Hymenicoides Kemp, 1917

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 : 18-20

Hymenicoides Kemp, 1917

Hymenicoides— Kemp, 1917: 267; Lucas, 1980: 196; Ng & Chuang, 1996: 50 (part); Guinot & Richer de Forges, 1997: 460 (part); Guinot & Bouchard, 1998: 685.

Type species. Hymenicoides carteri Kemp, 1917 , by monotypy; gender of genus masculine.

Diagnosis. Carapace oval, dorsal surface concave; grooves distinct; rostrum absent or very weak; antenna with base of basal article placed posterior-inner part of base of eye; eyes, antennae, antennules visible dorsally; third maxillipeds narrow, not covering more than three-quarters of mouth field when closed, merus rectangular, dactylus styliform, approximately twice length of propodus; male chelae relatively stout, manus with high, prominent, dorso-ventrally compressed tubercle on distal part of outer surface, tubercle absent in females. Female vulvae placed on imaginary line joining inner ends of sutures between sternites 5 and 6 on medially fused plate of thoracic sternum, vulva with basal mount. Male abdomen-pleotelson without fused segments, male pleotelson distinctly trilobed, inner surface thickened externally, forming socket for sternal button ( Fig. 1 a). G1 stout, bent outwards medially; distal part with prominent distal inner processes, swollen distal outer angle, long, dorsal bursiform projections. Female abdomen with 6 distinctly demarcated segments, boundary between first and second segments ( H. carteri ) or between second and third segments ( H. robertsi ) movable; long, biramous pleopods on second to fifth segments, developed from distal outer end of inner surface of each segment.

Remarks. The distinctive trilobate pleotelson of H. carteri has received some attention from taxonomists due to its unusual shape. Lucas (1980: 197) regarded that the lateral lobe of male pleotelson as “a cavity to receive the apice of the first pleopod”. Guinot & Richer de Forges (1997: 470) suggested that the lateral lobe of the pleotelson may be derived from the inserted plate (see Guinot & Richer de Forges, 1997: Fig. 6 , for mobile intercalated platelet of Odiomaris pilosus ). Guinot & Bouchard (1998: 685) subsequently hypothesized that the abdominal socket, which is placed on the inner surface of the lateral lobe (or the intercalated platelet), is homologous with the uropod.

Our examination of the type material of H. carteri and H. robertsi new species, shows that the inner surface of the lateral lobe of the male pleotelson is domed and that it is externally surrounded by a thickened rim, forming a semicircular cavity ( Fig. 1 b). When the abdomen is closed, the cavity overlays the sternal button ( Fig. 1 a). This does not support the premise that the lateral lobe of the pleotelson is to accommodate the unusual tip of the G1 ( Figs. 2 a, b, 5c, d). In fact, even when the abdomen is open, the G1 can easily be accommodated within the sterno-abdominal cavity ( Fig. 1 a). The sternal buttons and the lateral cavities of the pleotelson function instead to “lock” the abdomen with the thorax. It should be noted that the G1 of H. carteri closely resembles that of Cancrocaeca xenomorpha (see also Guinot & Richer de Forges, 1997), and in the latter species, the male abdomen is more typical in shape, lacking the trilobite pleotelson, as well as sternal buttons, seen in Hymenicoides (Naruse, T., Ng, P.K.L. & Guinot, D., in manuscript).

Limnopilos naiyanetri Chuang & Ng, 1991 (type species of Limnopilos Chuang & Ng, 1991 ), L. microrhynchus (Ng, 1995) new combination, and L. sumatranus new species, all possess the sternal button and the pleotelson lateral cavity ( Fig. 1 c–f). The lateral cavities of these three species, however, are relatively smaller and the rim is more distal in position on the cavity when compared with those of H. carteri and H. robertsi . This may be due to the different shape of the G1 of Hymenicoides . The lateral cavity of Hymenicoides needs to be thickened along the external margins to effectively engage the sternal button, three sides (anterior, inner and posterior) which are surrounded by the G 1 in situ ( Fig. 1 a). However, the sternal buttons of Limnopilos are placed more distantly from the distal outer angle of the G1 ( Fig. 1 c, e), and allow the lateral cavity to more easily lock onto the sternal button by way of the distally thickened rim.

The G1s of the Hymenicoides species ( Fig. 2 a, b, 5c, d) differ from those of Limnopilos in several key aspects: they have a proportionately stouter shaft (vs. moderately stout in Limnopilos ), more prominent distal inner processes (vs. moderately stout in Limnopilos ) and with the distal outer angle more swollen than in Limnopilos species ( Fig. 8 c; Ng & Chuang, 1996: Fig. 21H; Ng, 1995: Fig. 14A, B).

The marked differences in the chela, abdomen and G1 lead us to now conclude that Limnopilos should be resurrected as a valid genus.

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 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.

FIGURE 8. Limnopilos sumatranus new species. a, male abdomen-pleotelson; b, female abdomen-pleotelson; c, left G 1; d, left G 2. a, paratype male (ZRC 2007.0114), 5.0 × 5.9 mm; b, paratype female (MNHN-B 30395), 5.4 × 6.3 mm; c, d, holotype male (MZB Cru 1650), 4.7 × 5.5 mm. Scales, 1 mm.