Pleurophricus stellatus, Takeda, Masatsune & Tachikawa, Hiroyuki, 2015

Takeda, Masatsune & Tachikawa, Hiroyuki, 2015, Crabs of the families Palicidae and Crossotonotidae (Crustacea, Decapoda, Brachyura, Palicoidea) from the Ogasawara Islands, Japan, with the description of a new species, Zootaxa 3918 (1), pp. 141-150 : 144-148

Genus Pleurophricus A. Milne-Edwards, 1873

Pleurophricus stellatus sp. nov. ( Figs. 1 C, 2B, 3)

Material examined. Kominato, Chichi-jima I., Ogasawara Is., 3 m; 1 male (cb 5.7 mm, cl 5.2 mm), holotype, NSMT-Cr 22988; May 7, 1995; H. Tachikawa leg.

Description of holotype. Carapace ( Figs. 1 C, 2B, 3A) subcircular, only slightly wider than long; dorsal surface depressed, smooth only with microscopic pavement, divided into weakly swollen regions by shallow, wide depressions; anterior part of mesogastric region narrowly extended forward, widened posteriorly; posterior part indistinctly separated from anterior part, indistinctly subdivided into two subregions side by side; metagastric region narrower than mesogastric region, indistinctly subdivided into two at median part; hepatic region depressed behind supraorbital margin; gastro-cardiac transverse furrow shallow, but distinct, with an indistinct small depression at each lateral end; cardiac region slightly transverse, weakly convex dorsally, separated posteriorly from intestinal region by transversal shallow depression; intestinal region weakly convex dorsally, as long as cardiac region, its posterior half reinforced with median compressed, blunt keel; branchial region weakly convex dorsally for its main part behind hepatic region, depressed along lateral margin of carapace, flattened at posterior part.

Front ( Fig. 3 A, B) triangular, deeply incised by V-shaped notch at tip, protruded from level of inner end of supraorbital border; dorsal surface deeply undermined to form median, longitudinal depression between lateral borders of front; tubercle just inside of posterior end of lateral border of front.

Orbit ( Figs. 2 B, 3A, B) transversal, shallow to accommodate club-shaped, weakly tapering eyestalk. Supraorbital border raised dorsally, separated in 2 by median small, but distinct notch; inner part dorsally convex, subacute; outer part cut in 2 triangular, subequal teeth; external orbital tooth ( Fig. 3 A, B) elongated, directed obliquely dorsally as well as supraorbital teeth, weakly convex along outer margin, shallowly cupped to receive spherical cornea; infraorbital border deeply incised at both sides of median triangular tooth; its inner angle produced as elongated tapering tooth seen dorsally just inside of lobate external angle of buccal cavern.

Lateral margin of carapace ( Figs. 1 C, 2B, 3A) regularly arched, with 3 anterolateral and 3 posterolateral teeth behind external orbital tooth; second anterolateral tooth slightly larger than other teeth; anterolateral teeth depressed, subacute, equidistant, directed obliquely forward; posterolateral teeth smaller than anterolateral teeth, directed laterally and posterolaterally, second tooth placed closer to first tooth than to third tooth; posterior half of posterolateral margin unarmed, weakly concave just above base of last ambulatory leg.

Posterior margin of carapace ( Fig. 2 B, 3A) two thirds as wide as frontorbital width, with 6 rounded lobes; median pair largest, submedian pair similar to median pair in shape, but smaller; lateral pair smaller than other pairs.

Chelipeds ( Fig. 1 C, 3D, F) large, mostly visible in dorsal view. Anterior margin of ischium unarmed, as long as anterior margin of merus; articulation of ischium with merus oblique. Anterior margin of merus with tubercle at proximal end, prominent lobe over distal two thirds; tubercle placed just close to distal end of ischium; upper margin of merus with several small tubercles at basal part, with terminal tubercle. Lower margin of carpus produced subdistally as subacute tooth or lobe; carpus ornamented with prominent thin, high crest at outer margins, margin of lobe deeply incised to form 2 subequal lobes; inner margin of carpus armed with 3 thin lobes; median part of outer surface of carpus between thin crests at both margins armed with tubercle, basal part with small tubercle. Palms of both chelae ( Figs. 1 C, 3D-G) different in size, but basically similar in shape, high, only weakly inflated, slightly dorsoventrally depressed; both surfaces smooth, with broad tooth, subacute tubercle on upper rounded margin; both fingers about half as long as palm, with excavated inner surface, no distinct teeth; fingers of right (larger) chela stout, strongly curved; distal margin of palm with short hairs.

Ambulatory legs ( Figs. 1 C, 3H, I) stout, depressed, fringed with scant plumose hairs. In first 3 pairs upper surface of each ischium produced as flattened plate outside of lateral margin of carapace next of flattened merus. Both margins of each merus weakly crested, irregularly toothed, weakly constricted subdistally, with terminal tubercle on anterior margin directed obliquely forward. Each carpus ridged throughout on upper surface, with strong crest on anterior margin; crested anterior margin of first pair depressed weakly to form 2 distinct lobes. Each propodus only weakly convex, unarmed. Dactylus ends to sharp curved claw; inner margin armed with indistinct spinule at median part.

Ischium of third maxilliped ( Fig. 3 C) wide, articulating with merus at outer half of distal margin, inner half of distal margin developed forward to form obtuse lobe. Merus conspicuously smaller, narrower than ischium, with both distal ends of lateral margins protruded toward carpus; anterior (outer) parts of merus, carpus strongly depressed, slightly lobate.

Abdomen ( Fig. 3 J) with 7 somites, elongated, narrow from second to seventh somites. First pleopod ( Fig. 3 K- N) slender, straight, twisted 3 times, with a cupped tip.

Remarks. The genus Pleurophricus was validated and distinguished from Crossotonotus by Castro (2000) who mentioned some significant differences between the two genera. According to him (2000: 582), “Significant differences in the shape of the carapace (subcircular in shape, fewer and larger, salient tubercles along posterior border, two very high and conspicuous bosses on metagastric region in contrast to the subquadrate carapace with very low bosses and many, very small tubercles along the posterior border in Crossotonotus ) and walking legs (presumably shorter and more equal in length than in Crossotonotus ), however, support the separation of the two genera within the Crossotonotidiae.” In the original description of the type species of Pleurophricus , P. cristatipes A. Milne-Edwards, 1873 , the third maxilliped was figured as having the merus with extended (auriculate) anterolateral angle, and both chelipeds are similar in size and shape (A. Milne-Edwards, 1873: Fig. 6a, b). These characters are used in the key to distinguish Pleulophricus from Crossotonotus by Moosa & Serène (1981).

The holotype male of Pleurophricus stellatus sp. nov. applies well to the generic definition of Pleurophiricus in the subcircular carapace only with some lobate, posterior marginal teeth, but the metagastric region is not ornamented by a pair of prominent bosses, the anterolaterral angle of the third maxilliped merus is not auricular, and both chelipeds are different in size. As Castro (2000: 582) explained the third maxilliped of Pleurophricus as “meri of third maxillipeds smaller and narrower than ischia,” the anterolateral angle of the third maxilliped was possibly too emphasized in the original figure. As for the dorsal ornamentation of the carapace, or the presence or absence of a pair of the prominent bosses, it is considered to be not always generic comparing with the other palicoid species. In the holotype male of the present new species, the metagastric region is not armed with the prominent bosses, but shallowly subdivided into two subregions.

In the discussion for P. cristatipes, Castro (2000: 584) mentioned some doubts on the accuracy of the original figure in the nearly identical chelipeds of both sides. The known specimens of P. cristatipes are an incomplete male from Australia and a juvenile male from the Kai Islands, Indonesia, and another congener, P. longirostris ( Moosa & Serène, 1981) , is known only by a female from the Sunda Strait, Indonesia. The known specimens are insufficient to decide the generic value of the male cheliped handedness. The smaller (left) cheliped of the holotype male of P. stellatus sp. nov. seems to be complete and not regenerate in comparison with the larger (right) cheliped. Although, as discussed above, there is a discrepancy in the bilateral symmetry of the chelipeds, the present new species is referred not to Crossotonotus , but to Pleurophricus until the discovery of the male specimens referable to P. cristatipes .

Pleurophricus stellatus sp. nov. is close to P. cristatipes A. Milne-Edwards in the general morphology of the carapace, chelipeds, and ambulatory legs. The original figure of P. cristatipes is diagrammatic and may be inaccurate in some characters, but the holotype male, in which the carapace is said to be partly damaged and the chelipeds and ambulatory legs are missing, was finely figured by Castro (2000). In P. stellatus sp. nov., the metagastric region is only weakly raised and almost unarmed, with indistinct bosses (strongly raised and armed with two strong bosses side by side in P. cristatipes ), the front is two-lobed with a dorsal deep, V-shaped cleft (fourlobed and not so strongly cut dorsally), six subacute teeth on each lateral margin of the carapace (three distinct tubercles on the anterior half and some small indistinct tubercles on the posterior half), and six rounded lobes on the posterior margin of the carapace (five tubercles). According to the original description of P. cristatipes , the palm of the cheliped is armed with two rounded tubercles on the basal and distal parts of the dorsal margin, but in P. stellatus sp. nov., the upper margin of the palm are ornamented with thin and high crests.

Color in life. Intricate pattern with some subdued colors and camouflaged to the sandy bottom ( Fig. 1 C). It is unknown if the striking pink color of the right (larger) cheliped in the holotype male is a specific or individual character.

Etymology. Named from the Latin stella for “star” in reference to the general appearance of a star, having the rounded carapace armed with marginal subequal teeth or lobes, and the flattened and rounded appearance of the crab as a whole surrounded by the depressed chelipeds and ambulatory legs.

Distribution. The only specimen, the male holotype, was found on sandy bottom at a depth of 3 m in Chichijima Island, the Ogasawara Islands.

Figures

FIGURE 3. Pleurophricus stellatus sp. nov., male (cb 5.7 mm, cl 5.2 mm), holotype, NSMT-Cr 22988. A, carapace; B, frontorbital region; C, left third maxilliped; D, right cheliped in dorsal view; E, right chela in ventral view; F, left cheliped in dorsal view; G, left chela in ventral view; H, right third ambulatory leg; I, posterior margin of carapace and left fourth ambulatory leg; J, abdomen; K, M, left first pleopod in ventral and sternal view, respectively, with distal part (L, N) enlarged in different view. Scales: a = 1 mm in B; scale b = 1 mm in C, K, and M, 2 mm in A, D – J.

FIGURE 1. A, Neopalicus jukesii (White, 1847), male (cb 5.8 mm, cl 5.3 mm), CMNH − ZC 0 2475. B, Crossotonotus spinipes (De Man, 1888), immature female (cb 9.5 mm, cl 8.5 mm), NSMT-Cr 22987. C, Pleurophricus stellatus sp. nov., male (cb 5.7 mm, cl 5.2 mm), holotype, NSMT-Cr 22988.

FIGURE 2. A, Rectopalicus ampullatus Castro, 2000, juvenile (cb 6.7 mm, cl 6.5 mm), NSMT-Cr 22986. B, Pleurophricus stellatus sp. nov., male (cb 5.7 mm, cl 5.2 mm), holotype, NSMT-Cr 22988.