Proexotelson, 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 : 221-222

Proexotelson n. gen.

Type species. Proexotelson tokoroi n. sp. by present designation.

Diagnosis. Carapace oval, distinctly wider than long, dorsal surface convex longitudinally, transversely, region poorly defined. Orbital margins entire, infraorbital margin ending mesially with triangular, short inner orbital tooth, wide gape between tooth and lateral angle of front. Eyes oval, tightly filling orbit. Thoracic sternum wide; male sternites 2, 3 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 well when folded. Sternites 3, 4 indiscernible. Male sternoabdominal cavity narrow, long; cavity, telson, G1 reaching beyond suture 2/3 when folded. Penis sternal. Vulvae apart from each other, placed posterior to base of third maxilliped ischium, adjacent to suture 5/6 on sternite 6, sternal cover extending from posterolateral corner; operculum absent. Third maxilliped rectangular, 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; merus lateral margin auricular, produced laterally, with concave anterior margin, carpus attached to concave margin; propodus attached to distal end of carpus, dactylus articulated on subdistal portion of outer surface of propodus; carpus, propodus trigonalpyramid in cross-section, propodus thicker than carpus, dactylus flat, linguiform, distal end of 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. Chelipeds symmetrical in both male, female, male chelae slightly larger in female. Ambulatory legs moderately stout, P4 longest. Male abdomen narrow, third to sixth somites with sutures visible but functionally fused, telson long. G1 slender, long, distal process bent inwards, distal end exposed from telson when abdomen closed.

Etymology. The name of the genus is derived from an arbitrary combination of the two words proexeíkhe (“protruded” in Greek) and “telson”, alluding the protruded male telson from the sternoabdominal cavity. Gender neuter.

Remarks. Proexotelson n. gen. is morphologically distinct from all gaeticine genera by the long and narrow sternoabdominal cavity of the male that join the buccal cavity together with the telson and G1 ( Figs. 6 b, 8); as well as the sinuous and slender G1 which has a thin and perpendicularly bent distal end ( Fig. 9 d–f) that protrudes from the telson even when the abdomen is closed ( Fig. 8 ). Proexotelson n. gen. is morphologically most similar to Sestrostoma , but the sternoabdominal cavity of Sestrostoma always falls short from buccal cavity even in small males (e.g. S. toriumii , RUMF-ZC-3657, male, 3.4 × 3.9 mm). Table 2 summarizes the diagnostic characters of Proexotelson n. gen. The third maxilliped dactylus is attached to a subdistal portion of the propodus, the third maxilliped palp has moderately long setae on the distal end of the dactylus and the distoanterior margin of the propodus, the proximal margin of the third maxilliped ischium has a round and distinct concavity, the lateral end of the thoracic sternite 2 is distinctly produced anterolaterally, and the male abdominal somites 3–6 are functionally fused with visible sutures. Since Proexotelson n. gen. has these gaeticine characters, it is reasonable to place the genus in Gaeticinae .

Figures

FIGURE 6. Proexotelson tokoroi n. sp. (holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 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.