Samarplax, Husana, Daniel Edison M., Tan, Swee Hee & Kase, Tomoki, 2011

Husana, Daniel Edison M., Tan, Swee Hee & Kase, Tomoki, 2011, A new genus and species of anchialine Hymenosomatidae (Crustacea, Decapoda, Brachyura) from Samar, Philippines, Zootaxa 3109, pp. 49-59 : 51-52

Samarplax new genus

Type species. Samarplax principe new species, by present designation.

Diagnosis. Carapace sub-polygonal, as long as wide to slightly longer than wide; dorsal surface uneven due to regions, surrounded by continuous rim, regions slightly inflated, separated by deep grooves. Rostrum absent; frontal margin gently bilobed. Posterolateral margin entire. Antennular fossa very shallow. Epistome almost flat, not visible from dorsal view. Eyes strongly reduced, visible as vestigial structure only under scanning electron microscopy (SEM) ( Fig. 7 ); cornea absent. Male sterno-abdominal cavity relatively narrow; abdominal locking button indistinct. Male abdomen-pleotelson with somites 3–5 fused; somites 1, 2 with rounded lobes on disto-lateral angle on both sides; fused somites narrowed distally; pleotelson distinctly narrower than half width of abdomen; tip of pleotelson reaching to mid-point of abdominal somite 4. Female abdomen dome-shaped, with undivided plate formed by fusion of somite 2 to pleotelson, somite 2 and pleotelson marked by shallow suture on outer side. G1 slender, slightly sinuous, twisted at base, tapering at tip, curving inwards, distal portion with long stiff setae. G2 short, about half length of G1.

Etymology. The genus name Samarplax is derived from the locality, Samar province and plax for “plate”, the suffix of the cavernicolous genera, Sulaplax and Guaplax , alluding to their superficial resemblance. Gender feminine.

Remarks. Samarplax new genus, differs from other epigean genera of Hymenosomatidae in having several troglobitic features such as the extreme reduction of eyes, the absence of pigment and the disproportionately long legs. Otherwise, it resembles the epigean genus Neorhynchoplax Sakai, 1938 , in having well defined dorsal surface regions and the eggs being held in the brood cavity (see Ng & Chuang 1996; Naruse et al. 2005). However, in addition to the above-mentioned troglobitic features, Samarplax new genus, also differs from Neorhynchoplax in having no rostrum, the lateral walls of the carapace are not laterally expanded and the ambulatory dactyli are unarmed (see Naruse et al. 2008: Table 1). Externally, the new genus most closely resembles Sulaplax Naruse, Ng & Guinot, 2008 , and Guaplax Naruse, Ng & Guinot, 2008 , from Indonesia in possessing unpigmented eyes that are fused to the orbit, absence of a subterminal tooth on the ambulatory dactylus; having the dorsal carapace regions well defined, eggs contained in a brood cavity, a four-segmented pleotelson, and a G1 which is straight, tapering and having simple tip. The form of the third maxilliped is closer to that of Guaplax but overall, Samarplax new genus, appears to be more similar to Sulaplax , sharing characters such as the basal antennal article not been positioned directly below the base of the eye stalk but offset to the outer edge (versus directly below in Guaplax ) and in having very long chelipeds, with the merus especially elongate.

Samarplax new genus, nevertheless differs from Sulaplax in the following aspects: the inner distal edge of the ischium of the third maxilliped is slightly projected outwards and not as long as that observed on Sulaplax ( Fig. 4 e, versus Naruse et al. 2008: fig. 4c); the epistome is almost flat ( Fig. 7 ) (versus relatively long with produced posterior margin in Sulaplax, Naruse et al. 2008 : fig. 4b); and the fused abdominal piece consisting of somites 3-pleotelson is prominently triangular in shape with the lateral margins of somite 4 converging towards the telson ( Fig. 5 a) (versus with fused piece not subtriangular, with the lateral margins of somite 4 subparallel in Sulaplax, Naruse et al. 2008 : fig. 5d). In addition, Samarplax new genus, has a sub-polygonal carapace the length and width subequal (versus wider than long, Naruse et al. 2008: figs. 3, 4a), a simple straight, tapering distal part of the G1 that is gently curved towards the midline (versus more prominently twisted, Naruse et al. 2008: fig. 5e, f), and a relatively long chela armed with brush-like setae along the inner margin of its fingers instead of cutting teeth ( Fig. 7 ) (versus lined with low teeth and without brush-like setae, Naruse et al. 2008: fig. 5b, c). This chela structure is not known in other hymenosomatids so far. The G2 of Samarplax new genus, also differs from that of Sulaplax in that it is only about half the G1 length with a stout distal tip (versus about three-quarters of the G1 length with a slender and tapering tip in Sulaplax , present observation).

Overall, the absence of rostrum and eyes, well-defined dorsal surface of the carapace with two small spines or sharp teeth on its lateral margin, proportionally shorter projected merus of the third maxilliped, an almost flat epistome, long chelipeds without cutting teeth but lined instead with brush-like setae, a four-segmented pleotelson, relatively simple G1 and eggs that are held in the brood cavity are diagnostic characters of this new genus.

Figures

FIGURE 4. Samarplax principe new genus, new species. a, paratype female (ZRC 2011.0879; ex. NSMT-Cr 2007 - 0002),

FIGURE 5. Samarplax principe new genus, new species. a, holotype male (NMCR- 39033), abdomen, ventral view. b-c, paratype female (ZRC 2011.0879; ex. NSMT-Cr 2007 - 0002), abdomen: b, pleotelson; c, abdomen. Scale bars = 1 mm ..

FIGURE 7. Samarplax principe new genus, new species. SEM micrograph of the frontal region showing the vestigial eyestalk. Paratype male, 4.09 x 3.98 mm (NSMT-Cr 2007 - 00040).