Terrathelphusa secula, Ng, Peter K. L. & Tan, Leo W. H., 2015

Ng, Peter K. L. & Tan, Leo W. H., 2015, Terrathelphusa secula, a new species of semiterrestrial freshwater crab (Crustacea: Brachyura: Gecarcinucidae) from Sabah, East Malaysia, Borneo, Zootaxa 4007 (3), pp. 445-449 : 447-449

Terrathelphusa secula , new species

( Figs. 1–3)

Material examined. Holotype: male (29.2 × 20.4 mm) ( ZRC), found dead in pool adjacent to Borneo Rainforest Lodge, next to Danum Valley Conservation Area, Lahad Datu, Sabah, 458.2 ’N 11741.4 ’E, ca. 600 m asl, East Malaysia, Borneo, coll. local ranger, 28 May 2015.

Diagnosis. Carapace transversely ovate, distinctly wider than long, width to length ratio 1.43 ( Fig. 1 A, B). Cephalothorax prominently swollen; dorsal surface distinctly convex, median parts smooth, branchial surfaces with distinct oblique striae; cervical grooves deep; epigastric cristae low, barely visible, separated by shallow longitudinal groove; postorbital cristae not visible; H-groove distinct, almost confluent with cervical grooves; epibranchial tooth undiscernible, external orbital angle, anterolateral margin separated by very low depression, margin otherwise unarmed, appearing almost entire; anterolateral, frontal regions appearing somewhat compressed; supraorbital margin subparallel with broadly bilobed front, median part of frontal margin bent downwards but no median triangle visible ( Fig. 1 A, B). Epistome with median lobe broad, rounded ( Fig. 1 C). Ischium of third maxilliped rectangular, ca. 1.6 times maximum width ( Fig. 1 D). Cheliped carpus distinctly rugose ( Fig. 1 A), inner angle with sharp tooth, with basal sharp tubercle; fingers slightly shorter than palm, gap between fingers of major chela wide ( Fig. 2 D). Ambulatory legs smooth, relatively slender, second pair longest; length of fourth merus ca. 4.1 times maximum width ( Fig. 2 F, G). Male abdomen with linguiform telson, somite 6 elongated with deeply concave lateral margins ( Fig. 2 A–C). G 1 slender, C-shaped, curving outwards; terminal segment elongated, falciform, tapering gradually to subtruncate tip, ca. 0.5 times length of subterminal segment, surfaces with long, short stiff setae, distalmost surfaces with small scale-like spines ( Fig. 3 A–E). G 2 with distal segment short, ca. 0.3 times length of basal segment ( Fig. 3 F).

Life colour. The carapace was dark brown in the recently dead specimen.

Habitat. The specimen was found freshly dead in a small pool near a lodge by a local ranger. Like other Terrathelphusa species, T. secula n. sp. almost certainly digs deep burrows in the soil, coming out to forage only at night ( Collins 1979; Grinang & Ng 2015). Three other species of freshwater crabs are known from the Danum Valley Conservation Area: Thelphusula dicerophilus Ng & Stuebing, 1990 ( Gecarcinucidae ), Isolapotamon ingeri Ng & Tan, 1998 ( Potamidae ), and Geosesarma danumense Ng, 2002 ( Sesarmidae ) (see Ng 2002; Ng & Stuebing 1990; Ng & Tan 1998).

Etymology. The name is derived from the Latin secula for sickle; alluding to the strongly curved G 1 structure of this species. The name is used as a noun in apposition.

Remarks. Terrathelphusa secula n. sp. belongs to the group of Terrathelphusa species that have relatively low epigastric cristae, a third maxilliped ischium that is relatively short (length to width ratio 1.5–1.6), and a relatively short G 2 distal segment (0.5 times or less then the length of the basal segment). The two other species belonging to this group, T. ovis and T. telur are from eastern Sarawak and Borneo. All the other Terrathelphusa species have more prominent epigastric cristae, a relatively longer third maxilliped ischium (length to width ratio 1.8–2.1 times) and a much longer G 2 distal segment (0.6 length of basal segment or longer) ( Grinang & Ng 2015: Table 1). The carapace shape of T. secula n. sp., however, is much wider and more transversely ovate than those of T. ovis and T. telur , and is most similar to the condition observed for T. loxophthalma (cf. Ng 1997: fig. 3). It is certainly more tranversely ovate and proportionately wider than any of the species now known from western Sarawak (cf. Grinang & Ng 2015).

The most diagnostic feature of T. secula n. sp. is its distinctive sickle-shaped G 1. While other species of Terrathelphusa can have gently curved G 1 s (e.g., T. ovis and T. telur , cf. Ng 1997: figs. 6 E–H, J–L, 8 C–F), none have a G 1 that is as strongly curved and with a terminal segment that long ( Fig. 3 A, B, D). In fact, the length of the G 1 terminal segment of T. secula n. sp. is such that it closely resembles those of several species of the gecarcinucid genus Thelphusula Bott, 1969 (cf. Tan & Ng 1998; Grinang & Ng 2014). The carapace of Thelphusula species is nevertheless more quadrate, the epigastric and postorbital cristae are distinct, the male abdomens is relatively broader, and the distal segment of the G 2 is very short (see Tan & Ng 1998).