Ambiaxius abyssalis, Marin, 2022

Marin, Ivan, 2022, The first abyssal record of Axiidea (Crustacea: Decapoda) with a description of a new species of the genus Ambiaxius Sakai & de Saint Laurent, 1989 from the Banda Sea, Zootaxa 5178 (2), pp. 193-200 : 194-199

publication ID	https://doi.org/ 10.11646/zootaxa.5178.2.5
publication LSID	lsid:zoobank.org:pub:9135EAB2-7F0D-4AAB-A3CC-C489C0F2E724
DOI	https://doi.org/10.5281/zenodo.7065602
persistent identifier	https://treatment.plazi.org/id/038987C1-F650-FF8B-FF61-2129EEA2FA0D
treatment provided by	Plazi
scientific name	Ambiaxius abyssalis
status	sp. nov.

Treatment

Ambiaxius abyssalis sp. nov.

( Figs 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Material examined. Holotype, hermaphrodite (pcl. 8.0 mm, tl. 19.0 mm) ( ZMMU Ma-6220)— Southwestern Pacific , Banda Sea, 54th cruise of s/v “Vityaz”, st. 6778-2, 5°15’S 128°25’E, 3000–3374 m, Sigsbee trawl, 04 Nov. 1973 GoogleMaps . Paratype, hermaphrodite (pcl. 7.8 mm, tl. 18.5 mm) ( ZMMU Ma-6221)—same locality and data as for holotype. GoogleMaps

Etymology. The species is named after the environ (abyss), where the type specimens of the new species were collected.

Description. Carapace ( Figs 1A–C View FIGURE 1 ; 2A–C View FIGURE 2 ) smooth; branchiostegite smooth above posteroventral region; gastric region weakly convex; anterior dorsal carina low but well-marked, without teeth, extending from base of rostrum to about midlength of gastric region; supraocular spine prominent, sharp, obliquely directed, arising from lateral gastric carina extending from rostral lateral margin to about half of gastric region; pterygostomial angle produced anteriorly, bluntly rounded. Cervical groove located about at mid-length of carapace, extending laterally to anterolateral corner. Rostrum ( Figs 1B, C View FIGURE 1 ; 2B, C View FIGURE 2 ) styliform, about 0.4 times length of front-to-cervical groove, 0.2–0.25 times as long as carapace, reaching to distal margin of antennular peduncle, triangular, broad at base and gradually tapering to acute, nearly straight, with slightly upturned apex; dorsal surface concave; lateral margin with 3–4 minute but well-marked teeth; ventral surface unarmed.

Pleon ( Fig. 1A View FIGURE 1 ) smooth. Pleomere I short; pleuron produced ventrally as small, rounded projection ( Fig. 1D View FIGURE 1 ). Pleura of pleomeres II–VI unarmed, with long simple setae (perhaps, unnecessary) ( Fig. 1D View FIGURE 1 ); ventral margin of pleomere VI rounded posteroventrally.

Telson ( Fig. 2D View FIGURE 2 ) about 2 times as long as proximal width, approximately parallel-sided; dorsal face unarmed, with median depression and, pair of oblique ridges; lateral margin unarmed; posterior margin produced, broadly rounded, without posteromedian spine.

Eyes reduced, immovably attached to cephalothorax; division between cornea and eyestalk unclear, cornea unpigmented, unfaceted, ( Figs 1B View FIGURE 1 , 2C View FIGURE 2 ), hardly visible in dorsal view ( Fig. 1C View FIGURE 1 , 2A View FIGURE 2 ).

Antennular peduncle ( Fig. 2E, F View FIGURE 2 ) overreaching midlength of article 4 of antennal peduncle.Article 1 (basal) with small spine on lateral surface of statocyst lobe. Article 2 (penultimate) as long as wide, unarmed.Article 3 (ultimate) slightly longer than wide, slightly longer than penultimate one. Dorsal and ventral flagella multiarticulated, about as long as carapace.

Antennal peduncle ( Fig. 2G, H View FIGURE 2 ) with article 1 (basal) unarmed, about as long as wide. Article 2 about as long as wide, with small blunt projection on ventrodistal margin. Scaphocerite small, spine-like, directed slightly upwards less, about 1/2 of length of article 2. Article 3 about 1.5 times as long as wide, with distodorsal spine and smaller sharp spine on distomesial angle. Article 4 ( Figs 1B View FIGURE 1 ; 2C View FIGURE 2 ) cylindrical, 3.0–3.5 times as long as wide, about 1.2 times longer than article 3, unarmed.Article 5 about as long as wide, 1/3 of length of article 4, unarmed. Scaphcerite small, sharp, directed slightly upwards. Antennal flagellum multiarticulated, about as long as body.

Maxilliped III ( Fig. 2I View FIGURE 2 ) overreaching distal margin of antennal peduncle. Coxa about 1.3 times as long as wide, with podobranch and slender epipod. Basis about as long as wide, with blunt projection distomedially. Ischium unarmed, about 3.5 times as long as wide; dorsal margin nearly straight, ventral margin slightly sinuous; crista dentate feebly developed, composed of small blunt teeth. Merus about 2.5 times as long as wide, with 1 small subdistal spine on ventral margin. Carpus moderately slender, about 2 times as long as wide, unarmed. Propodus shorter than carpus, unarmed, with straight margins. Dactyls shorter than propodus, tapering distally. Carpus, propodus and dactylus fringed with dense long setae along ventral margins. Exopod with multiarticulate flagellum, slightly overreaching distal margin of merus of endopod.

Pereopods I (chelipeds) ( Figs 1F, E View FIGURE 1 ; 3F View FIGURE 3 ) slightly dissimilar in shape and slightly unequal in length, with smooth segments. Coxa about 1.3 times as long as wide, bearing small blunt projection distoventrally, with podobranch and epipod. Basis about as long as wide, unarmed. Ischium about as long as wide, unarmed. Merus 4.0–4.5 times as long as wide, with parallel margins and small subdistal spine on dorsal margin; ventral margin unarmed. Carpus short, cup-shaped, about 0.25 times as long as merus, unarmed. Chela smooth, slightly compressed laterally, about 2 times as long as wide, about 1.5 times as long as merus; dorsal margin terminating in small subdistal spine; ventral surface unarmed, fringed with moderately long setae; fingers equal in length, simple, similar in shape, slightly longer than palm, gradually tapering to acute apex, 5.0–6.0 times as long as proximal width, straight cutting edge straight, unarmed, ventral margin of pollex and dorsal margin of dactylus densely fringed with moderately long setae.

Pereopod II ( Fig. 3B View FIGURE 3 ) chelate. Coxa unarmed, about as long as wide. Basis quadrate, as long as wide. Ischium unarmed, about 1.3 times as long as wide, with concave ventral margin. Merus about 6.5 times as long as wide, slightly compressed laterally; dorsal and ventral margins straight, parallel, unarmed. Carpus widening distally, about 2.5 times as long as distal width, about 0.3 times of merus, unarmed. Chela about as long as carpus, compressed, unarmed; dorsal and ventral margins straight; ventral margin fringed with simple setae; fingers (polex and dactylus) similar, about 1.2 times as long as palm, gradually tapering with sharp tips, cutting margins straight, ventral margin of pollex and dorsal margin of dactylus densely fringed with setae.

Pereopod III ( Fig. 3C View FIGURE 3 ) slender, consisting of simple, unarmed segments; coxa about as long as wide, with podobranch and epipod. Basis short, about as long as wide. Ischium about 1.8 times as long as wide, subcylindrical; margins straight. Merus about 9.5 times as long as wide, straight. Carpus about 3.5 times as long as wide, about 0.2 times as long as merus. Propodus about 8.5 times as long as wide, about 3 times longer than carpus and 0.8 times as long as merus, straight, fringed in distal part with long simple setae. Dactylus simple, faintly sinuous, about 0.3 times as long as propodus, curved, tapering distally, densely fringed with long simple setae.

Pereopod IV ( Fig. 3D View FIGURE 3 ) generally similar to pereopod III, unarmed. Coxa unarmed, about as long as wide. Basis about as long as wide. Ischium about 2.5 times as long as wide. Merus about 10 times as long as wide. Carpus about 3.5 times as long as wide, about 0.2 times as long as merus, slightly widening distally, with straight margins fringed with short simple setae. Propodus about 8 times as long as wide, about 3 times longer than carpus and 0.7 times as long as merus, fringed in distal part with simple setae. Dactylus slightly sinuous, tapering distally to subacute tip, about 0.3 times as long as propodus, curved, densely fringed with simple setae.

Pereopod V ( Fig. 3E View FIGURE 3 ) as slender as pereopods III and IV, similarly unarmed. Coxa unarmed, about as long as wide. Basis about as long as wide. Ischium about 1.5 times as long as wide. Merus straight, about 8 times as long as wide. Carpus about 2.5 times as long as wide, about 0.25 times as long as merus, and about 0.2 times as long as propodus, widening distally. Propodus about 7.0–7.5 times as long as wide, about 1.2 times as long as merus, 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 flexor distal margin ( Fig. 3F View FIGURE 3 ). Dactylus ( Fig. 3F View FIGURE 3 ) slender, faintly sinuous, tapering to subacute tip, about 0.25 times as long as propodus; flexor surface deeply excavated near base; proximomesial margin slightly expanded; dorsal surface with numerous stiff setae.

Branchial formula similar to that of other congeneric species: pleurobranchs absent; 1 simple arthrobranch above maxillipeds I–II; 2 simple, non-lamellate arthrobranchs above bases of maxilliped III to pereopod IV; podobranchs on maxilliped III to pereopod III, simple, non-lamellate. Epipods on maxilliped II to pereopod IV.

Gonopores present on coxae of pereopods III and V.

Pleopod I (Fig. 5L) well developed. Protopod strongly flattened, slightly twisted. Ramus (segment 2) about 0.35 length of protopod, flattened, triangular, terminally divided into two slightly unequal lobes by deep incision, small proximomesial protrusion, representing appendix interna and bearing cluster of adhesive hooks.

Pleopod II (Fig. 5M) equal in size to pleopod I. Basal segment smooth, about 5 times as long as wide. Distal segment, represented by remnant of endopodal ramus fused with appendix masculina and appendix interna, tapering distally, with proximoventral part armed with dense row of simple setae, distoventral part armed with a row of spiniform setae, with several terminal setae.Appendix interna thumb-like in shape, located at proximomesial margin of distal segment.

Pleopods III–V slender, without appendices internae.

Uropod ( Fig. 2D View FIGURE 2 ) with unarmed protopodite. Endopod with straight, unarmed lateral margin, submedian carina on dorsal surface feebly developed, unarmed. Exopod with straight, unarmed lateral margin; posterolateral angle with 1 spine, without spiniform setae; transverse suture unarmed; lateral and submedian carinae on dorsal surface unarmed.

Distribution. The species is presently known only from the type locality in the Banda Sea, southwestern Pacific, at a depth of 3000–3374 m.

Taxonomic position. The genus Ambiaxius was originally established for two species theretofore assigned to the genus Calocaris Bell, 1846 , C. alcocki McArdle, 1901 and C. aberrans Bouvier, 1905 . Lately, Sakai & Ohta (2005) proposed a new genus, Briancarins Sakai & Ohta, 2005, for Ambiaxius aberrans , A. japonicus Kensley, 1996 (type species) and A. foveolatus Kensley, Lin & Yu, 2000 . Poore (2020) synonymized Briancaris under Ambiaxius , noting “ Recognition of Briancaris would almost certainly introduce at least one paraphyletic taxon ”. Presently, the following 9 species are assigned to the genus Ambiaxius (e.g., Poore, 2020; Komai et al., 2010), separating into two species groups differing in the shape/armature of the rostrum, article 4 of the antennal peduncle as well as shape both of pleopods, especially distal segment of pleopod I (see below). The new species possibly occupies an intermediate position between these species groups.

Ambiaxius abyssalis sp. nov. appears morphologically close to Ambiaxius japonica Kensley, 1996 ( Japan, 472 m) ( Kensley, 1996) and Ambiaxius foveolatus Kensley, Lin & Yu, 2000 (NE Taiwan, 400–500 m) ( Kensley et al., 2000), which also possess a short rostrum (about 0.2 times as long as the carapace) armed with small but wellmarked lateral teeth (see Kensley, 1996: Fig. 11; 2000: Fig. 6), a relatively short article 4 of the antennal peduncle, less than 3 times as long as wide and not longer than 1.5 times of the length of article 3, only slightly reaching the antennular peduncle (see Kensley, 1996: Fig. 11; Kensley et al., 2000: Fig. 6). Such features clearly separate these species from other species of the genus (see below) and correspond to the diagnosis of the synonymized genus Briancaris (see Sakai & Ohta, 2005).

At the same time, the new species can be easily separated from A. japonicus and A. foveolatus by 1) less developed lateral rostral teeth; 2) the absence of a distoventral spine on the merus of the pereopod I (chelipeds); 3) a relatively longer propodus of the pereopod II, which is equal in the length to the carpus (vs. distinctly shorter than the carpus in A. japonicus and A. foveolatus ); 4) the relatively longer fingers of the pereopod II, which are longer than carpus and propodus for about 1.5 and 1.2 times, respectively (vs. shorter than carpus in A. japonicus and A. foveolatus ), and 5) different shape of the pleopod I with a leaf-like, triangular and flattened ramus (vs. ramus widening distally to slender folded (protruding) area in A. japonicus (see Kensley, 1996: Fig. 11E) and A. foveolatus (see Kensley et al., 2000: Fig. 6I)).

The other species group within the genus includes Ambiaxius alcocki (McArdle, 1900) (SW Indian Ocean, 792– 1630 m) (after Sakai, 2011; see also Poore, 2020), Ambiaxius aberrans ( Bouvier, 1905) (Lesser Antilles, 809 m) ( Bouvier, 1905; Kensley, 1996), Ambiaxius franklinae K. Sakai, 1994 (northwestern Australia, 1194-1357 m) ( Sakai, 1994; Poore & Collins, 2009; Poore, 2020), Ambiaxius surugaensis K. Sakai & Ohta, 2005 (Suruga Bay, 490–547 m) ( Sakai & Ohta, 2005), Ambiaxius propinquus Komai, Lin & Chan, 2010 ( Taiwan, 1271–1275 m) ( Komai et al., 2010), Ambiaxius meriman Poore, 2020 (Bismarck Sea, 695–899 m) ( Poore, 2020) and Ambiaxius tudak Poore, 2020 (Bismarck Sea and Indonesia, 650–891 m) ( Poore, 2020). These species are characterized by a long, slender, poorly armed or unarmed, distally upturned rostrum, reaching the distal margin of the antennular peduncle; article 4 of antennal peduncle is long and slender, about 5–6 times as long as wide, overreaching article 3 of antennal peduncle for 2–3 times, and the presence of large meral spine on pereiopod I (chelipeds). Abovementioned features clearly separate these species from Ambiaxius abyssalis sp. nov., while the new species is closer to the latter species by the shape of pleopods I–II and can be clearly separated from A. japonicus and A. foveolatus .