A new species of trogloplacine crab of the genus Australocarcinus Davie, 1988 from a freshwater stream in Mahé, Seychelles (Crustacea, Brachyura, Chasmocarcinidae)

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.


Introduction
The Chasmocarcinidae Serène, 1964, is a predominantly marine family, with most of the species occurring in subtidal habitats, and some reaching depths of over 2000 metres (Ng and Castro 2016). One subfamily, the Trogloplacinae Guinot, 1986, however, occurs exclusively in freshwater habitats, sometimes several kilometres from the sea as well as in aquatic inland limestone caves (Davie and Guinot 1996;Ng and Castro 2016). Trogloplacines are also unusual in practising direct development, lacking planktotrophic larvae (Davie and Guinot 1996). Only two genera of Trogloplacinae are known, the monotypic Trogloplax Guinot, 1986 (which lives in caves in New Britain, southwestern Pacific), and Australocarcinus Davie, 1988 (with three epigeal species from northeastern Australia, New Caledonia and Palau). The first Australocarcinus species is reported here from the Indian Ocean, A. insperatus sp. n., from southern Mahé in the Seychelles archipelago.

Materials and methods
Material examined is deposited in the Zoological Reference Collection (ZRC) of the Lee Kong Chian Natural History Museum, National University of Singapore. Measurements provided (in millimetres) are of the carapace width and length, respectively. The terminology used follows that in Ng and Castro (2016) and Davie et al. (2015). The following abbreviations are used: G1 male first pleopod; G2 male second pleopod.
Etymology. From the Latin "insperatus" for "unforeseen", alluding to the unexpected discovery of a species of Australocarcinus in the western Indian Ocean.
Remarks. 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)   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 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 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), 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.
Biology. 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.
Amended key to species of Australocarcinus