Proexotelson tokoroi, Naruse, Tohru, 2015

Naruse, Tohru, 2015, Description of a new genus and a new species of gaeticine crab (Crustacea: Brachyura: Varunidae) from the Ryukyu Islands, and a review of Acmaeopleura Stimpson, 1858, and Sestrostoma Davie & N. K. Ng, 2007, Zootaxa 3925 (2), pp. 211-228 : 222-224

Proexotelson tokoroi n. sp.

( Figs. 2 c, 6–9)

Material examined. All specimens of Proexotelson tokoroi n. sp. were collected with the aid of a yabbie pump while SCUBA diving at Iriomote Island, Ryukyu Islands, Japan,

Holotype: RUMF-ZC-3669, 1 male, 3.0 × 3.8 mm, Amitori Bay, - 4 m, coll. T. Naruse, 20 Aug. 2014.

Paratypes: RUMF-ZC-3653, 1 male, 2.6 × 3.4 mm, Midara, - 7–9 m, coll. T. Naruse, 15 Aug. 2014; RUMF- ZC-3654, 1 female, 3.4 × 4.7 mm, data same as RUMF-ZC-3653; RUMF-ZC-3655, 2 females, 3.2 × 4.2 mm, 3.9 × 5.8 mm (with epicaridian parasite), Amitori Bay, - 1 m, coll. T. Naruse et al., 17 Jul. 2014; RUMF-ZC-3656, 1 female, 2.7 × 3.6 mm, Amitori Bay, - 8 m, coll. T. Naruse, 5 Aug. 2014; ZRC 2015.0007, 1 male, 2.5 × 3.0 mm, 3 females, 2.8 × 3.6–3.1 × 4.4 mm, Midara, coll. T. Naruse, 14 Aug. 2014; RUMF-ZC-3664, 1 female, 3.5 × 4.5 mm, data same as holotype.

Description. Carapace oval ( Figs. 6 a, 7a), distinctly wider than long, CW 1.22–1.36 (mean 1.30, n = 8) CL, widest at slightly posterior to middle of carapace. Dorsal surface convex longitudinally, transversely, region poorly defined. Front gently sloping anteroventrally, frontal margin straight in anterior view, frontal width 0.28–0.31 CW (mean 0.29, n = 8). Orbital margins entire, oval in anterior view, external orbital corner not pointed, confluent with anterolateral margin. Infraorbital margin ending mesially with triangular, short inner orbital tooth, wide gape between tooth and lateral angle of front. Suborbital crest consist of one long, thin, low, straight crest, laterally followed with minute granules, in both male, female. Milne Edwards openings distinct. Lateral margins of carapace entire, weakly cristate; anterolateral margins weakly convex, divergent posteriorly; posterolateral margin straight, convergent posteriorly.

Epistome posterior margin only slightly produced medially.

Eyes oval, tightly filling orbit ( Fig. 6 a). Antennule with relatively high basal segment. Antenna entering orbit, with relatively short flagellum.

Thoracic sternum wide. Male thoracic sternites 2, 3 ( Fig. 8 ) with wide, deep depression for telson; female anterior sternal plate almost flat. Lateral end of sternite 2 produced anterolaterally, projection fitting concavity of third maxilliped ischium when folded ( Figs. 8 , 9 a). Sternites 3, 4 indiscernible. Male sternoabdominal cavity narrow, long; cavity, telson, G1 reaching beyond suture 2/3 when folded; lateral margin of sternoabdominal cavity on sternite 4 slightly convex, meeting proximal concavity of telson when closed ( Fig. 8 ). Simple, large granule for locking mechanism present near suture 4/5 on sternite 5. Penis sternal. Female sternoabdominal cavity not reaching suture 2/3. Vulvae apart from each other, placed posterior to base of third maxilliped ischium, adjacent to suture 5/ 6 on sternite 6, sternal cover developed from posterolateral corner.

Third maxilliped ( Figs. 8 , 9 a) rectangular, tightly covering buccal cavity when closed. Ischium as long as merus; ischium, merus each as long as its width, border between ischium, merus horizontal. Ischium with rounded, distinct concavity on proximal margin medially, mesial margin slightly dentate. Merus lateral margin auricular, produced laterally, with concave anterior margin, carpus attached to concave margin. Carpus, propodus attached to distal end of carpus, dactylus articulated on subdistal portion of outer surface of propodus ( Fig. 1 c), carpus longest, carpus, propodus trigonal-pyramid in cross-section, propodus thicker than carpus, dactylus flat, linguiform, propodus as long as dactylus, dactylus not reaching proximomesial corner of merus (reaching to approximately proximal fifth of mesial margin of merus) when folded, with long setae, setae reaching proximomesial corner of ischium. Exopod with long flagellum, reaching distal end of carpus, with long setae on tip.

Chelipeds ( Figs. 6 , 7 ) symmetrical in both male, female, male chelae slightly larger than in female. Ischium short, with row of minute granules on mesial margins. Merus triangular in cross-section, with slight dentation near lower outer margin, otherwise smooth. Carpus with smooth upper surface, mesial margin weakly granular, but no distinct inner tooth. Chela almost smooth except for sparse granulation on upper half of inner surface; no tuft of setae between fingers. Lower portion of chela with 1 long row of granules from base of manus to just before tip of immovable finger, 1 short row on distal portion of immovable finger below long row.

Ambulatory legs moderately stout ( Figs. 6 , 7 ), P4 longest; no marginal spine or tooth. Meri longest among segments, straight, with rectangular cross-section, anterior, posterior margins covered with minute granules. Dactyli acicular.

Male abdomen ( Figs. 6 a, 8, 9) narrow. First, second somites short; second, third widest, third to sixth with sutures visible but functionally fused. Telson long, proximal third of lateral margins weakly concave.

G1 ( Fig. 9 d–f) slender, long, weakly sinuous, setose on distal third, distal process bent inwards, distal end exposed from telson when abdomen closed. G2 ( Fig. 9 g) short, opening on distal end.

Coloration. In life, carapace mottled brown, pereopods whitish with brownish patterns on meri ( Figs. 6 , 7 ).

Ecology. Proexotelson tokoroi n. sp. was collected with the aid of yabbie pump from burrows on very fine sandy bottoms at depths of 1 to 8 m. No particular associated symbionts were collected from the burrows.

Etymology. The species is dedicated to Mr. Tokoro George. He hosts the television program Tokoro-san no megaten that has contributed to generate interest in science for the public. The program also supported the Iriomote Island survey that led to the discovery of the new species. A standard Japanese name Megaten-gani is also proposed here.

Remarks. Proexotelson tokoroi n. sp. is superficially similar to the species of Acmaeopleura and Sestrostoma . Other than the characters discussed in the remarks of the genus, Proexotelson tokoroi n. sp. can be easily distinguished from S. balssi and S. depressum by the oval contour of the carapace, with the widest portion of the carapace at posterior to the middle portion ( Figs. 6 a, 7a). Sestrostoma balssi has a nearly circular shape, with the widest portion in the middle of the carapace ( Shen, 1932: fig 92, pl. 6-1; Itani et al. 2002: fig. 2; Marin et al. 2011: figs. a–c), whereas S. depressum has a nearly subovate shape with the widest portion at slightly anterior to the middle of the carapace ( Sakai 1965: fig. 26, pl. 9-4). Sestrostoma toriumii is morphologically close to P. t o k oro i n. sp. in that the widest portion of the carapace is slightly posterior to the middle portion. Proexotelson tokoroi n. sp. can be, however, differentiated from S. toriumii in having a relatively wider carapace CW 1.22–1.36 CL (mean 1.30, n = 8) in P. t ok o ro i, CW 1.13–1.33 CL (mean 1.19, n=12) in S. toriumii (calculated from material examined by Takeda 1974; Itani et al. 2002; and the present study).

Itani (2001) reported an undescribed Sestrostoma species that is usually clinging to the abdomen of Upogebia major ( Itani 2001: fig. 2; Itani 2012). This S. sp. appears to differ from P. t o ko ro i n. sp. in the shape of ambulatory dactyli (G. Itani, pers. comm.). Itani et al. (2002) questioned the identity of Crosnier’s (1965) S. cf. balsii from Madagascar and Ghani & Tirmizi’s (1991) S. balssi from Pakistan. These specimens differ from P. t o k oro i n. sp. as they have different carapace outlines ( Crosnier 1965: fig. 60, pl. 5-3; Ghani & Tirmizi 1991: fig. 1A). Furthermore, Ghani & Tirmizi’s (1991) specimens also differ from P. t o ko ro i n. sp. in their distally curved cheliped fingers and the dactyli of at least P3 and P4 ( Ghani & Tirmizi 1991: fig. 1A, D–G). Crosnier’s (1965) specimen has a less concave proximal margin of the third maxilliped ischium ( Crosnier 1965: fig. 61). This may suggest that the Madagascar specimen may not even belong to Sestrostoma and Proexotelson n. gen. The identities of Pakistani and Malagasy species should be reconsidered in future studies.

Figures

FIGURE 1. Anterior thoracic sternum and third maxillipeds. a, Gaetice depressus (De Haan, 1835) (RUMF-ZC- 1814, male, 24.0 × 29.3 mm); b, Sestrostoma balssi (Shen, 1932) (CBM ZC 8146, male, 9.2 × 12.1 mm); c, Sestrostoma depressum (Sakai, 1965) (CBM ZC 7012, male, 5.0 × 6.1 mm); d, Acmaeopleura parvula Stimpson, 1858 (RUMF-ZC- 2939, male, 6.9 × 8.5 mm). 2 / 3, suture between thoracic sternites 2 and 3; G 1, male first gonopod; ms, median sulcus; S 2 l, lateral end of thoracic sternite 2; S 3, 4, thoracic sternite 3 and 4.

FIGURE 2. Palp of left third maxilliped in anterior view. a, Gaetice depressus (De Haan, 1835) (RUMF-ZC- 1814, male, 24.0 × 29.3 mm); b, Sestrostoma balssi (Shen, 1932) (CBM ZC 8146, male, 9.2 × 12.1 mm); c, Proexotelson tokoroi n. sp. (holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 mm); d, Acmaeopleura parvula Stimpson, 1858 (neotype, RUMF-ZC- 2939, male, 6.9 × 8.5 mm); e, Gopkittisak angustum Komai, 2011 (RUMF-ZC- 3673, female, 5.5 × 10.0 mm). Ca, carpus; Da, dactylus; Pr, propodus; Pr-d, distal end of propodus.

FIGURE 6. Proexotelson tokoroi n. sp. (holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 mm). a, dorsal view; b, ventral view.

FIGURE 7. Proexotelson tokoroi n. sp. (paratype, RUMF-ZC- 3654, female, 3.4 × 4.7 mm). a, dorsal view; b, ventral view.

FIGURE 8. Proexotelson tokoroi n. sp. (holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 mm). Buccal area, anterior portion of thoracic sternum, sixth abdominal somite and telson. G 1, tips of male first gonopods; S 2 l, lateral end of thoracic sternite 2; 2 / 3, suture between thoracic sternite 2 and 3.

FIGURE 9. Proexotelson tokoroi n. sp. (a – e, g, holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 mm; f, ZRC 2015.0007, male, 2.5 × 3.0 mm). a, left third maxilliped; b, left chela; c, posterior margin of carapace, abdomen and telson (setae not drawn); d, f, left G 1, ventral view; e, left G 1, mesial view); g, G 2. Scales: 1.0 mm.