Ambiaxius propinquus, Lin, Tomoyuki Komai Feng-Jiau & Chan, Tin-Yam, 2010

Lin, Tomoyuki Komai Feng-Jiau & Chan, Tin-Yam, 2010, Five new species of Axiidae (Crustacea: Decapoda: Axiidea) from deep-water off Taiwan, with description of a new genus, Zootaxa 2352, pp. 1-28 : 8-13

publication ID	https://doi.org/ 10.5281/zenodo.193489
DOI	https://doi.org/10.5281/zenodo.6211753
persistent identifier	https://treatment.plazi.org/id/038C87AC-5E4F-FFA4-43E0-CCD453BACEBD
treatment provided by	Plazi
scientific name	Ambiaxius propinquus
status	sp. nov.

Treatment

Ambiaxius propinquus n. sp.

( Figs. 4 View FIGURE 4 , 5, 13C)

Type material. Holotype: hermaphrodite (cl 11.8 mm), TAIWAN 2001, stn CD 129, 6 May 2001, 22o56.13’N, 122 o3.69’E, 1271–1275 m, ( NTOU A00044 View Materials ).

Description of holotype. Rostrum ( Fig. 4 View FIGURE 4 A–C) 0.35 as long as carapace, slender, upturned, extending to midlength of fourth segment of antennal peduncle, armed with 1 (left) or 2 (right) tiny lateral spines, continuous with base of supraocular spine; dorsal surface channeled medially. Carapace ( Fig. 4 View FIGURE 4 A–C) smooth except for slight rugosity on branchiostegite; supraocular spines prominent; gastric region convex, sloping to rostral base; lateral gastric carina very short, limited to base of supraocular spine; submedian gastric carina blunt, slightly rugose anteriorly, gradually becoming obsolete posteriorly; median gastric carina also blunt, unarmed, becoming obsolete gradually, not reaching cervical groove; cervical groove very shallow, but extending to pterygostomial region; very shallow branchial groove also present; no postcervical groove on dorsal midline; pterygostomial margin rounded.

Thoracic sternum ( Fig. 4 View FIGURE 4 D) with shield on seventh somite weakly delimited, medially divided by deep groove.

First abdominal pleuron ( Fig. 4 View FIGURE 4 E) short, produced ventrally as projection with narrowly rounded ventral margin; second pleuron broad, anteroventrally rounded, lateral surface shallowly depressed; third to fifth pleura rounded; sixth pleura also rounded. Telson ( Fig. 4 View FIGURE 4 F) 1.8 times longer than wide, widest proximally, then approximately parallel-sided, lateral margin unarmed, posterior margin strongly convex without posteromedian spine, posterolateral region unarmed; dorsal face without spines on obsolete oblique ridges.

Eyestalks ( Fig. 4 View FIGURE 4 A, C) very short, immovably attached to cephalothorax, contiguous, hardly visible in dorsal view; cornea depressed dorsoventrally, unpigmented, unfaceted, division between cornea and eyestalk unclear. Antennular peduncle ( Fig. 4 View FIGURE 4 A, C) slightly falling short of midlength of fourth segment of antennal peduncle; first segment with small spine on statocyst lobe distolaterally; flagella missing. Antennal peduncle ( Fig. 4 View FIGURE 4 A, C) with first segment unarmed; second segment with very small dorsolateral distal spine; scaphocerite small, directed slightly upwards, less than half length of second segment; third segment with sharp spine on distomesial angle; fourth segment about 1.8 times longer than second segment; fifth segment about 0.3 length of fourth segment; flagellum missing.

Third maxilliped (Fig. 5A) moderately slender; coxa with 1 ventromesial spinule; basis also with very small ventromesial spinule; ischium unarmed on ventral margin; crista dentata with about 30 small, corneoustipped teeth; merus with 1 small subdistal spine on ventral margin; carpus unarmed; exopod with multiarticulate flagellum.

Only left cheliped preserved. Cheliped (Fig. 5B, C) elongate. Coxa bearing minute denticle on ventrodistal margin. Basis unarmed. Ischium with 2 minute denticles on ventral margin medially. Merus 4.9 times longer than high, with 1 subdistal spine (but fairly close to distal margin) on dorsal margin, unarmed on ventral margin. Carpus about 2.3 times longer than high, unarmed. Chela subequal in length to carapace and about 4.0 times longer than high (dorsal spine excluded), ventral margin faintly sinuous. Palm slightly becoming higher distally, 2.1 times longer than high, with 1 subdistal spine; lateral surface slightly convex, smooth, with few tufts of stiff setae, no carina along ventral margin; mesial face also smooth, with row of long stiff setae along ventral margin extending onto fixed finger. Fixed finger (Fig. 5D) smooth on lateral surface, with row of tufts of setae medially; mesial face faintly elevated along midline; cutting edge with row of very small triangular teeth interspersing 1 to 4 minute denticles in proximal 0.7 and row of tiny denticles in distal 0.3. Dactylus subequal in length to palm, with rows of tufts of long stiff setae on lateral and mesial faces; mesial face slightly elevated along midline; cutting edge thin, sharply edge, with row of minute denticles, tip crossing with that of fixed finger when closed; small hiatus formed proximally between fingers when dactylus closed.

Second pereopod (Fig. 5E) slender, unarmed on ischium to carpus; carpus about 0.6 length of chela; chela wider than carpus, with tufts of long setae on margins, fingers slightly longer than palm, each with row of minute corneous spinules on cutting edge. Third pereopod missing. Fourth pereopod (Fig. 5F) somewhat elongate, unarmed on ischium to carpus; propodus distally with cluster consisting of stiff setae and few stout setulose setae located distally (Fig. 5G, H), possibly representing grooming apparatus, but otherwise nearly smooth on lateral surface; dactylus (Fig. 5G, H) slender, about 0.3 times as long as propodus, slightly twisted, bearing numerous tufts of stiff setae along extensor and flexor margins. Fifth pereopod (Fig. 5I) not subchelate, unarmed on ischium to carpus; propodus distally with grooming apparatus consisting of cluster of short to long setae, extending onto lateral and mesial faces, and transverse row of short, setulose, stout setae on ventrodistal margin (Fig. 5J, K); dactylus (Fig. 5J, K) very slender, elongate, about 0.4 times as long as propodus, flexor surface excavated near base, proximomesial margin slightly expanded, dorsal surface with numerous stiff setae.

Gonopores present on coxae of third and fifth pereopods ( Fig. 4 View FIGURE 4 D).

Pleurobranchs absent. Two arthrobranchs above bases of third maxilliped to fourth pereopod. Podobranchs on third maxilliped to third pereopod slender, simple, without trace of gill elements.

First pleopod (Fig. 5L) with first segment (protopod) strongly flattened, slightly twisted; second segment (ramus) about 0.3 length of first segment, rounded triangular, leaf-like, small proximomesial protrusion representing appendix interna, terminally divided into two lobes by deep V-shaped incision. Second pleopod (Fig. 5M) with inner ramus consisting of elongate, somewhat twisted first segment of endopod while distal segment of endopod fused with appendix masculina (= distal fused segment); thumb-like appendix interna located at proximomesial margin of distal fused segment; distal fused segment tapering distally, boot-like in shape, distinctly shorter than first segment of endopod, with double row of spiniform setae on strongly sinuous mesial margin and with several terminal setae. Third to fifth pleopods very slender, without appendices internae.

Uropodal endopod ( Fig. 4 View FIGURE 4 G) 2.4 times as long as wide, without lateral spines, dorsal ridge unarmed. Uropodal exopod ( Fig. 4 View FIGURE 4 G) without lateral spines, posterolateral angle with 1 spine, but no spiniform setae apparent; transverse suture unarmed.

Coloration. Body generally ivory-whitish, with anterior part of carapace including eyes somewhat dark greenish ( Fig. 13 View FIGURE 13. A C).

Distribution. Known only from southeastern Taiwan at depths of 1271–1275 m.

Remarks. The present specimen from Taiwan is very similar to the respective holotypes of A. alcocki and A. franklinae . However, it differs from the descriptions of the holotype of Ambiaxius alcocki from the Bay of Bengal in two points ( McArdle 1900; Alcock 1901). In the Taiwanese specimen, the postcervical carapace is rounded in the dorsal surface, whereas it was mentioned that the carapace was bluntly carinate along the midline in the holotype, clearly suggesting the presence of a postcervical carina on the carapace. In fact, Sakai (1995) identified a Japanese specimen, which has a clearly delimited postcervical carina on the carapace, as A. alcocki , although he later referred it to a new species, A. surugaensis (cf. Sakai & Ohta 2005). The second point is the absence of an enlarged tooth on the cutting edge of the dactylus of the cheliped in the present specimen, which was said to be present in the holotype. The significance of the second point is rather questionable, because only the left cheliped is preserved in the present specimen. However, the development of the postcervical carina on the carapace is considered to be species specific in Axiidae and Calocarididae , and its absence alone seems to justify the separation of the Taiwanese specimen from A. alcocki . Direct comparison with the holotype likely will reveal further morphological differences. Moreover, the identities of A. alcocki reported from discrete localities such as South Africa and New Caledonia (cf. Stebbing 1915, 1917; Barnard 1950; Sakai & de Saint Laurent 1989; Sakai & Ohta 1995) will need to be verified (e.g., Kensley 1996a). The lack of the postcervical carina on the carapace aligns the Taiwanese specimen with A. franklinae . However, it differs from the latter by the shorter and less curved rostrum. In the Taiwanese specimen, the rostrum does not reach the distal end of the fourth segment of the antennal peduncle, whereas in the holotype of A. franklinae it is longer, nearly reaching the distal end of the fifth segment of the antennal peduncle. Furthermore, the spiniform setae on the concave anterior margin of the boot-shaped appendix masculina of the second pleopod seem to be more numerous in the Taiwanese specimen than in A. franklinae . Therefore, it is concluded that the Taiwanese form is a separate species.

Another species of the genus, Ambiaxius surugaensis , can be immediately distinguished from the present new species and the other two species by the arthrobranchs bearing rudimentary gill elements. In the other three species, the arthrobranchs are distinctly lamellate. The presence of the postcervical carina also distinguishes A. surugaensis from A. propinquus n. sp. and A. franklinae . The much broader distal segment of the first pleopod is also characteristic to A. surugaensis , although the detailed structure of the appendage remains unknown for the holotype of A. alcocki .

FIGURE 5. Ambiaxius propinquus n. sp., holotype hermaphrodite (cl 11.8 mm), NTOU A0 0 0 44. A, right third maxilliped, lateral view; B, chela and carpus of left cheliped, lateral view; C, merus of left cheliped, lateral view; D, fixed finger of left chela, lateral view; E, left second pereopod, lateral view; F, left fourth pereopod; G, same, distal part of propodus and dactylus, lateral view; H, same, mesial view; I, left fifth pereopod, lateral view; J, same, distal part of propodus and dactylus, lateral view; K, same, mesial view. Scale bars: A–C, E, F, I = 2 mm; D, G, H, J, K–M = 1 mm.

Etymology. From the Latin, propinquus , meaning similar or related, in reference to the close similarity of the new species to A. alcocki and A. franklinae .