Australocarcinus riparius Davie, 1988, by original designation.

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, 4°41'32.42"S, 55°31'2.90"E, 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.

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).

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.

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.

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

Davie (1988) originally established Australocarcinus for one freshwater species from northern Queensland in Australia, A. riparius Davie, 1988. Davie & Guinot (1996) subsequently described two more species, A. kanaka Davie & Guinot, 1996, and A. palauensis Davie & Guinot, 1996, from New Caledonia and Palau, respectively. Davie & Guinot (1996) showed that Australocarcinus was in the same subfamily as the more apomorphic cavernicolous species Trogloplax joliveti Guinot, 1986, from New Britain; that it belonged to the family Chasmocarcinidae; and provided evidence that their larval development was truncated with the eggs hatching directly into juvenile crabs or megalopas.

Australocarcinus insperatus sp. n., is morphologically most similar to A. riparius Davie, 1988, in the anterolateral margin possessing four low teeth, the anteroexternal angle of the merus of the third maxilliped is clearly auriculiform and the male telson is relatively longer. Australocarcinus insperatus sp. n., however, can easily be separated by possessing a more sub-hexagonal carapace (Fig. 1B) (vs. carapace more subquadrate in A. riparius, Fig. 4B); a distinctly convergent posterolateral margin (Fig. 1B) (vs. posterolateral margins subparallel in A. riparius, Fig. 4B); the last anterolateral tooth is triangular and protrudes laterally beyond the carapace margin (Fig. 1B) (vs. last tooth truncate and not extending beyond carapace margin in A. riparius, Fig. 4B); the ischium of third maxilliped is wider than long (Fig. 1C) (vs. ischium longer than wide in A. riparius, Fig. 4C); the ambulatory merus is more elongate and slender (Figs 1A, 2B, C, E) (vs. meri proportionately shorter in A. riparius, Fig. 4A); and the G1 is relatively more slender (Fig. 3A–D) (vs. G1 stouter in A. riparius, cf. Ng and Castro 2016: fig. 98A).

Figure 3. 

Australocarcinus insperatus sp. n., left G1 and G2; holotype male (10.7 × 8.6 mm) (ZRC 2017.1072), Seychelles. A ventral view B ventral view C distal part (ventral view) D distal part (dorsal view) E ventral view F area between basal and distal segments. Scale bars 0.50 mm (A, B, E); 0.25 mm (C, D, F).

Figure 4. 

Australocarcinus riparius, male (8.8 × 10.2 mm) (ZRC 2006.167), Australia. A overall dorsal habitus B dorsal view of carapace (right side denuded) C right third maxilliped (denuded) D posterior thoracic sternum and pleon E anterior thoracic sternum and pleon F posterior thoracic sternum showing supplementary plate.

All three specimens of A. insperatus sp. n. have a distinct cutting or peeling tooth at the base of the dactylus of the right chela (Fig. 2A), a character which Ng and Tan (1984, 1985) have suggested is used to specially feed on gastropod snails. As most gastropod snails have dextral coiling (opening on the right side when viewed frontally), Ng and Tan (1984, 1985) observed that crabs with the enlarged basal dactylar tooth always have this structure on the right chela to make peeling of the shell more efficient. The other three species of Australocarcinus also have this tooth on the right chela (see Ng and Castro 2016: figs 95A, C, E) and on both sexes. This suggests that one of the main food items of Australocarcinus are freshwater gastropods.

The discovery of A. insperatus sp. n. is surprising as all the members of the Trogloplacinae have been previously found in Australasian and Palau waters. Davie (1988) found juvenile crabs under the pleon of a female A. riparius, with ovigerous specimens possessing some 70 large eggs. Davie and Guinot (1996) found megalopa under a female pleon of A. kanaka, suggesting that the development was direct, like those in primary freshwater crabs like Potamidae, Potamonautidae and Gecarcinucidae (and some Sesarmidae). All trogloplacines also have large vulvae (Ng and Castro 2016: figs 99B, D, F, H), suggesting the eggs of the other two species, A. palauensis and Trogloplax joliveti also have large eggs and do not have free-swimming larvae. The vulvae of A. insperatus sp. n. are also large (Fig. 2G). If all trogloplacines have abbreviated (or at least a semi-abbreviated) development and there are no free-swimming larvae, how did they disperse so widely? Despite hypotheses that primary freshwater crabs may have dispersed through Gondwanic connections (Ng et al. 1995), the available evidence is that they are not old enough to have done so (see Daniels et al. 2006; Cumberlidge et al. 2008; Klaus et al. 2009; Cumberlidge and Ng 2009; Daniels 2011; Cumberlidge and Daniels 2014); and as such, the disjunct distribution of A. insperatus sp. n. begs further studies. A complete molecular phylogeny of the Chasmocarcinidae is now being undertaken by L. M. Tsang (Chinese University of Hong Kong) and the results should throw some light on this matter in the future.

The freshwater stream where the specimens were collected was shallow, the water flowing over a sandy bottom, with scattered rocks and construction rubble from past development works in the area. The crabs attempted to bury into the soft sand when disturbed.