Tuesday, February 20, 2018

[Crustacea • 2018] Australocarcinus insperatus • A New Species of Trogloplacine Crab of the Genus Australocarcinus Davie, 1988 (Brachyura, Chasmocarcinidae) from A Freshwater Stream in Mahé, Seychelles


Australocarcinus insperatus 
Ng & Daniels, 2018 


Abstract
A new species of freshwater chasmocarcinid crab, Australocarcinus insperatus sp. n., is described from the Seychelles Islands in the Indian Ocean. This is the first record of the genus and the subfamily Trogloplacinae Guinot, 1986, from the Indian Ocean, with all other members previously recorded from Australia, New Britain, New Caledonia, and Palau in the Pacific Ocean. The disjunct distribution of Australocarcinus is unexpected considering all trogoplacines are believed to practice direct development, lacking free-swimming larval stages. The new species is morphologically most similar to A. riparius Davie, 1988, from Queensland, Australia, but can be distinguished from its three congeners on the basis of the structures of its carapace, ambulatory legs and male first gonopod.

Keywords: Chasmocarcinidae, freshwater, Indian Ocean, new species, Trogloplacinae, taxonomy


Figure 1. Australocarcinus insperatus sp. n., holotype male (10.7 × 8.6 mm) (ZRC 2017.1072), Seychelles. A overall dorsal habitus B dorsal view of carapace (right side denuded) C right third maxilliped (denuded) D anterior thoracic sternum and pleon E posterior thoracic sternum and pleon F frontal view of cephalothorax G posterior margin of epistome. 

Australocarcinus insperatus sp. n., holotype male (10.7 × 8.6 mm) (ZRC 2017.1072), Seychelles.
Figure 1.  A overall dorsal habitus B dorsal view of carapace (right side denuded) C right third maxilliped (denuded) D anterior thoracic sternum and pleon E posterior thoracic sternum and pleon F frontal view of cephalothorax G posterior margin of epistome.
Figure 2.  A outer surfaces of chelae B right first ambulatory leg showing setose posterior margin on propodus and dactylus C left fourth ambulatory leg D posterior thoracic sternum showing supplementary plate

Figure 2. Australocarcinus insperatus sp. n. A–D holotype male (10.7 × 8.6 mm) (ZRC 2017.1072), Seychelles E–G paratype female (9.5 × 7.8 mm) (ZRC 2017.1073), Seychelles. A outer surfaces of chelae B right first ambulatory leg showing setose posterior margin on propodus and dactylus C left fourth ambulatory leg D posterior thoracic sternum showing supplementary plate E female overall dorsal habitus F female posterior thoracic sternum and pleon G female sterno-pleonal cavity showing vulvae.

Systematics
Family Chasmocarcinidae Serène, 1964
Subfamily Trogloplacinae Guinot, 1986
Genus Australocarcinus Davie, 1988
Type species: Australocarcinus riparius Davie, 1988, by original designation.

Australocarcinus insperatus sp. n.

Material examined: Holotype: male (10.7 × 8.6 mm) (ZRC 2017.1072), in shallow stream, ca. 800 m from sea, about 2 km south-southeast of international airport, Mahé, Seychelles, coll. SR Daniels, May 2010. Paratypes: 1 male (8.5 × 7.2 mm), 1 female (9.5 × 7.8 mm) (ZRC 2017.1073), same data as holotype.

Diagnosis: Carapace subquadrate, front weakly bilobed, with shallow median concavity (Fig. 1A, B); dorsal surface gently convex (Fig. 1F); dorsal surfaces and margins covered with short uneven tomentum (Fig. 1A, B); anterolateral margins arcuate, with four low teeth: first widest with gently sinuous margin, second lobiform, third wide, fourth (at junction of antero- and posterolateral margins) dentate, directed laterally, protruding beyond margin (Fig. 1B). Posterolateral margin converging towards gently convex posterior carapace margin (Fig. 1B). Epistome compressed, posterior margin with distinct triangular median lobe with median fissure, lateral margins gently sinuous (Fig. 1G). Eye peduncle completely filling orbit, relatively short, mobile; cornea distinct, pigmented (Fig. 1B, F). Third maxillipeds leaving gap when closed; merus quadrate, anteroexternal angle auriculiform; ischium quadrate, slightly longer than merus with very shallow median sulcus (Fig. 1C, D). Chelipeds subequal, relatively stouter in males (Figs 1A, 2E); cutting margins of both chelae with distinct teeth in both sexes, base of fingers with tuft of stiff setae; proximal part of dactylus of right chela with large, triangular tooth directed towards palm (Fig. 2A); ventral surface of cheliped merus with tubercles. Ambulatory legs moderately short; meri unarmed but setose to varying degrees; P2 carpus, propodus and dactylus with very long coarse setae which obscures margins (Figs 1A, 2B); P3–P5 propodus and dactylus setose but setae shorter than on P5 (Fig. 2C); P5 dactylus straight (Fig. 2C). Thoracic sternites 1, 2 fused, broadly triangular, short; separated from sternite 3 by sinuous groove; sternites 3, 4 fused, relatively broad (Fig. 1D). Male pleon with lateral margins of somite 6 and fused somites 3‒5 gently sinuous; telson slightly longer than broad (Fig. 1D, E). Sterno-pleonal cavity of male deep, press-button for pleonal holding small, short tubercle posterior to thoracic sternal suture 4/5 near edge of sterno-pleonal cavity. Male thoracic sternite 8 short, rectangular; supplementary plate narrow, wider along outer part (Figs 1E, 2D). G1 stout; basal part truncate; distal part cylindrical, with rounded tip, covered with short spinules (Fig. 3A–D). G2 prominently longer than G1, basal segment curved; distal segment slightly longer than basal segment, apex cup-like (Fig. 3E, F). Somites of female pleon with slightly convex lateral margins; telson wider than long (Fig. 2F). Sterno-pleonal cavity of female moderately deep, with large vulvae distinctly separated from each other, covering most of thoracic sternite 5, ovate, with low raised lip on outer margin, opening slit-like (Fig. 2G).


Etymology: From the Latin “insperatus” for “unforeseen”, alluding to the unexpected discovery of a species of Australocarcinus in the western Indian Ocean.


 Peter K. L. Ng and Savel R. Daniels. 2018. A New Species of Trogloplacine Crab of the Genus Australocarcinus Davie, 1988 from A Freshwater Stream in Mahé, Seychelles (Crustacea, Brachyura, Chasmocarcinidae).  ZooKeys. 738; 27-35.   DOI:  10.3897/zookeys.738.23708

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