Pilumnus swajayai Ng & Rahayu, 2021

Takeda, Masatsune & Komatsu, Hironori, 2024, Actumnus ngankeeae sp. nov. and Pilumnus swajayai Ng & Rahayu, 2021, pilumnid crabs from Japan (Crustacea: Decapoda: Brachyura), Zootaxa 5476 (1), pp. 424-433 : 428-432

Pilumnus swajayai Ng & Rahayu, 2021

( Figs. 3D , 5 )

Material examined. Male (cb 5.8 mm, cl 5.0 mm; NSMT-Cr 31015), ovigerous female (cb 5.5 mm, cl 4.6 mm, with some remaining eggs after releasing larvae; NSMT-Cr 31016), off Kado , Amami-Oshima Island, northern Ryukyu Islands, tugboat Daiyu Maru No. 38, sta. DY-03-10, 28°29.90'N 129°33.44'E to 28°30.09'N 129°33.73'E, 155⎼ 155 m depth, 20 June 2003 . Male (cb 8.4 mm, cl 6.3 mm), NSMT-Cr 31017, off Okinawa-jima Island, central Ryukyu Islands, 60⎼ 80 m depth, TV Tokai Daigaku Maru II, 02⎼ 05 August 1979 .

Description of male (NSMT-Cr 31015). Carapace dorsal surface ( Fig. 5A, D ) wholly covered with velvety tomentum, moderately convex in both directions, not granulated, but uneven as a whole, weakly divided into regions; median longitudinal, bifurcating furrow distinct, dividing frontal, epigastric and protogastric regions on both sides; transverse furrow from post-orbital furrow to anterior end of mesogastric region; transverse furrow from last anterolateral tooth to median part of protogastric region; branchial posterior region divided into two by transverse furrow. Hepatic region produced as dentiform lobe ( Fig. 5D ). Cardiac region not distinctly demarcated from branchial regions. Anterolateral teeth ( Figs. 5C, D ) strong, with small horny tip; first tooth placed a little lower plane than line from external orbital tooth to second tooth, being distinctly tuberculated with thick basal part, directed anterolaterally; second tooth prominent, elongated, weakly curved dorsally; third tooth directed dorsally. Posterolateral margin ( Figs. 5A, C, D ) not strongly convergent toward carapace posterior margin, weakly bulged behind last anterolateral tooth. Subhepatic region ( Fig. 5D ) armed with conical tubercle, smaller than external orbital tooth, subequal to hepatic dentiform lobe. Front ( Fig. 5B–D ) strongly deflexed, margin divided into two with median wide, large V-shaped notch; each lobe weakly convex forward for inner two-thirds, deeply concave for outer one-third. Supraorbital margin ( Fig. 5B–D ) moderately raised, without granules; inner part directed posteriorly, main part transverse toward external orbital tooth, with two deep interruptions; small triangular lobe at inside of inner interruption; triangular tooth tipped with spinule between two interruptions; external orbital tooth ( Fig. 5D ) well developed, nearly triangular, directed forward, with obtusely angulated tip and weakly convex outer margin. Infraorbital margin sharply incised below external orbital angle, deeply concave at median part; inner end of infraorbital margin strongly developed as triangular lobe with sharp tip.

Third maxilliped wide, sparsely hairy. Ischium becomes slightly narrower anteriorly, with oblique shallow furrow from anterior inner part to basal outer part. Merus widens anteriorly, leaving narrow space between both sides at articulation with ischium.

Chelipeds ( Figs. 5 ) stout, right being larger, with general shape being basically same in both chelipeds. Upper margin of ischium armed with three spines. Upper margin of merus with some spines of irregular size. Entire surface of carpus uneven, with several dispersed granules of good size; submarginal deep furrow on outer surface along distal margin; inner angle of carpus with recurved tubercle. Larger palm ( Fig. 5E ) with outer upper surface covered with tomentum from distal end of upper margin to basal part of lower margin, bare for lower surface; sharp tips of several tuberculate granules through dense tomentum, some granules on bare surface. Smaller palm ( Fig. 5E ), with outer surface nearly covered with tomentum. Both chelae with cutting edges toothed, fingers with teeth closely occluding; larger chela with fingers thickened; smaller chela with outer surfaces of both fingers each narrowly grooved throughout length.

Ambulatory legs ( Fig. 5A ) stout, last pair little shorter than first three pairs, covered tightly with velvety tomentum on surfaces of all articles, margins of distal three articles with long silky or club-shaped setae; meri unarmed, carpi and propodi each with some spinules on dorsal margin.

Male pleon ( Fig. 5E ) with free somites covered with short setae. G1 ( Fig. 3D ) strongly curved dorsally for its median one-third, ventrally for its distal one-third, with long, subtruncated tip. G2 short.

Notes on the male (NSMT-Cr 31017) and the female (NSMT-Cr 31016). In the male, the carapace is somewhat decalcified and depressed, and the right cheliped and all the right ambulatory legs are missing. The velvety tomentum of the carapace was partly removed along the anterolateral margins. The carapace anterolateral teeth are sharper than those of the other male (NSMT-Cr 31015), and the ambulatory carpi and propodi are armed each with one or two spinules on dorsal margin. The remaining left cheliped is the larger, with the smooth palm from the basal part of the lower margin to the distal part of the upper margin; both fingers are stout, with thick teeth. The G1 is well developed and strongly curved, being similar to the G1 ( Fig. 3D ) of the male (NSMT-Cr 31015).

In the female, the ambulatory legs other than the last leg of left side are detached, and the left cheliped is missing. The carapace dorsal surface, chelipeds and ambulatory legs are similarly covered with velvety tomentum as in the male specimens. The arrangement of the spinules of the ambulatory carpi and propodi is also close to that of the male specimen (NSMT-Cr 31015).

Remarks. Based on the velvety tomentum covering the carapace, chelipeds and ambulatory legs, the taxonomic position of this species was first considered to be in the genus Serenepilumnus Türkay & Schuhmacher, 1985 , which is a replacement name for Leopoldius Serène, 1971 . The presence of the hepatic dentiform lobe and the toothed anterolateral margin of the carapace were also indicative of Serenepilumnus . According to Ng et al. (2008), the genus Serenepilumnus is represented by four species, S. kuekenthali (De Man, 1902) [originally Pilumnus ], S. leopoldi ( Gordon, 1934) [originally Parapilumnus ], S. pisifer ( MacLeay, 1838) [originally Halimede ] and S. velasquiezi ( Serène, 1971) [originally Leopoldius ]. In addition, Ghory et al. (2013) mentioned that Pilumnus sinensis Balss, 1933 , is probably referable to this genus. They are all small species and covered with tomentum. Ng & Rahayu (2021) examined the type specimen of Leopoldius velasquezi Serène, 1971 and indicated that it may even be conspecific with S. leopoldi .

The genus Serenepilumnus was noted by Ng & Rahayu (2021: 270) in the paper describing a new species of Pilumnus , P. swajayai from Indonesian waters: a dense tomentum completely obscures the surfaces and margins of the carapace, chelipeds and ambulatory legs, the carapace is prominently wide, the subhepatic region is unarmed; the carpus of the cheliped is without grooves, the cutting edges of the fingers of the chela are blade-like, and the ambulatory legs are very short. Although, at present, it may be difficult to determine whether all of these features are definitely generic, or may be specific or individual, the Indonesian species was referred not to Serenepilumnus , but to Pilumnus .

The specimens from Japanese waters seem to be more densely covered with tomentum than the Indonesian specimens of P. swajayai as reported by Ng & Rahayu (2021), but on denudation, the fronto-orbital margin, the carapace anterolateral teeth, the chelipeds, and the ambulatory legs are close in the forms from both areas. They are considered to be the same species, with some minor variations. However, most species of Pilumnus are covered with setae of various lengths, without velvety tomentum, the carapace dorsal surface is shallowly divided into regions and sparsely granulated or minutely tuberculated, and the carapace anterolateral margin is usually armed with spines or spiniform teeth with broad base. Ng & Rahayu (2021) mentioned of P. swajayai that its appearance is close to many Indo-West Pacific species of Pilumnus , and thus referred it to that genus. The species was also said to be distinct in possessing a hepatic dentiform lobe, the presence of a distinct supraorbital tooth, and the carinate and lobiform infraorbital margin. The validity of velvety tomentum tightly covering the carapace, chelipeds and ambulatory legs was not mentioned, but might be also peculiar in Pilumnus .

Ng & Rahayu (2021) also discussed the affinity of P. swajayai with the genus Nanopilumnus Takeda, 1974 , which was erected based on Medaeus rouxi Balss, 1935 , and at present includes six species ( Ng et al. 2008; Takeda & Komatsu 2018): N. barbatus (A. Milne-Edwards, 1873) , N. rouxi ( Balss, 1935) , N. heterodon ( Sakai, 1934) , N. coralliophilus ( Takeda & Miyake, 1969) , N. hondai ( Takeda & Miyake, 1969) , and N. modestus Takeda & Komatsu, 2018 . Parapilumnus boletifer Monod, 1956 , formerly included in Nanopilumnus ( Ng et al. 2008) , has been designated as the type species of the genus Balssopilumnus by Števčić (2011). As mentioned by Ng & Rahayu (2021), species of Nanopilumnus share the common characters of a small adult size with the carapace smaller than cb 6 mm, the relatively dense, soft setae covering the carapace, chelipeds and ambulatory legs, the presence of three distinct, stout anterolateral teeth of the carapace, and no subhepatic tooth. However, the presence of a distinct subhepatic tooth and the deep transverse groove along the distal part of the cheliped carpus argue against its placement in Nanopilumnus . In the genus Pilumnus , the subhepatic surface of the carapace is unarmed or armed with a prominent tooth or cluster of some granules, and the cheliped merus is usually provided with a shallow marginal furrow. The type species of the genus Nanopilumnus , N. rouxi , was well documented by Serène & Umali (1972), Tirmizi & Ghani (1986), and Asada & Sato (2020), and the Japanese species of Nanopilumnus , including N. rouxi , were taxonomically clarified by Maenosono (2022), but it is still insufficient to delimit the genus from some related genera. As pointed by Türkay & Schuhmacher (1985) and Davie (1989), and Ng & Rahayu (2021), the definition of Nanopilumnus should be clarified. As mentioned above, the present species from Indonesian and Japanese waters has some mosaic characters among Serenepilumnus , Nanopilumnus and Pilumnus . In this paper, the species is referred to Pilumnus for the present, following Ng & Rahayu (2021).

Distribution. This species has originally been reported from the Sunda Strait, Java, Indonesia (6°09.803'S 104°57.976'E to 6°09.606'S 104°58.208'E; 92⎼ 103 m), and south of Cilacap, southern Java (8°07.462'S 109°05.639'E to 8°07.864'S 109°06.470'E; 163⎼ 166 m).

Figures

FIGURE 3. A–C, Actumnus ngankeeae sp. nov., holotype, male (cb 4.6 mm, cl 3.5 mm), Ogasawara Islands, NSMT-Cr 31014. D, Pilumnus swajayai Ng & Rahayu, 2021, male (cb 5.8 mm, cl 5.0 mm), Ryukyu Islands, NSMT-Cr 31015.A, right chela, outer view; B, right fourth ambulatory leg, dorsal view; C–D, left G1, inner view.

FIGURE 5. Pilumnus swajayai Ng & Rahayu, 2021, male (cb 5.8 mm, cl 5.0 mm), Ryukyu Islands, NSMT-Cr 31015: A, habitus, dorsal view; habitus, anterior view; C, habitus, oblique dorsal view (carapace, right-half denuded); D, habitus, dorsal view (carapace, right-half denuded); E, both chelae, ventral view.