Thelphusula capillodigitus, Ng, Peter K. L. & Ng, Paul Y. C., 2018

Ng, Peter K. L. & Ng, Paul Y. C., 2018, The freshwater crabs of Danum Valley Conservation Area in Sabah, East Malaysia, with a description of a new species of Thelphusula Bott, 1969 (Crustacea, Brachyura, Gecarcinucidae, Potamidae, Sesarmidae), ZooKeys 760, pp. 89-112 : 89

Thelphusula capillodigitus sp. n. Figures 1, 2, 3, 4

Material examined.

Holotype: male (23.9 × 18.4 mm) (ZRC 2017.1294), coll. Danum Valley, Lahad Datu, Sabah, Borneo, Malaysia, 22 July 2017. Paratypes: 1 male (18.8 × 15.5 mm) (ZRC 2017.1295), same data as holotype; 1 male (19.9 × 16.4 mm) (ZRC 2009.0080), in pitfall trap, Danum Valley Research Centre, Sabah, coll. C. Colón, October 1996. Others: 3 juveniles (3.7 × 3.0 mm, 6.5 × 5.4 mm, 6.7 × 5.5 mm), 1 young female (10.7 × 9.1 mm) (ZRC 1990.0548-0551), Danum Valley Research Centre, Lahad Datu, Sabah, coll. R. Stuebing, 23 July 1989.

Diagnosis.

Carapace broader than long, not raised; dorsal surface with regions clearly demarcated; frontal median triangle absent (Figs 2 A–C, 3E); epibranchial tooth low, distinct, separated from external orbital tooth by shallow cleft; epigastric regions raised, rugose, not cristate; postorbital cristae low, distinct, rugose, not confluent with epigastric cristae, not reaching anterolateral margin; cervical grooves and H-shaped gastric depression deep; gastric regions with prominent transverse striae; antero- and posterolateral regions with strong oblique striae (Fig. 2A, B); median lobe on posterior margin of epistome triangular, tip rounded (Figs 2C, 3E). Third maxilliped with subrectangular ischium, distinctly longer than broad (Fig. 2D). Chelipeds with outer surface of palm almost smooth, dorsal and lateral surfaces of adult male dactylus covered with dense short setae (Fig. 3 A–D). Ambulatory legs not prominently elongate, dorsal margin of merus gently serrated (Figs 2A, 3 F–I). Thoracic sternum with surface evenly pitted to smooth, sternopleonal cavity reaching imaginary line joining anterior edges cheliped coxae (Fig. 2F, G); pleon distinctly T-shaped, somite 6 rectangular, slightly more than twice as long as broad, telson triangular, longer than broad (Fig. 2E, F). G1 relatively slender, almost straight; terminal segment approx. a quarter length of subterminal segment (Fig. 4 A–C). G2 approx. two-thirds length of G1, distal article short (Fig. 4D).

Description of male holotype.

Carapace broader than long, not raised; dorsal surface gently convex, regions clearly demarcated, covered with very short setae which does not obscure surface; frontal margin almost straight, without distinct median concavity, not deflexed, approx. a third carapace width; frontal median triangle absent (Figs 2 A–C, 3E); anterolateral margin not clearly separated from posterolateral margin; external orbital tooth low, broadly triangular; epibranchial tooth low but distinct, separated from external orbital tooth by shallow cleft; postfrontal surface slightly rugose; postorbital region surface; epigastric regions raised, rugose, not cristate, divided into 2 parts by narrow, deep median groove; postorbital cristae low but distinct, sharp, not confluent with epigastric cristae, not reaching anterolateral margin; cervical grooves and H-shaped gastric depression deep; gastric regions with prominent transverse striae; antero- and posterolateral regions with strong oblique striae; posterolateral margins concave, gently converging towards posterior carapace margin; posterior carapace margin straight (Fig. 2A, B); pterygostomial, suborbital, sub-branchial, and subhepatic regions covered with striae (Fig. 2C); orbits large, eyes occupying entire width, supra and suborbital margins entire, cristate; eyes pigmented, well developed, peduncle with low, sinuous median ridge (Fig. 2B, C); median lobe on posterior margin of epistome triangular, tip rounded, lateral margins sinuous (Figs 2C, 3C).

Mandibular palp 2-segmented, terminal one distinctly bilobed. Third maxilliped covering majority of buccal cavity when closed; ischium subrectangular, distinctly longer than broad, with shallow submedian groove; merus quadrate, slightly broader than long; exopod long, slender, reaching median part of merus, flagellum long, exceeding width of merus (Fig. 2D).

Chelipeds asymmetrical, right larger; surface of merus slightly rugose, relatively long, trigonal in cross section, margins without teeth or spines; carpus surface distinctly rugose, subovate, inner distal angle with sharp spine with basal tubercle; palm relatively stout, longer than broad, outer surface slightly rugose to almost smooth; fingers subequal in length to palm, dactylus marginally longer than pollex, curving inwards, cutting margin of fingers lined with numerous denticles, fingers pitted (Figs 2A, 3 A–D); dorsal and lateral surfaces of most of dactylus covered with dense short setae; lateral surface of pollex with mat of short, relatively less dense setae; tips of fingers strongly curved, corneous, glabrous (Fig. 3 B–D).

Ambulatory legs not prominently elongate, third pair longest, fourth leg shortest; segments laterally flattened laterally, surfaces mildly rugose; dorsal margin of merus gently serrated, no visible subdistal tooth; carpus of first to third legs with low median ridge, absent on carpus of fourth leg; margins of propodus and dactylus lined with numerous short spines (Figs 2A, 3 F–I).

Thoracic sternum surface evenly pitted to smooth; sternites 1 and 2 completely fused forming triangular structure; suture separating sternites 2 and 3 relatively shallow, sinuous, medially convex with lateral parts concave (towards buccal cavity); sternites 3 and 4 completely fused; sternopleonal cavity almost reaching imaginary line joining anterior edges cheliped coxae, near suture between sternites 2 and 3; part of sternite 8 exposed when pleon closed; tubercle of male pleonal locking mechanism prominent, peg-like, on anterior third of sternite 5 (Fig. 2F, G).

Pleon distinctly T-shaped; somite 1 short, broad, reaching coxae of fourth ambulatory legs; somite 2 slightly longer than somite 1, as broad as somite 1; somite 3 short, broadest, with prominently convex lateral margins; somites 4 and 5 trapezoidal; somite 5 notably narrower than 4, trapezoidal with concave lateral margins; somite 6 rectangular, slightly more than twice as long as broad, lateral margins concave; telson triangular, longer than broad, tip rounded (Fig. 2E, F).

G1 relatively slender, entire structure almost straight; terminal and subterminal segments clearly separated; terminal segment relatively short, approx. a quarter length of subterminal segment, cylindrical with tip tapering to subtruncate tip, margins with short stiff setae, surface just before tip with numerous squamiform setae; lower half of subterminal segment with numerous short setae (Fig. 4 A–C). G2 approx. two-thirds length of G1; basal segment long; distal segment short (Fig. 4D).

Variation.

Unlike the male holotype (the largest specimen), the degree and extent of the setation on the fingers of the chelae of the two smaller paratype males are the same in both chelipeds. Male specimens less than 15 mm in carapace width do not have the setae on the fingers of the chelae. The outer surface of the chela in smaller specimens is also relatively more rugose compared to larger ones.

Etymology.

The name is derived from the Latin capillus for hair and digitus for finger. The name is used as a noun in apposition.

Colour.

In life, the carapace is mostly dark reddish brown; the sub-branchial regions, third maxillipeds, pleon and thoracic sternum is pale yellow; the ambulatory legs dark brown, faintly marmorated, with exception of pale yellow, faintly spotted merus; and the chelipeds are yellowish orange, with the inner surfaces paler and the setose patches on the surface of the male fingers light brown (Fig. 1).

Remarks.

Thelphusula capillodigitus sp. n. can easily be distinguished from all congeners by the adult male possessing dense setae on the dorsal surfaces of the fingers of the chelipeds (Fig. 3 B–D), a character also absent in genera allied to Thelphusula : Adeleana Bott, 1969, Balssiathelphusa Bott, 1969, Stygothelphusa Ng, 1989, Arachnothelphusa Ng, 1991, and Coccusa Tan & Ng, 1998 (cf. Bott 1969, 1970; Ng 1989b, 1991; Tan and Ng 1998; Ng and Guinot 2014). The absence of a clearly discernible frontal median triangle is a character T. capillodigitus shares with T. pueh , T. cristicervix and T. styx , but it can be distinguished from them by the presence of setose patches on the fingers of the adult male chelipeds (Fig. 3 B–D) as well as a G1 which is only slightly curved with a relatively shorter terminal segment that is approx. a third the length of the subterminal segment (Fig. 4 A–C). In contrast, the G1s in T. pueh and T. cristicervix possess a prominently curved terminal segment which is proportionately longer, being approx. half the length of the subterminal segment (cf. Ng and Grinang 2014: fig 3). In T. styx , the G1 has a relatively broader subterminal segment with the terminal segment distinctly upturned (cf. Ng 1989a: figs 2E, F). Thelphusula capillodigitus can further be distinguished from T. styx by its relatively more shallow cervical grooves which end at the H-shaped median depression with a level and straight frontal margin (Fig. 2B) (versus with deeper and distinctly longer cervical grooves that extend to the posterolateral region of the carapace, and the frontal margin deflexed in T. styx ; cf. Ng 1989a: fig.1). The carapace of T. capillodigitus is gently convex (Fig. 2B, C) whereas in both T. pueh and T. cristicervix , the carapaces are distinctly inflated (cf. Ng and Grinang 2014: figs 1C, 2C). In addition, T. pueh , T. cristicervix , and T. styx are only known from Sarawak.

In the general form of the carapace (not raised and relatively low) and relatively shorter ambulatory legs, T. capillodigitus most closely resembles T. sabana from Lahad Datu and T. hulu from the Maliau Basin, both in Sabah. Other than in the setose adult male cheliped fingers, T. capillodigitus can also be distinguished by the gastric regions prominently lined with transverse striae (Fig. 2B) (versus gastric regions rugose to smooth in T. hulu ; cf. Tan and Ng 1997: fig. 5); the absence of a frontal median triangle (Figs 2C, 3E) (versus frontal median triangle distinct in T. hulu ; Tan and Ng 1997: fig. 3B); the ischium of the third maxilliped being relatively longer (Fig. 2D) (versus ischium relatively shorter in T. hulu ; cf. Tan and Ng 1997: fig. 3D); the outer surface of chela being almost smooth (Fig. 3A, B) (versus covered with prominent striae and scattered granules in T. hulu ; cf. Tan and Ng 1997: fig. 3C); the male pleonal somite 6 being proportionately longer (Fig. 2E) (versus male pleonal somite 6 proportionately shorter in T. hulu ; cf. Tan and Ng 1997: fig. 4B); and the G1 being almost straight (Fig. 4 A–C) (versus G1 terminal segment strongly curved outwards in T. hulu ; cf. Tan and Ng 1997: fig. 4C, D). Thelphusula capillodigitus resembles T. sabana from Lahad Datu in possessing strong striae and granules on the carapace surface, but can be separated by lacking a frontal median triangle (Figs 2C, 3E) (versus frontal median triangle discernible but incomplete in T. sabana ; cf. Tan and Ng 1998: fig. 3C); the male pleonal somite 6 is proportionately longer (Fig. 2E) (versus male pleonal somite 6 proportionately shorter in T. sabana ; cf. Tan and Ng 1998: fig. 3B); and the G1 is almost straight with a short terminal segment (Fig. 4 A–C) (versus G1 prominently curved outwards with the terminal segment very long in T. sabana ; cf. Tan and Ng 1998: fig. 3 D–G).

Two other species of Thelphusula are present in Sabah, T. dicerophilus (which occurs in the same area as T. capillodigitus ) and T. tawauensis which occurs to the east. Thelphusula capillodigitus can be separated from T. dicerophilus easily by its relatively flatter carapace (Fig. 2B, C) (versus carapace very high and raised in T. dicerophilus ; Fig. 6B; cf. Ng and Stuebing 1990: pl. 1B); and from T. tawauensis by the gastric regions covered with prominent striae and the frontal median triangle being absent (Figs 2C, 3E) (versus gastric regions smooth with the frontal median triangle prominent in T. tawauensis ; cf. Tan and Ng 1998: fig. 4A, C).

Thelphusula capillodigitus was collected in a clear flowing shaded jungle stream with an average temperature range of 26-28 degrees Celsius and near neutral pH. All specimens were collected during the day, under rocks, and appear to be mostly aquatic in habits, although one specimen was collected from a pitfall trap (ZRC 2009.0080). Parathelphusa valida was also present in the same stream in larger numbers. The presence of a second species of Thelphusula in Danum Valley is not surprising, considering that T. capillodigitus has more aquatic habits than T. dicerophilus (see next species).