Neorhynchoplax yaeyamaensis, Naruse, Tohru, Shokita, Shigemitsu & Kawahara, Takeshi, 2005

Naruse, Tohru, Shokita, Shigemitsu & Kawahara, Takeshi, 2005, Neorhynchoplax yaeyamaensis, a new false spider crab (Decapoda: Brachyura: Hymenosomatidae) from the Yaeyama Group, the Ryukyu Islands, Japan, Zootaxa 877, pp. 1-7 : 2-6

publication ID	https://doi.org/ 10.5281/zenodo.170894
DOI	https://doi.org/10.5281/zenodo.6267100
persistent identifier	https://treatment.plazi.org/id/03A787E6-FFE8-FFA5-FED8-FC434B47FAD5
treatment provided by	Plazi
scientific name	Neorhynchoplax yaeyamaensis
status	sp. nov.

Treatment

Neorhynchoplax yaeyamaensis View in CoL , new species

( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 )

Neorhynchoplax okinawaensis View in CoL — Ng & Chuang, 1996: 77 (part).

Material examined. Holotype. 1 male, CW 3.5 mm, RUMFZC24, Tsuru River, Ishigaki Island, coll. Tohru Naruse & Takashi Nagai, 21 Dec. 2004.

Paratypes. 1 male, CW 2.3 mm, RUMFZC132, Shiira River, Iriomote Island, coll. Natsuki Okuda, 24 July 2003; 1 male, CW 1.8 mm, 3 females, CW 1.9–2.9 mm, RUMF ZC133, data same as RUMFZC132; 1 female, CW 3.6 mm, RUMFZC134, Shiira River, Iriomote Island, coll. Natsuki Okuda, 22 July 2003; 2 females, CW 2.7, 3.1 mm, NSMTCr. 15954, NSMTCr. 15955, data same as RUMFZC132; 1 male, CW 2.1 mm, NSMTCr. 15956, mangrove tide pool, Fukari River, Iriomote Island, coll. Takeshi Kawahara, 23 Sept. 2002; 1 female, CW 4.4 mm, RUMFZC23, data same as holotype; 1 male, CW 3.4 mm, RUMFZC25, Tsuru River, Ishigaki Island, coll. Tohru Naruse & Takashi Nagai, 21 Dec. 2004; 2 females, CW 2.5, 2.8 mm, RUMFZC26, Shira River, Iriomote Island, coll. Tohru Naruse & Takashi Nagai, 22 Dec. 2004; 2 males, CW 2.0, 2.2 mm, 2 females, CW 2.8, 2.9 mm, RUMFZC27, Urauchi River, Iriomote Island, coll. Tohru Naruse & Takashi Nagai, 23 Dec. 2004; 5 males, CW 1.9–2.2, 1 female, CW 2.4 mm, 2 juveniles, CW 1.4, 1.5 mm, ZRC 1994.4246, Shiira River, Iriomote Island, coll. K. Nakamura.

Comparative material. Neorhynchoplax okinawaensis ( Nakasone & Takeda, 1994) : 1 female, CW 3.2 mm, RUMFZC135, mouth of Nagura River, Ishigaki Island, coll. Yoshihisa Fujita, Akane Ito & Takeshi Kawahara, 14 May 2003; 2 females, CW 3.4, 4.2 mm, RUMFZC136, Awase intertidal flat, Okinawa Island, coll. Tohru Naruse, 2 Jul.

2003; 2 males, CW 2.2, 2.8 mm, 1 female, CW 3.0 mm, RUMFZC28, Oura River, Okinawa Island, coll. Tohru Naruse, 8 Dec. 2004; 1 female, CW 4.2 mm, RUMFZC29, Nakama River, Iriomote Island, coll. Masayuki Osawa, 17 Jun. 1995; 1 male, 1 juvenile, ZRC 1994.4248, Goga, Okinawa Island, coll. T. Kosuge, 16 Aug. 1993.

General Description. Carapace ( Figs. 1 View FIGURE 1 a–c, 2a) oval; dorsal surface flat, gastrocardiac grooves clearly delineated, bordered by continuous subcircular rim, anterolateral bor der slightly concave, no anterior or posterior lateral teeth, branchiostegite somewhat swollen laterally. Rostrum trilobed, basally fused, median lobe longest, placed slightly below lateral lobes in frontal view. Epistome relatively wide longitudinally; lower border widely projecting medially.

Eye ( Fig. 2 View FIGURE 2 a) with well developed cornea, eyestalk short.

Third maxilliped ( Fig. 2 View FIGURE 2 d) with relatively narrow ischium and merus, leaving wide medial gape when closed, inner margin of merus lined with strong setae; palp long, reaching proximal margin of merus when folded backwards; exopod with developed flagellum. Chelipeds stouter than ambulatory legs; merus and carpus without teeth; chelae symmetrical; manus ( Fig. 2 View FIGURE 2 b) thick, high, cutting edges of fingers with alternating strong triangular teeth.

Ambulatory legs long, slender, elliptical in crosssection, anterior 3 pairs subequal in length, with short to long sparse setae. Dactylus ( Fig. 2 View FIGURE 2 c) of all ambulatory legs weakly incurved, apex sharply pointed, incurved, first leg with 1 subterminal large spine, sometimes followed by 0–7 (median, 0; n=15) smaller spine(s), second to fourth leg with 1 subterminal large spine and 0–7 (median, 4; n=48) smaller spine(s).

Abdomen ( Fig. 2 View FIGURE 2 e) with first segment, second segment and telson fully, broadly convex, abruptly narrowed over distal 2/3.

G1 ( Fig. 2 View FIGURE 2 f), in situ, curved towards midline of thoracic sternum over proximal 2/3, distal 1/3 directed anteriorly; distal margin blunt, with row of long setae extending to dorsal surface.

Brooding habit. Females of several species of Elamenopsis A. Milne Edwards, 1873 , Neorhynchoplax Sakai, 1938 , and Crustaenia Ng & Chuang, 1996 , have been observed to brood their eggs inside a cephalothorax brood cavity (e.g. Lucas, 1980; Lucas & Davie 1982, Ali et al. 1995, Ng & Chuang 1996, Davie & Richer de Forges 1996). Neorhynchoplax yaeyamaensis also possesses such an internal brood cavity. The single ovigerous female (RUMFZC134, CW 3.6 mm) has a swollen dorsal surface, branchiostegites, and abdomen, and eggs are visible through both dorsal and abdominal surfaces ( Fig. 1 View FIGURE 1 c, d). When the abdomen of the preserved ovigerous female was opened, more than 10 zoeae were released from the pseudovulvae. The remaining eggs at the back of the brood cavity had not yet hatched, and this might suggest ovovivipary, as Ng & Chuang (1996) had reported for N. mangalis . But it is also possible that the eggshells of the “hatched” zoeae had in fact been dissolved by formalin. In any case, rearing experiment are necessary to confirm whether N. yaeyamaensis is indeed ovoviviparous in the sense of Ng & Chuang (1996). Mean of egg sizes is 0.53 mm (S. D. 0.015) by 0.47 mm (S. D. 0.015), and there was a total of 166 eggs and zoeae.

Habitat. Neorhynchoplax yaeyamaensis is found on pebbly bottoms in upper estuarine basins (Dr. Natsuki Okuda, pers. comm.). One individual (NSMTCr. 15956, male, CW 2.4 mm) was caught from a tide pool in a Kandelia obovata forest on the Fukari River.

Distribution. Neorhynchoplax yaeyamaensis is currently known only from Ishigaki Island and Iriomote Island, the Yaeyama Group, the Ryukyu Islands, Japan.

Etymology. The new species is named after its distributional area, the Yaeyama Group.

Remarks. Lucas (1980) regarded Neorhynchoplax Sakai, 1938 , as a junior subjective synonym of Elamenopsis A. Milne Edwards, 1873 . Subsequently Ng and Chuang (1996) resurrected Neorhynchoplax , and among other characters, commenting that it can be easily distinguished from Elamenopsis by the shape of the dorsal surface of the carapace. We follow there action here.

Neorhynchoplax yaeyamaensis is easily assigned to Neorhynchoplax because of the subcircular shape of the carapace, the presence of distinct grooves on the dorsal surface, presence of a trilobed rostrum, the narrow third maxillipeds, and the slender ambulatory legs.

From 25 known congeners ( Ng & Chuang 1996, Rahayu & Ng 2004), N. yaeyamaensis can easily be recognised by its more circular carapace, the absence of anterior and posterior lateral teeth, and the trilobed rostrum. Neorhynchoplax yaeyamaensis is perhaps closest to N. exigua in the similar shape of the rostrum, and also to N. aspinifera with regards to the similar male abdomen. Neorhynchoplax yaeyamaensis can be distinguished from N. exigua by the dentition of the inner margin of the ambulatory dactyli (one large subterminal spine and 0–7 following smaller spines vs. 3–9 exceptionally and equally large spines) ( Kemp 1917: 261, fig. 10). Neorhynchoplax yaeyamaensis can be differentiated from N. aspinifera by the presence of the spines on the inner margin of the ambulatory legs (absent in N. aspinifera ), and the shape of the distal end of the G1 (blunt vs. narrow with uniform thickness) ( Lucas 1980: 195, figs. 3I, 6R, 10G). Neorhynchoplax yaeyamaensis may be allied to N. octagonalis , but the former can be easily distinguished from the latter by the shape of the trilobed rostrum (basally fused vs. isolated in dorsal view), the absence of posterior lateral tooth (present in N. octagonalis ), the shape of the dorsal surface of carapace (subcircular vs. octagonal), and the dentition of the inner margin of the ambulatory dactyli (one large and 0–7 smaller spines vs. no spine on first leg and one spine on second to fourth legs) ( Kemp 1917: 256, fig. 5).

Neorhynchoplax okinawaensis was originally described from Okinawa Island. Recently N. okinawaensis has been found living on the same island with N. yaeyamaensis . Neorhynchoplax yaeyamaensis is easily distinguished from N. okinawaensis by: the absence of the posterior lateral tooth (present in N. okinawaensis ); the shape of the dorsal surface of the carapace (subcircular vs. subcircular with carapace widening posteriorly); the shape of the male abdomen (fused segments abruptly narrowed over distal 2/3 vs. gradually narrowed over distal 1/2), the dentition of the ambulatory inner margin of dactyli (one subterminal large spine followed by 0–7 smaller spines vs. only a single subterminal spine), and the shape of the G1 (blunt vs. tapered). Ng & Chuang (1996) recorded several specimens (ZRC 1994.4246, ZRC 1994.4248) from the Ryukyus as N. okinawaensis . We have rechecked this material and there are two species. All the specimens from ZRC 1994.4246 from Iriomote (5 males, 1 female, 2 juveniles, CW 1.4–2.4 mm) lack a posterior lateral tooth, which is one of the most important characters of N. okinawaensis . We have on hand a small male of N. okinawaensis (RUMFZC28, CW 2.2 mm) which already possesses distinct posterior lateral teeth, and it is thus clear that this character is not associated with size. With regards to the relative carapace shape, ambulatory leg dentition and G1 structure, they also differ; and as such these specimens should now be referred to N. yaeyamaensis . The one specimen from ZRC 1994.4248 from Okinawa has a lateral tooth and is regarded here as true N. okinawaensis .