNH.CRUS.D.1219; B, D, G, I, K = Parvonotus meridionalis sp. nov., holotype (12.6×10.9 mm), QM-W29654; E, P. spinosus (H. Milne Edwards, 1853) comb. nov., holotype female (MNHP B11017). A, B, dorsal views; C, D, outer views of left male chela; E, F, G, anterior views of P5; H, I, sternal views; J, K, pleonal somites 5, 6 and telson.