Opusia new genus

Type species. Tylodiplax indica Alcock, 1900, by present designation.

Diagnosis. Carapace transversely subovate, broader than long, weakly convex, regions well demarcated by grooves; anterolateral margin not separated from posterolateral margin by clearly demarcated angle; pterygostomial region swollen, produced beyond anterolateral margin, visible dorsally; dorsolateral margin not demarcated; posterolateral margin swollen, rounded; posterior margin weakly sinuous, thickened, with submarginal ridge (Figs. 1 A, 2A). Front straight, entire from dorsal view, slightly thickened, lateral angles rounded, junction between base, supraorbital margin rounded; frontomedial projection broadly triangular, ending in truncate tip (Figs. 1 A, 2A). Eyestalks long, slender, with long, plumose setae (Fig. 2 A). Orbits sparsely setose, about as wide as front, external orbital margin distinct, triangular; supraorbital margin sinuous; inner two-thirds of infraorbital margin moderately cristate, inner infraorbital tooth distinctly demarcated from rest of infraorbital margin by broadly triangular notch (Fig. 2 A); suborbital ridge moderately developed, not visible dorsally; transverse pterygostomial sulcus present, open. Epistome with anterior half separated from posterior half by ridge, anterior half sunken slightly; posteromedial tooth large, long, acute, posterior margin on either side lobate under basal segment of antenna (Fig. 2 D). Ischium of third maxillipeds longer than merus, with inner distal angle slightly produced; outer distal angle of merus produced, lobate, inner angle excavated for insertion of palp (Fig. 2 B). Outer margins of chelipeds with long setae, fingers narrow, about twice palm length, setose, tips blade-like, sharp (Fig. 2 F); chelipeds in male similar to female, but slightly stouter. Ambulatory legs robust, setose, P3 broadest, P5 with dactylus directed upwards (Fig. 1 A); segments relatively less setose in females. Anterior part of male thoracic sternum rounded (Fig. 2 E). Male abdomen with all sutures distinct, complete, somite 1 slightly wider than somite 2, not reaching P5 coxae; somite 5 slightly constricted close to proximal end; telson about as long as broad (Fig. 2 C). Female abdomen with all somites free, telson broadly triangular. G1 gently bent, apex conical with numerous strong subapical spines (Fig. 2 G, H).

Etymology. The genus name is arbitrarily derived from the Latin opimus meaning "well-fed, fat", alluding to the rounded and swollen appearance of the type species. The gender is feminine.

Remarks. The monotypic Opusia new genus is only superficially similar to Tylodiplax De Man, 1895 (type species Tylodiplax tetratylophora De Man, 1895). Members of these two genera do not show strong sexual dimorphism of the chelipeds, have a straight front, possess long, plumose setae on their eyestalks, the posterior margin of the epistome is lobed on either side, the dactylus of the P5 is upturned and the carapace lateral margins diverge posteriorly. Opusia new genus can easily be distinguished from Tylodiplax by several key characters. Most obvious is the differently shaped carapace. The carapace in Opusia new genus is broadly ovate with no prominent angles on the anterolateral margins with the exception of the external orbital tooth, and its dorsal surface has no distinct ridges or tubercles (Figs. 1 A, 2A). The anterolateral margin of the carapace of Tylodiplax, however, bears a distinct angle with the posterolateral margins straighter (Fig. 3A; Manning & Holthuis 1981: fig. 58a, c), and the anterolateral margin is not as swollen as in Opusia new genus .

In addition, the dorsal carapace surface of Tylodiplax bears several prominent ridges and protuberances (Fig. 3A; Manning & Holthuis 1981: 217, fig. 58a, c). The structures of the inner infraorbital tooth and suborbital ridge are also markedly different. In Opusia new genus, the inner infraorbital tooth is distinctly demarcated from the rest of the infraorbital margin by a triangular notch and the suborbital ridge is not distinctly visible dorsally (Fig. 2 D). In Tylodiplax, however, the inner infraorbital tooth is weak and its margin is contiguous with the rest of the infraorbital margin with the suborbital ridge prominent, produced and clearly visible dorsally (Fig. 3A, B). The anterior part of the male thoracic sternum (first two sternites) in Opusia new genus is rounded whereas in Tylodiplax, it forms a distinct triangle (Fig. 3 E). In Opusia new genus, all the sutures between the male abdominal somites are complete and visible, although somites 3–5 are functionally fused, with somite 5 constricted at the proximal end (Fig. 2 C), while in Tylodiplax somites 2–4 are fused with the sutures shallow and/or incomplete with somite 5 not constricted, with only a slight constriction close to the proximal end of somite 6 (Fig. 3C). The structure of the G 1 in Opusia new genus ends in a conical apex (Fig. 2 G, H), while in Tylodiplax the distal part of the G1 broadens before reaching the apex, forming a bulbous structure (Fig. 3D).

Opusia new genus cannot be included in Paratylodiplax Serène, 1974 (type species Cleistostoma blephariskios Stebbing, 1924) as there are also key differences in terms of the structure of the orbital structure (supraorbital margin sinuous in Opusia new genus, Fig. 2 A, against almost straight in Paratylodiplax, Fig. 3 E), epistome (posteromedial tooth triangular with lateral margins gently sinuous in Opusia new genus, Fig. 2 D, against posteromedial tooth acutely triangular with lateral margins strongly sinuous in Paratylodiplax, Fig. 3 F), male abdominal segmentation (sutures between all somites visible in Opusia new genus, Fig. 2 C, against somites 3–5 completely fused without trace of sutures in Paratylodiplax, Fig. 3 G), and G1 (bends gradually with several large subterminal spines covered with lateral spinules in Opusia new genus, Fig. 2 G, H, against bending sharply with subdistal part only with one large subterminal spine in Paratylodiplax, Fig. 3 H), as well as absence of sexual dimorphism in the chelipeds (Fig. 3 E–H; Manning & Holthuis 1981: 209, fig. 54a–e).

Manning & Holthuis (1981) were unable to resolve the generic position of Tylodiplax indica, so they provisionally placed the species in Serenella . Examination of the present material, however, indicates that T. indica does not belong to Serenella because of important differences in several characters. The inner infraorbital tooth of the frontal region of T. indica, is clearly demarcated from the rest of the infraorbital margin, and the frontomedial margin is triangular, ending in a truncate apex (Fig. 2 A, D); but in Serenella the inner infraorbital tooth is not sharply demarcated, and the frontomedial margin is bilobed (Manning & Holthuis 1981: fig. 55a). The front of T. indica is straight and entire from dorsal view and the anterolateral angles are rounded (Fig. 2 A), whereas for Serenella the front is distinctly bilobed from dorsal view and the anterolateral angles are pointed (Manning & Holthuis 1981: fig. 55a). There is no obvious sexual dimorphism of the chelae in T. indica (Fig. 2 F), but this is clearly present in Serenella (Manning & Holthuis 1981: fig. 55a). The supraorbital margins of T. indica are slightly sinuous but generally parallel to the posterior margin of the carapace (Fig. 2 A, D), but in Serenella, the supraorbital margins are oblique relative to the posterior margin (Manning & Holthuis 1981: fig. 55a). The junction between the recurved and straighter portions of the G 1 in T. indica, is rounded and gradual (Fig. 2 G), while in Serenella it is abrupt (Manning & Holthuis 1981: fig. 55b).

Comparative material. Tylodiplax tetratylophora De Man, 1895 – THAILAND: 1 male (6.9 x 11.0 mm) (PMBC 2227), Phang Nga mangrove forest, leg. S. Lundoer, 3 January 1972; 4 males (5.9–7.7 mm x 8.4–11.7 mm) (PMBC 4384), mangrove shore, Ao Nam Bor, Phuket; 7 males (5.0– 7.2 mm x 8.2–10.2 mm), 3 females (6.4–7.2 mm x 11.0– 11.7 mm) (PMBC 4390), mangrove shore, Ao Nam Bor, Phuket; 2 females (5.1 x 7.5 mm, 6.6 x 10.3 mm), 1 juvenile (3.7 x 5.8 mm) (PMBC 4394), mudflat, Ko Yao Yai, Phang Nga, Phuket; PENINSULAR MALAYSIA: 10 males, 20 females (ZRC), mudflat, Matang mangroves, Perak, coll. A. Ramli, F. Mohamad, C. G. S. Tan, 10 June 1995. Paratylodiplax blephariskios (Stebbing, 1924) – SOUTH AFRICA: 1 male (5.4 x 8.8 mm), 1 female (4.7 x 7.0 mm) (USNM 137247), Cape Town, coll. J. Day, September 1957.