Heteropilumnus planus Ng & Lin 2023, n. sp.
Creators: name: Ng, Peter K. L.
name: Lin, Chia-Wei
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Pilumnidae
Heteropilumnus
Heteropilumnus planus

description: Heteropilumnus planus n. sp.
(Figs. 1A–C, G, 2–4)
Type material. Holotype: male (10.7 × 8.0 mm) (NMMBCD5625), Toucheng, Yilan County, Taiwan, coll. SCUBA, C.-W. Lin, 1 September 2018. Paratypes: 1 female (11.4 × 8.2 mm) (NMMBCD 5626), same data as holotype; 1 female (10.5 × 7.9 mm) (ZRC 2017.1044), muddy substrate under rocks, Northeast Cape, Taiwan, coll. SCUBA, C.-W. Lin, 18 July 2014.
Diagnosis. Carapace subhexagonal, wider than long; dorsal surface of carapace, chelipeds and ambulatory legs with low, covered with felt-like tomentum; anterior part of carapace and anterolateral margins with relatively dense, long, plumose setae, obscuring surface, setae lining postfrontal ridge relatively shorter, more plumose; carapace not high, with dorsal surface gently convex to almost flat in frontal view (Figs. 2C, 4C); anterolateral margin with low external orbital tooth and 3 low anterolateral teeth separated by clear concavities; first anterolateral tooth relatively low, triangular in shape (Figs. 2B, 4B); median lobes of posterior margin of epistome projecting downwards, truncate in shape with each margin concave, separated from lateral part by deep V-shaped cleft (Figs. 2C, E, 4C).Adult male chelipeds heterochelous, outer surface of major chela smooth and glabrous on ventral half (Figs. 2A); dorsal surface of cheliped carpus and chela with mixture of long and short plumose setae obscuring surface. Extensor and flexor margins of ambulatory legs with mixture of long and short plumose setae obscuring margins (Figs. 1A–C, 2A, C, E, G, H); meri relatively long, slender (Figs. 2A, H, 4A, F). Male pleon triangular, somite 1 subrectangular reaching to coxae of last ambulatory legs, somite 3 trapezoidal with rounded lateral margins, as wide as somite 1 (Fig. 3A); telson relatively short, semicircular in shape (Figs. 2F, 3A). G1 gently sinuous, with proximal half almost straight with only distal half curved, distal part distinctly hooked downwards (Fig. 3B–E).
Colour in life. Overall orange-brown setae covering carapace, with surface white; surface of ambulatory legs with patches and streaks of red and orange; distal half of fingers pigmented brown (Fig. 1A–C, G).
Etymology. The name is derived from the Latin for flat, alluding to the relatively flat carapace of the species.
Remarks. Heteropilumnus planus n. sp. is distinctive in that the carapace has a felt-like tomentum overall with the long plumose setae protruding from the anterior surfaces. In aspect and overall appearance, H. planus n. sp. most closely resembles H. hirsutior, but the carapace is lower, with the dorsal surface distinctly flatter in frontal view (Figs. 2C, 4C) (carapace higher with the dorsal surface gently convex in frontal view in H. hirsutior; Fig. 5A, B, D; Ng & Tan 1988; fig. 3A, B); the first anterolateral tooth is relatively lower and more triangular in shape, with the teeth separated by concavities (Figs. 2B, 4B) (first anterolateral tooth more truncate, with the teeth separated by deeper clefts in H. hirsutior; Fig. 5A, B; Ng & Tan 1988: fig. 1A); the two median lobes of the posterior margin of the epistome project downwards, are truncate with each margin concave and separated from the lateral part by a prominent V-shaped cleft (Figs. 2C, E, 4C) (median lobes forming one triangular structure, joining lateral part as a concavity and separated by a narrow fissure in H. hirsutior; Fig. 5D); the chelipeds are heterochelous in adult males with the outer surface of the major chela smooth on the ventral half (Figs. 2A) (chelipeds more or less homochelous with the outer surfaces of both distinctly and evenly granulated in adult male H. hirsutior; Ng & Tan 1988; fig. 3C, G); the ambulatory meri are relatively longer and more slender (Figs. 2A, H, 4A, F) (shorter and stouter in H. hirsutior; Fig. 5A; Ng & Tan 1988; fig. 3E); the male telson in similarly sized specimens is more rounded and proportionately shorter (Fig. 3A) (distinctly more elongate and linguiform in H. hirsutior; Fig. 5C; Ng & Tan 1988; fig. 3D; see later for discussion of this character); and the G1 is less sinuous, with the proximal half almost straight and only the distal half curved, with the distal part distinctly hooked downwards (Fig. 3B–E) (G1 is uniformly sinuous with the distal part directed more laterally in H. hirsutior; Ng & Tan 1988; fig. 4A–D).
Maenosono (2019) reported H. hirsutior from the Ryukyus, Japan. We compared one of his specimens with the material from Singapore and they are very similar. The male chela structure requires comment. The large holotype male has only one chela, which appears to be the minor one (Fig. 5A; E; Ng & Tan 1988: fig. 3C, G), and all the other male specimens from Singapore we have examined so far have homochelous chelipeds with both chelae similarly structured. Even in the largest male examined (11.7 × 8.9 mm, ZRC 1992.10569), both chelae are similar and closely resemble that of the holotype male. For the Japanese material, however, Maenosono (2019) figured a larger male (8.6 × 5.9 mm) as having distinctly heterochelous chelipeds, with the major one having the lower part of the outer surface mostly glabrous and smooth (Maenosono 2019: fig. 4D), whereas a smaller male (7.1 × 4.8 mm) was homochelous, with chelae like those from Singapore specimens. The ambulatory legs of the Japanese specimens are slightly longer (Maenosono 2019: fig. 1C) than those from Singapore (Fig. 5A; Ng & Tan 1988: fig. 3E) but this does not seem significant. The male pleon figured by Maenosono (2019: fig. 6C) is comparatively shorter and more rounded than that of the type (Fig. 5C; Ng & Tan 1988: fig. 3D) but there appears to be some variation, with smaller males possessing a more rounded telson. It must be noted that the holotype male of H. hirsutior is very large (10.8 × 8.0 mm) and a similarly sized male in Singapore (ZRC 1992.10569) also has the same telson shape. Maenosono (2019: fig. 14E, F) figured the G1 of the species and his figure gives the appearance that the proximal half is straighter with the distalmost part more hooked compared to that of the Singapore material (Ng & Tan 1988: fig. 4A–D). Examination of the Japanese male on hand shows its G1 to be almost identical to the Singapore specimens. As such, we treat all the Singapore and Japanese material as conspecific for now. It would be useful to compare all of the Japanese and Singapore specimens to determine if they belong to the same species.
Biology. The specimens were all collected from a coral reef flat covered with muddy sediments (Fig. 1G).
description: Published as part of Ng, Peter K. L. & Lin, Chia-Wei, 2023, Two new species of hairy crabs (Pilumnidae) and the first record of Crinitocinus alcocki (Borradaile, 1900) (Acidopsidae) (Crustacea: Brachyura) from Taiwan, pp. 101-114 in Zootaxa 5297 (1) on pages 102-106, DOI: 10.11646/zootaxa.5297.1.5, http://zenodo.org/record/7989367