Pilumnus caerulescens A. Milne-Edwards, 1873

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 : 110-113

Pilumnus caerulescens A. Milne-Edwards, 1873

( Figs. 2 , 5B )

Pilumnus caerulescens A. Milne-Edwards, 1873, p. 242 , pl. 9 fig. 3. — Rathbun, 1910, p. 355, pl. 1 fig. 15. — Takeda & Miyake, 1968, pp. 6 (in key), 30, fig. 7, pl. 2 fig. D.

Pilumnus forskalii coerulescens : Balss, 1933, p. 14.

Material examined. Dive site “Secret Bay”, Anilao, Batangas, Luzon I., Philippines, 3 m depth, gravelly bottom;1 Ə (NMCR-92507; cb 11.5 mm including lateral spines, cl 14.0 mm); 2-X-2018; H. Takakura leg.

Diagnosis. Carapace dorsal surface ( Fig. 2A ) strongly convex in both directions, distinctly separated into regions, with a tuft of stiff, long hairs together with some short setae around each granule; anterolateral margin with four teeth including external orbital tooth tipped each with a spiniform spinule; subhepatic region with a stout tooth and some accessory granules. Both chelipeds ( Fig. 2A–B ) unequal; carpus with scattered conical granules of good sizes fringed and interspaced with short and long setae and hairs; larger palm with conical granules distinctly beaded to some longitudinal lines on outer upper part. Ambulatory legs ( Fig. 2A–B ) comparatively stout, heavily fringed with stiff hairs of variable lengths on margins.

Color in life ( Fig. 5B ). The carapace surface is dark brownish, making the camouflage strongly effective against the surroundings together with long stiff, grayish hairs. The morphologically close congener, Pilumnus hirsutissimus Takeda and Komatsu, 2020 ( Fig. 3 ) is rather brick red with irregular, darker red speckles on the carapace and cheliped margins.

Remarks. This species has only been recorded without comments in most of the old literature. However, the present male specimen from the Philippines agrees well with the description of Pilumnus caerulescens by Takeda and Miyake (1968) in the distinct dorsal areolation of the carapace, the arrangements of tufts of some setae and hairs arising from the bases of granules on the areolae, the four sharp anterolateral teeth tipped each with a horny spinule, and the stout subhepatic tooth associated with some granules.

In the male from the Philippines, the hairs covering the carapace are long and dense, making the rough appearance. Takeda and Miyake (1968) mentioned that the tufts of hairs are typically rather sparse, but variable individually. The ambulatory legs are unarmed in the present male, whereas Takeda and Miyake (1968) mentioned that the first three pairs have a small terminal granule on each merus, and usually a very small terminal granule on each carpus. Considering such subtle comments on granules or spinules, the presence or absence of the lsmall terminal granulez of each ambulatory merus and carpus may be also referable to the individual variation.

For the comparative study with P. caerulescens , the authors examined three males and one female identified as P. hirsutissimus (1Ə, NSMT-Cr 31481, cb 7.1×cl 5.0 mm; 1Ə, NSMTCr 31482, 8.3× 6.2 mm, Fig. 4B ; 1 Ə, NSMT-Cr 31483, 10.2×8.0 mm, Figs. 3 , 4A, C–E; 1 8 , NSMT-Cr 31484, 13.8×cl 9.6 mm) from Ose-zaki, the northwestern coast of the Izu Penisula in Suruga Bay, Pacific coast of central Honshu, Japan, at 4.5–6.5 m depth. In the original description of P. hirustissimus by Takeda and Komatsu (2020) based on two females and one ovigerous female from the Ryukyu Islands, southwestern Japan, the species was compared only with P. kempi Deb, 1987 , and without comments on P. caerulescens which is also heavily covered with long hairs on the carapace, chelipeds and ambulatory legs. The specimens from Suruga Bay agree well with the type specimens of P. hirsutissimus in the numerous, long hairs completely disguising the details of the carapace, chelipeds and ambulatory legs, and when denuded, the carapace dorsal surface is well sculptured into areolae and covered with sparse small granules ( Fig. 4A ). The hairs of P. hirsutissimus ( Fig. 4A–B ) are longer and denser than those of P. caerulescence , and arise singularly around the granules, unlike consisting of tufts which make non-rough appearance as in P. caerulescence ( Fig. 2A ).

The male first gonopod (G1) of the present P. hirsutissimus specimen is fully developed ( Fig. 4C–E ), and the median part is strongly convex along the sternal surface of the abdominal cavity; the distal beak is short and subtruncated at the tip, without special taxonomic importance. However, the tip of the distal beak of G1 is subtruncated in P. hirsutissimus , whereas it is narrow and sharp in P. caerulescence as illustrated by Takeda and Miyake (1968: fig. 7).

Distribution. Tropical and subtropical Indo- West Pacific, from the Andaman Islands to Aus- tralia and New Caledonia, and also from the Gulf of Thailand and Micronesian islands to the Ryukyu Islands. Records from the Philippines include Miers (1886: 155, Mindanao), Balss (1933: 24, Sulu Sea), Estampador (1937: 530, Mindanao), Garth and Kim (1983: 693, south side of Marongas I., vicinity of Jolo, scattered coral, sand).

Figures

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. 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. 3. Pilumnus hirsutissimus Takeda and Komatsu, Ə (NSMT-Cr 31483; cb 10.2×cl 8.0mm) from Suruga Bay. Posture in field (A), and habitus in dorsal view (B).

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.

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.