A new species of micro-mangrove crab of the genus Haberma Ng & Schubart, 2002 (Crustacea, Brachyura, Sesarmidae) from Hong Kong

Abstract The sesarmid genus Haberma Ng & Schubart, 2002, currently contains two species of small mangrove crabs with the first two pairs of the male ambulatory legs possessing characteristic subchelate dactyli and propodi. A new species, H. tingkok, is here described from Hong Kong. It can be separated from H. nanum Ng & Schubart, 2002 (from Singapore), and H. kamora Rahayu & Ng, 2005 (from Indonesian Papua) by its carapace shape, proportions of the ambulatory legs, and structures of the male pleon and male first gonopod.


Introduction
The genus Haberma was established by Ng and Schubart (2002) for a small species of mangrove sesarmid crab from Singapore, H. nanum; and was characterised by an entire lateral carapace margin, the absence of stridulatory ridges and tubercles on the male chela and dactylus, and the possession of subchelate male first and second ambulatory legs. A second species, H. kamora Rahayu & Ng, 2005, was subsequently described from Papua in Indonesia. In this paper, a third species from mangroves in Hong Kong is described.
The sesarmid fauna of Hong Kong is not well studied. Stimpson (1858Stimpson ( , 1907 reported four species of Sesarmidae Dana, 1851, s. str., andShen (1940) recognised only seven species from the territories. Soh (1978) was the first to do a major study, recording 15 species of which four were new records and three were new species. Of the new species, Naruse and Ng (2008) synonymised Holometopus serenei Soh, 1978, under Chiromantes haematocheir (De Haan, 1833. One recent taxonomic change is recognising what had been called "Parasesarma plicatum (Latreille, 1803)" as Parasesarma affine (De Haan, 1837) instead . Perisesarma maipoensis (Soh, 1978), previously regarded as a Hong Kong endemic, is now also known from Vietnam . Rahayu and Ng (2005: 24) recorded Parasesarma tripectinis (Shen, 1940) from Hong Kong, and noted it was a senior synonym of Parasesarma acis Davie, 1993 (see also Ng et al. 2008). It can be expected that with more intensive surveys, additional species of Sesarmidae will be discovered.
Ischium of third maxilliped with shallow, oblique median sulcus; merus shorter than ischium, with distinct oblique median ridge; exopod slender, tip reaching to more than half length of outer margin of merus, flagellum long (Fig. 4A).
Eggs small, subovate (Fig. 3E), ca. 0.25 mm in freshly preserved material. Variation. The adults examined do not vary substantially. The smallest specimen, a young male measuring 4.6 × 4.5 mm (ZRC 2016.622), differs from the adult males in having the lateral margins of the carapace gently concave, resembling the condition observed in H. kamora (Fig. 6B) but the carapace still looks relatively quadrate and the supraorbital margin is less sloping. Its subchelate P2 and P3 are less prominent, mainly because the setae on the distal margin of the propodus and that of the dactylus are less developed, being very short. In addition, its pleon is less semicircular in shape, being more triangular and closer to that observed in H. kamora (Fig. 6D).
Colour. In life, carapace dark brown with light brown mottling; ambulatory legs mottled brown with darker bands on carpi and propodi; chelae light-orange to orange, with fingers darker coloured (Fig. 1A, B). Ventral surfaces of cephalothorax and ambulatory legs light yellow with numerous fine brown spots (Fig. 1B).
Etymology. The species is named after the Ting Kok mangrove area, which has been designated a "Site of Special Scientific Interest" in Hong Kong. The name is used as a noun in apposition.
Remarks. Haberma tingkok sp. n. can easily be separated from H. nanum Schubart, 2002, andH. kamora Rahayu &Ng, 2005, by the carapace appearing proportionately broader (Fig. 3A) rather than more distinctly quadrate (Fig. 6A, B). While their carapace width to length proportions do not differ substantially, this difference in carapace shape is due mainly to the lateral margins of H. tingkok being straighter and less sinuous (Fig. 3A); which in H. nanum and H. kamora, are distinctly more sinuous (Fig. 6A, B). This also affects the shape of the supraorbital margin. In H. tingkok, the margin is relatively longer and gently curves laterally to the external orbital tooth (Fig. 3A). In H. nanum and H. kamora, the supraorbital margin appears shorter as it curves obliquely to the margin (Fig. 6A, B). This difference in carapace shape applies for both sexes (Fig. 2). In addition, the male P4 and P5 propodi are prominently more elongated ( Fig. 2A, 4D, E) than those of H. nanum and H. kamora (Fig. 6A, B). The margin of the frontal lobe of H. tingkok is the most convex in the genus (Fig. 3A); in H. nanum, the margin is almost straight (Fig. 6A) whilst in H. kamora, it is gently convex (Fig. 6B). The male telson of H. tingkok is distinctive as it is semicircular (Figs 3B, 4F); in H. nanum and H. kamora, the telsons are relatively longer and more triangular (Fig. 6C, D). The distal chitinous part of the G1 of H. tingkok (Fig. 4H-L) is more curved than that of H. nanum (cf. Ng and Schubart 2002: fig. 4B, C); and while it is more similar in form to that of H. kamora, the chitinous distal part of the G1 of this species is relatively longer (cf. Rahayu and Ng 2005: fig. 3H, I).
One character not described in Ng and Schubart (2002) and Rahayu and Ng (2005) is the presence of a short, uneven ridge on the outer surface of the ambulatory merus, at the subdistal part just before the carpus (Fig. 4B-E). It is present on all the legs, being more prominent in P2 and P3. This ridge is present also in H. nanum and H. kamora, but is relatively less well developed compared to H. tingkok.
Ecology. The specimens were found climbing trees of Kandelia obovata Sheue, Liu & Yong, 2003, and Aegiceras corniculatus (L.) Bianco, 1837, in the mid intertidal area of the Ting Kok mangrove stand, in Tolo Harbour. The area is the largest mangrove stand on the eastern coast of Hong Kong and is largely dominated by K. obovata trees, up to 3 m tall. All specimens, including the ovigerous females, were collected at a height of approximately 1.5-1.8 m above the substrate, walking on the bark of the branches at ebbing and low tides.