Borneosa serrata sp. nov.

(Figs. 17, 18, 22M–P) urn:lsid:zoobank.org:act: B65416DA-4501-43C1-A0FB-31C9D46816A0

Material examined: 1 male (25.5 × 22.0 mm) (ZRC 2006.57), Sungei Pangean (= Pengean), Long Yiu Hulu, Kabupaten Bulungan, Kalimantan Utara, ca. 2.7°N 116.7°E, Indonesia, coll. R. Diesel, 20 August 1995.

Diagnosis: Adult carapace transversely subquadrate, width to length ratio 1.16 (Fig. 17A, B); striae on lateral parts of carapace distinct (Fig. 17B); branchial regions gently convex in frontal view (Fig. 17C, F); dorsal margin of frontal median triangle contiguous with lateral margins (Fig. 17C–E); exorbital tooth with outer margin serrated (Fig. 17B); epibranchial tooth prominent, separated from rest of margin by distinct cleft (Fig. 17B); median lobe of posterior margin of epistome acutely triangular (Fig. 17C–E); ischium of third maxilliped subrectangular (Fig. 18A); fingers of adult male cheliped closing along entire cutting margins (Fig. 18E); P3 and P4 dactyli subequal in length (Fig. 18G, H); male pleonal somite 6 subquadrate, slightly longer than broad (Fig. 18C); male sternopleonal cavity prominently anterior, distance between tip of cavity and suture of thoracic sternites 2 and 3 distinctly shorter than length of thoracic sternite 2 (Fig. 18B); adult Gl subterminal segment relatively slender; terminal segment subconical, distal half gently curved outwards, tapering gradually to subtruncate tip, 0.31 times length of subterminal segment, distal opening large, ovate (Fig. 22M–O); females not known.

Colour: When freshly collected, the specimen was a uniform dark brown on all the dorsal surfaces (R. Diesel, pers. comm.).

Etymology: The species is named for the serrated appearance of the anterolateral margin, with the epibranchial tooth prominent.

Remarks: Morphologically, B. serrata sp. nov. most closely resembles B. niah sp. nov., especially in the presence of a more well-developed epibranchial tooth separated from the anterolateral margin by a deep cleft; and the male sternopleonal cavity projects deeper into thoracic sternite 3 with only a narrow space present between the tip of the cavity and the suture between sternites 1 and 3. Borneosa serrata, however, can be distinguished in having the outer margin of the exorbital tooth serrated (Fig. 17A) (versus almost smooth in B. niah; Fig. 8B); the median tooth on the posterior margin of the epistome is sharply triangular in shape (Fig. 17C– E) (versus blunter in B. niah; Fig. 8C–E); the third maxilliped ischium is proportionately longer (Fig. 18A) (versus proportionately shorter in B. niah; Fig. 9A); the male pleon (notably somites 3–5) is relatively more narrow (Fig. 18C) (versus relatively wider in B. niah; Fig. 9C); and the G1 terminal segment is relatively longer (Fig. 22M–O) (versus relatively shorter in B. niah; Fig. 21A–C).

The G1 structure of B. serrata is very close to that of B. brachyphallus, but B. serrata can be separated in possessing a distinct epibranchial tooth (Fig. 17B) (versus tooth very low to almost absent in B. brachyphallus with the anterolateral margin appearing almost entire; Figs. 6B, 7I); the outer margin of the exorbital tooth is tuberculate and appears serrated (Fig. 17B) (versus almost smooth in B. brachyphallus; Figs. 6B, 7I); the third maxilliped ischium is proportionately longer (Fig. 18A) (versus shorter in B. brachyphallus; Fig. 7A); the male sternopleonal cavity projects deeper into thoracic sternite 3, with only a narrow space present between the cavity and the suture between sternites 1 and 3 (Fig. 18B) (versus there is a wider gap between the tip of the male sternopleonal cavity and the suture between sternites 2 and 3 in B. brachyphallus; Fig. 7B); male pleonal somite 6 is distinctly longer (Fig. 18C) (versus proportionately shorter and more quadrate in B. brachyphallus; Fig. 7C); and the G1 subterminal segment is slightly longer (Fig. 22M–O) (versus relatively shorter in B. brachyphallus; Fig. 20Q–S).

Biology: The specimen was collected in a small tributary with rock substrates and dense forest cover in the centre of Kalimantan Utara (R. Diesel, pers. comm.).