Hymenicoides robertsi, Naruse, Tohru & Ng, Peter K. L., 2007

Naruse, Tohru & Ng, Peter K. L., 2007, On the taxonomy of the genus Hymenicoides Kemp, 1917 (Crustacea: Decapoda: Brachyura: Hymenosomatidae), with resurrection of Limnopilos Chuang & Ng, 1991, and descriptions of two new species, Zootaxa 1621, pp. 17-31 : 21-26

Hymenicoides robertsi new species

( Figs. 1 a, b, 3–6)

Material examined. Holotype: 1 male, 4.9 × 6.0 mm, ZRC 2007.0108, Kyaukdaw market on lower Kaladan River (tidal), Rakhine, Myanmar, coll. T. R. Roberts, 26 Mar. 2004.

Paratypes: 1 male, 5.0 × 6.2 mm, ZRC 2007.0109; 42 males, 3.0 × 3.3 – 5.2 × 6.2 mm, ZRC 2007.0110; 1 female, 4.8 × 5.6 mm, ZRC 2007.0111; 4 females, 3.9 × 5.5 mm – 4.8 × 5.6 mm, ZRC 2007.0112; 16 ovig., 4.1 × 4.9 mm – 4.5 × 5.4 mm, ZRC 2007.0113; 5 males, 4.2 × 5.0 – 5.0 × 5.8 mm, 2 ovig., 3.8 × 4.2, 3.8 × 4.3 mm, NHM 2007.600–606; 5 males, 4.0 × 4.8 – 4.9 × 5.7 mm, 2 ovig., 3.8 × 4.4 mm, 4.2 × 4.8 mm, MNHN- B30393. All paratypes were collected together with the holotype.

Description. Carapace ( Fig. 4 a) oval, CW 1.13–1.24 times CL (mean 1.18, n = 9) CL; dorsal surface flat, surrounded by continuous rim, regions well demarcated by grooves, H-shaped gastric groove continuous with cervical groove, cervical groove branching anteriorly, branches confluent with anterolateral rim. Rostrum vestigial, triangular. Lateral margin of carapace lacking tooth or lobe, side wall of posterolateral region slightly expanded laterally, with longitudinal groove along posterolateral rim below. Conical tooth present between antennules, completely disconnected from vestigial rostrum; orbit indiscernible, without tooth on outer part of eye. Epistome long, placed anterior-dorsally to buccal cavern, ischium of third maxilliped partially covering posterior margin; posterior margin with trapezoidal convexity with median notch.

Eyes moderately developed, visible dorsally. Antennule with long coxa and basis; basis more than half length of coxa. Third maxilliped ( Fig. 4 b) narrow, covering about one-quarter of buccal cavern; mid-length of ischium about three-quarters of merus, distal inner angle produced; palp long, propodus about half length of dactylus, dactylus as long as merus, tip of dactylus almost reaching proximal end of ischium in situ; exopod short, reaching about proximal two-thirds of merus, with distinct flagellum.

Male with relatively wide abdominal cavity ( Fig. 1 a), sternal button narrow, high, just posterior to imaginary line joining inner ends of sutures between sternites 5 and 6. Female thoracic sternite 3 separated from sternite 4 by posteriorly convex rim, sterites 4–8 medially fused, vulva on imaginary line joining inner ends of sutures between sternites 5 and 6 on medial fused plate of thoracic sternum, vulva with longitudinally elliptical basal mount.

Chelipeds symmetrical, relatively stouter in males; male cheliped with short merus, ventral outer margin with subdistal tooth, inner margin of merus to carpus lined with long stiff setae, outer surface of carpus rounded, not strongly elevated, outer margin with subdistal tooth; chela ( Fig. 4 c, d) with rounded palm, almost glabrous, outer surface inflated, with dorso-ventrally flattened tubercle on outer surface of proximal lower part of base of immovable finger, tubercle absent in females; fingers with ovoid gape when closed, tips slightly hoof-like; immovable finger with ca. 6 low teeth; teeth of movable finger similar to those of immovable finger.

Ambulatory legs ( Fig. 4 a) slender, long, second longest, inner margin of propodi to dactyli fringed densely lined with long plumose setae; meri slightly longer than respective propodi, distal anterior angle not produced; dactyli ( Fig. 4 e) terminating in sharp tooth, with subterminal inner tooth placed far from distal tooth.

Male abdomen-pleotelson ( Fig. 5 a) 6-segmented; first segment thick, hard, distal margin widely concave; second to fifth segments with distal margin concave medially, third segment widest; pleotelson trilobed, lateral lobes distinct, auriculate, inner surface of lateral lobes thickened externally, forming socket for sternal button on inner surface ( Fig. 1 a, b). G1 ( Fig. 5 c, d) stout, strongly bent on distal half, with longitudinal line of long, stiff setae on distal to medial inner angle of shaft; distal inner end with beak-like chitinous projection, distal outer angle swollen, covered by tiny granule, distal outer angle dorsally connected with thumb-like projection. G2 ( Fig. 5 e) short, less than half length of G1. Female abdomen-pleotelson ( Fig. 5 b) demarcated to 6 segments, boundary between second to fifth segments movable, more or less fixed in other boundaries; second to fifth segments with corneous wide ridge medially, that of pleotelson only on proximal half, pleotelson longest; pleopods ( Fig. 6 ) on second to fifth segments, long, biramous from near base, developing from distal outer angle of inner surface of each segment.

Eggs spherical, small, diameter 0.27 – 0.30 mm [mean = 0.28, n = 15 from three females (3.9 × 4.4 mm, 4.1 × 4.7 mm, 4.2 × 5.0 mm)], attached on endopods of pleopods, exopods growing inwards, partially covering dome-shaped abdomen.

Habitat and distribution. All specimens of Hymenicoides robertsi were obtained from the Kyaukdaw market on the lower Kaladan River (with tidal influence), Rakhine, Myanmar. The specimens were found crawling over a large number of young Pisodonophis snake eels in a tray in the market (T. R. Roberts, pers. comm.) and had been apparently collected in the nearby area. The species is known only from Myanmar at present.

Etymology. We are pleased to name this species for the well known ichthyologist, Tyson R. Roberts, who very generously provided the specimens of this new species as well as those of H. carteri from Bangladesh for our study.

Remarks. Hymenicoides robertsi new species, is clearly different from H. carteri in its proportionately more slender ambulatory legs ( Fig. 3 a; Kemp, 1917: Fig. 17). In addition, both species can be distinguished by the armature of the inner margin of the ambulatory dactyli. In H. robertsi , all the dactyli have only one subterminal tooth, which is placed some distance from the tip, while in H. carteri , the third to fifth dactyli have 3–11 small teeth (posterior legs with fewer teeth) and a subdistal large tooth which is placed closer to the tip. Another major difference is the single beak-like chitinous projection of the G1 of H. robertsi (biramous shorter projection in H. carteri ) ( Figs. 2 a, b, 4a, e, 5c, d; Kemp, 1917: Figs. 17, 20).

Figures

FIGURE 1. Male abdominal cavity and pleotelson of Hymenicoides robertsi new species, and Limnopilos species. a, b, Hymenicoides robertsi new species (paratype, ZRC 2007.0109, 5.0 × 6.2 mm); c, d, L. naiyanetri Chuang & Ng, 1991 (ZRC 1991.6521 – 6555, 4.9 × 5.3 mm); e, f, L. sumatranus new species (paratype, ZRC 2007.0114, 5.0 × 5.9 mm). a, c, e, male abdominal cavity with sternal button (sb) and G 1; b, d, f, inner surface of male pleotelson.

FIGURE 2. Male first and second gonopods of Hymenicoides carteri Kemp, 1917. Lectotype, male (NHM 1919.11.1.121), 4.0 × 4.8 mm. a, left G 1, ventral view; b, left G 1, outer view; c, left G 2. Scale bar, 1 mm.

FIGURE 3. Live colour of Hymenicoides robertsi new species. Photograph was taken by Tyson Roberts.

FIGURE 4. Hymenicoides robertsi new species. a, habitus; b, left third maxilliped; c, male left chela, outer view; d, male left chela, dorsal view; e, right second ambulatory dactylus. a, c – e, holotype male (ZRC 2007.0108), 4.9 × 6.0 mm; b, paratype, male (ZRC 2007.0109), 5.0 × 6.2 mm. Scale bars, 1 mm.

FIGURE 5. Hymenicoides robertsi new species. a, male abdomen-pleotelson; b, female abdomen-pleotelson; c, left G 1, ventral view; d, left G 1, outer view; e, left G 2. a, paratype, male (ZRC 2007.0109), 5.0 × 6.2 mm; b, paratype, female (ZRC 2007.0111), 4.8 × 5.6 mm; c – e, holotype, male (ZRC 2007.0108), 4.9 × 6.0 mm. Scale bars, 1 mm.

FIGURE 6. Hymenicoides robertsi new species. a – d, right second to fifth pleopods; e, exopods. Paratype, ovigerous female (ZRC 2007.0113), 4.2 × 5.0 mm.