Heteropilumnus planus Ng and Lin, 2023

Sunil, Navami, Unnathpadi, Rajesh & Pullithadathil, Biji, 2023, Five Subtidal Species of the Family Pilumnidae (Crustacea, Decapoda, Brachyura) from the Philippines, with Comparative Notes on Pilumnus hirsutissimus Takeda and Komatsu, 2020, from Japan, Bulletin of the National Museum of Nature and Science. Series A, Zoology 49 (3), pp. 107-122 : 108-110

Heteropilumnus planus Ng and Lin, 2023

( Figs. 1 , 5A )

Heteropilumnus planus Ng & Lin, 2023, p. 102 , figs. 1A–C, G, 2–4.

Material examined. Dive site lSecret Bayz, Anilao, Batangas, Luzon I., Philippines, 6 m depth, gravelly bottom; 1 ovig.8 (NMCR-92506; cb 10.9 mm, cl 7.3 mm); 30–IX–2018; H. Takakura leg.

Diagnosis. Carapace ( Fig. 1A ) quadrilateral rather than oval in contour; dorsal surface ( Fig. 1A–B ) flattened, densely covered with short soft hairs; frontal margin fringed with a row of long club-shaped hairs; anterolateral margin cut into four weakly convex lobes with three small, but distinct notches; external orbital angle poorly developed, confluent with anterior end of first lobe. Chelipeds ( Fig. 1A ) heavily covered with shaggy hairs of various lengths disguising their details. Ambulatory legs ( Fig. 1A ) marginally fringed with numerous hairs similar to those of chelipeds; upper surface of last pair covered with short soft hairs similar to those of carapace dorsal surface.

Color in life ( Fig. 5A ). The carapace ground color is more or less pinkish, and the dense hairs of various lengths covering the carapace, chelipeds and ambulatory legs are grayish. Upper surfaces of first three pairs of the ambulatory legs are nearly naked and pinkish creamy with some reddish blurred patterns.

Remarks. This species was first identified as

Heteropilumnus hirsutior ( Lanchester, 1900) characterized by short soft hairs and long club-shaped hairs covering the carapace dorsal surface, chelipeds and ambulatory legs which are well represented by Ng and Tan (1988) and Maenosono (2019). As already pointed by them, the general appearance of this species is close to those of two congeners, Heteropilumnus ciliatus ( Stimpson,1858) and H. satriai Yeo, Rahayu and Ng, 2004 . In H. hirsutior , the carapace is said to be proportionally wider, and the carapace anterolateral margin is divided into four lobes, the median two of which are weakly toothed and armed with some marginal spinules, differing from the more developed, but unarmed two lobes in H. ciliatus as seen in Maenosono (2019, fig. 2B), and also from the indistinct two lobes margined with spinules in H. satriai . In the ovigerous female examined, both chelipeds are heavily covered with club-shaped hairs of variable lengths.

However, very recently, Ng and Lin (2023) described a new Heteropilumnus species from Taiwan, H. planus , based on comparison with the specimens of H. hirsutior from Singapore including the male holotype and one of the specimens from the Ryukyu Islands identified as H. hirsutior by Maenosono (2019). Both species are really similar in the general formation of the carapace, chelipeds and ambulatory legs, but Ng and Lin (2023) recorded the differences in 1) the carapace (higher with the dorsal surface gently convex in frontal view in H. hirsutior / lower, with the dorsal surface distinctly flatter in frontal view in H. planus ), 2) the first anterolateral tooth of the carapace (more truncate, and the following teeth separated by deeper clefts in H. hirsutior / more triangular in shape, with the teeth separated by concavities in H. planus ), 3) two median lobes of the epistome posterior margin (median lobes forming one triangular structure, joining lateral part as a concavity and separated by a narrow fissure in H. hirsutior / two lobes project downwards, truncate with each margin concave and separated from the lateral part by a prominent V-shaped cleft in H. planus ), 4) the chelipeds of the adult male (more or less homochelous, with the granulated outer surface of the major palm in H. hirsutior / heterochelous, with the smooth outer surface of the major palm in H. planus ), 5) the ambulatory meri (shorter and stouter in H. hirsutior / relatively longer and more slender in H. planus ), 6) the male telson (distinctly more elongate and linguiform in H. hirsutior / more rounded and proportionately shorter in H. planus ), and 7) the G1 (uniformly sinuous, with the distal part directed more laterally in H. hirsutior / less sinuous, with the proximal half almost straight and only the distal half curved, with the distal part distinctly hooked downwards in H. planus ). All of these differences may be not always clear and specific, indicating the importance of direct comparison of the specimens for definite identification. As regards the present Philippine ovigerous female, only the items 1 ( Fig. 1B ), 2 ( Fig. 1A ) and 3 ( Fig. 1B ) are applicable for comparison with the figures ( Figs. 2B–C, E , 4B–E ) in the original description of H. planus , and with Fig. 5B–C of H. hirsutior given by Ng and Lin (2023), but as a result, its first identification was changed from H. hirsutior to H. planus , with aid of the good photographs by Ng and Lin (2023).

Distribution. The type locality of H. planus is Taiwan, and the closest relative, H. hirsutior is known from Singapore and many local places of Okinawa-jima Island, the Ryukyu Islands. The present record extended the geographical range of H. planus from Taiwan southwards to the Philippines.

Figures

Fig. 1. Heteropilumnus planus Ng and Lin, ovig.8 (NMCR-92506; cb 10.9×cl 7.3mm). A: Dorsal view with the right half denuded. B: Frontal view to show the flattened carapace dorsal surface and the indented poste- rior margin of the epistome.

Fig. 5. Posture in field, with color in life. A: Heteropilumnus planus Ng and Lin, ovig.8 (NMCR-92506; cb 10.9×cl 7.3mm). B: Pilumnus caerulescens A. Milne-Edwards, Ə (NMCR-92507; cb 11.5×cl 14.0mm). C: Pilumnus neglectus Balss, Ə (NMCR-92508; cb 8.3×cl 6.9mm). D: Pilumnus aff. purpureus A. Milne- Edwards, ovig. 8 (NMCR-92510; cb 11.4×cl 7.9 mm).

Fig. 2. Pilumnus caerulescens A. Milne-Edwards, Ə (NMCR-92507; cb 11.5×cl 14.0mm). Habitus in dorsal (A) and ventral (B) views.

Fig. 4. Pilumnus hirsutissimus Takeda and Komatsu, Ə (A, C–E: NSMT-Cr 31483; cb 10.2×cl 8.0mm), Ə (B: NSMT-Cr 31482; 8.3×6.2mm) from Suruga Bay. Habitus in dorsal view (A), chelae in outer view (B), Left G1in ventral view (C), enlarged distal part of left G1 in ventral (D) and sternal (E) views. Scale for C=2 mm.