Stephonyx arabiensis, Diffenthal, Mark & Horton, Tammy, 2007

Diffenthal, Mark & Horton, Tammy, 2007, Stephonyx arabiensis (Crustacea: Amphipoda: Lysianassoidea: Uristidae), a new deep-water scavenger species from the Indian Ocean, with a key to the genus Stephonyx, Zootaxa 1665, pp. 31-41 : 33-39

Stephonyx arabiensis sp. nov.

Figures 1–4 .

Type material. Holotype: female, 26.7 mm, (NHM Reg. N o. 2007.799), Allotype: male, 24.9 mm (NHM Reg. N o. 2007. 800) Paratypes – 6 large mature females (NHM Reg. N os. 2007.801–806), 31 males (NHM Reg. N os. 2007.807–816), 14 immature females (NHM Reg. N os. 2007.817–826), 14 juveniles (NHM Reg. N os. 2007.827–836) collected off the coast of Pakistan in the northern Arabian Sea, RRV Charles Darwin, (cruise 151), station number 56137#5, baited trap attached to Profilur autonomous lander; deployed at 1322 ( UTC) 13/10/03, 22º 51.067' N, 65º 59.916' E at 1864m, recovered at 0 448 hrs, 14/10/03; bottom time, 14.2 hrs; bottom temperatures max = 3.55, min = 3.52, mean = 3.53).

Description. Based on adult female, 26.7 mm. Head: deeper than long, lateral cephalic lobes well developed tapering to a rounded point; rostrum absent; eyes absent in holotype. Antenna 1: short, about 0.2 times as long as body, about 0.75 times as long as antenna 2; peduncular article 1 short, slightly deeper than long; accessory flagellum 11-articulate, not forming cap; callynophore well-developed, 2-field; flagellum long 26- articulate; calceoli absent. Antenna 2: peduncle with brush setae, peduncular articles 4 and 5 not swollen; flagellum not elongate, 40-articulate; calceoli absent.

Mouthpart bundle: subquadrate. Epistome and upper lip separated by notch, subequal; epistome acute; upper lip rounded. Mandible: only left incisor examined; left lacinia mobilis 6-denticulate; left accessory setal row with 4 stout setae; molar setose with distal vestigial triturating surface; palp attached midway, article 1 short, slightly longer than broad, article 2 elongate, slender with 43 A2 setae, article 3 blade-like with one B3 seta and 28 D3 plumose setae and 5 E3 plumose setae. Maxilla 1: inner plate not observed, damaged; outer plate narrow with 7/4 crown arrangement of setal teeth, ST1 stout, bi-denticulate, ST2 stout tri-denticulate, ST3 slender tri-denticulate, ST4 slender 4-denticulate, ST5 slender 5-denticulate, ST6 slender 7-denticulate, ST7 slender, short, 5-denticulate, STA short 3-denticulate, STB slender 4-denticulate, STC slender 7-denticulate, STD slender 7-denticulate; palp large, 2-articulate with 7 terminal short stout setae and 1 flag seta. Maxilla 2: inner plate slightly shorter than outer plate (ratio = 0.88: 1). Maxilliped: inner plate large, subrectangular, with 2 nodular setae near inner corner and 1 near outer corner (damaged), oblique setal row well-developed; outer plate large, subovate, apical setae present, about 13 robust apical setae and 5 pappose apical setae, 17 small medial facial stout setae present; palp well developed, dactylus well developed, unguis present, terminal setae absent.

Gnathopod 1: chelate; coxa damaged (not observed); carpus linear, elongate, 5 times as long as deep, subequal in length to propodus; propodus linear, elongate, 6 times as long as deep, margins subparallel, palm obtuse with convex margin; postero-distal corner with 1 lateral and 1 medial tooth; dactylus simple, without subterminal tooth. Gnathopod 2: parachelate, coxa large; subequal in size to coxa 3, carpus linear, elongate, subparallel, nearly 4 times as long as deep; propodus subrectangular, short, (length 1.5 x depth); palm with slightly concave margin, postero-distal corner with 6 medial stout setae; dactyl not quite reaching palmar corner.

Pereopod 3: simple, coxa large. Pereopod 4: simple, coxa with well-developed postero-ventral lobe, anterior and posterior margins converging. Pereopod 5: coxa bilobate, posterior lobe more produced ventrally; basis expanded with posterior lobe rounded. Pereopod 6: coxa damaged, subrectangular; basis, posterior margin weakly convex with slightly squared postero-ventral corner; merus broadly expanded posteriorly. Pereopod 7: coxa damaged, ventral and dorsal margins converging; basis, posterior margin weakly convex, posteroventral corner slightly squared; merus broadly expanded posteriorly.

Oostegites: present on gnathopod 2 to pereopod 5. Gills: from gnathopod 2 to pereopod 7, not pleated.

Epimeron 1: antero-ventral corner rounded with 4 robust setae; posterior margin sinuous, convex. Epimeron 2: postero-ventral corner with significant tooth. Epimeron 3: postero-ventral corner with slight tooth.

Uropod 1: without fine setae; peduncle with 7 dorso-lateral, 1 apico-lateral, 9 dorso-medial and 1 apicomedial robust setae; rami subequal in length; outer ramus with 7 lateral and 3 medial robust setae; inner ramus with 11 lateral and 3 medial robust setae. Uropod 2: damaged, not observed. Uropod 3: peduncle short, breadth 0.6 x length, with 3 apico-lateral robust setae, 1 dorso-medial robust setae and 5 apico-medial robust setae; rami lanceolate, inner ramus with 14 lateral plumose, 9 lateral and 7 medial robust setae; outer ramus 2- articulate, article 2 short, article 1 with 4 medial plumose, 4 medial robust setae, 7 lateral and 2 apico-lateral robust setae. Telson: length 1.7 x width, deeply (75%) cleft, with 2 pairs of dorsal stout setae on each lobe and several very short, fine setae, distal margins with small notch in tip with 1 pappose and 2 short stout setae in each lobe.

Other Materials: Allotype male, 24.9 mm. As holotype except: Head: eyes visible: L-shaped, fading in alcohol. Antennae 1: about 0.4 times as long as antenna 2; accessory flagellum 9-articulate; callynophore 2- field, denser than holotype, aesthetascs longer than holotype; flagellum 28-articulate; calceoli present. Antenna 2: brush setae denser; flagellum elongate, 60-articulate; calceoli present.

Mandibles: incisors symmetrical; setal row on right mandible with 4 small stout setae. Maxilla 1: inner plate with 4 plumose setae; outer plate showing variation in denticulation of STA. Maxilliped: inner plate with oblique setal row well-developed with 20 plumose setae; outer plate with 9 facial setae lying underneath inner plate.

Pereopod 6: coxa subrectangular. Pereopod 7: coxa trapezoidal.

Epimeron 1: antero-ventral corner rounded, posterior margin convex. Epimeron 2: postero-ventral corner with significant tooth. Epimeron 3: postero-ventral corner with slight tooth. Uropod 2: peduncle with 9 dorsolateral and 9 dorso-medial robust setae; inner ramus longer than outer ramus (ratio = 1: 0.8); outer ramus with 8 lateral and 4 medial robust setae; inner ramus with 17 lateral and 6 medial robust setae. Uropod 3: peduncle with 6 short slender setae at base of lateral margin of inner rami, with 20 additional elongate slender plumose and 13 stout setae on lateral margin; outer ramus 2-articulate; article 1 with 8 medial plumose setae.

Immature female paratype 18.3 mm. As for holotype except coxa 1: very reduced, with anterior margin straight, posterior margin slightly concave, postero-ventral margin angled towards anterior margin, 4 setae on antero-ventral corner.

Etymology. This species is named after the Arabian Sea where it was collected.

Distribution. Off the coast of Pakistan in the northern Arabian Sea, Indian Ocean at depths ~ 1800 m.

Remarks. This species is placed within the genus Stephonyx Lowry and Stoddart (1989) rather than within the closely related genus Euonyx Norman, 1867 , because of the presence of a vestigial triturating molar and cuspidate setal teeth on maxilla 1.

Stephonyx scutatus , recorded from 2900m off the Cape of Good Hope, can be distinguished from the new species of Stephonyx by the carinate body, pronounced dorsal teeth on the pereon and pleon and the shield-like coxa 5. Stephonyx carinatus , recorded from 1947m in the Barbados Trench, also has a carinate body but can be distinguished from S. scutatus by the rounded keel on urosomite 1 (with a posterior tooth in S. scutatus ).

The species appears to lack a lacinia mobilis, although this could be an artefact of the illustration and/or description. The inner ramus of uropod 3 is shorter than the outer ramus (whereas the rami are equal in S. arabiensis ).

S. pirloti , recorded from South Australia, can be distinguished from the new species by the elongate carpus of gnathopod 2, (carpus: propodus = 4:1) and the elongate article 1 of the mandibular palp (article 1 length 0.75 x article 2).

Stephonyx incertus , recorded from 1947m in the Barbados Trench, can be distinguished from S. arabiensis by the strongly produced merus on pereopods 5–7, prominent quadrate lateral lobe of the head, and the ovate propodus of gnathopod 1 with a straight palm. Interestingly, this species has a reduced number of setal teeth on the outer plate of maxilla 1, with only 6 setal teeth in a 4/2 crown arrangement in contrast to the typical 7/4 crown arrangement of setal teeth, found in all other species in the genus.

Stephonyx mytilus , taken from 2447m – 2635m, Galapagos vents and 13°N vents, has 5 apico-medial stout setae on the maxilliped inner plate compared with none in S. arabiensis , the palm of gnathopod 2 is slightly convex, rather than concave.

Stephonyx normani , taken in a trawl at 1150m off the Kermadec Islands, can be distinguished by characters of the gnathopods. Gnathopod 1 is robust with a compressed carpus and a strongly curved propodus versus slen- der and elongate carpus and straight propodus in S. arabiensis .

Stephonyx talismani , taken in a trawl off Cape Bojador, Western Sahara, between 698m – 882m, and SW of the Faroe Islands, between 850–970m ( Stephensen, 1923), differs from the new species in the characters of gnathopod 2, with the dactyl not reaching the palm corner and the palm being strongly acute and excavate. The species also possesses large prominent eyes.

S. arabiensis sp. nov. is most closely allied to S. biscayensis and S. laqueus . Although similar to S. biscayensis (Bay of Biscay, 1455m and SW Ireland, 1180–1275m) in the structure of the mandibles, gnathopod 1 and the structure of the maxilliped inner plate; and to S. laqueus in the structure of the mandibles, gnathopod 1 and the shape of the palm of gnathopod 2, S. arabiensis can be easily distinguished from both species. Stephonyx laqueus ( Barnard, 1967) (Baja, California, 1187m) has a gnathopod 2 palm which is more excavate than the new species. The inner plate of the maxilliped has only 2 nodular stout setae and the mandible appears to lack a lacinia mobilis, while S. arabiensis has 3 nodular setae on the inner plate of the maxilliped and possesses a lacinia mobilis. There are further differences in the shape of lateral cephalic lobe and the antero-ventral corner of coxa 4 (more rounded in S. arabiensis ).

Stephonyx biscayensis ( Chevreux, 1908) has a smooth lacinia mobilis, 11 terminal stout setae on the palp of maxilla 1 and the palm of gnathopod 2 is convex. In contrast, S. arabiensis has 6 denticulations on the lacinia mobilis, 7 terminal stout setae on the palp of maxilla 1 and a weakly concave palm of gnathopod 2.

In addition to the known species of Stephonyx a number of entities from Eastern and Southern African waters have been ascribed to the species S. biscayensis resulting in some confusion. Lowry & Stoddart (1989), note that the S. biscayensis specimens reported by Barnard (1961) & Ledoyer (1986), from eastern Africa are more setose than those found in the Atlantic, and therefore probably represent an ‘undescribed species of Stephonyx’ ( Lowry & Stoddart, 1989). These entities, although from the same ocean basin as S. arabiensis , differ from it in the shape of coxa 1, subrectangular in S. biscayensis sensu Barnard, 1961 , and more elongate and rectangular in S. biscayensis sensu Ledoyer, 1986 . S. arabiensis has a triturating surface on the molar, absent in S. biscayensis sensu Barnard, 1961 and a concave palm on gnathopod 2, convex on S. biscayensis sensu Ledoyer, 1986 .

The specimen of S. biscayensis described by Griffiths (1975) from Southern Africa was not illustrated. Griffiths’ description was short and based upon a single record. He describes the ratios of the length of the chela of gnathopod 1 to be 40% of the propodus (it is only 30% in S. arabiensis ) and the propodus of gnathopod 2 to be half as long as the carpus (in S. arabiensis the propodus is 0.4 times as long as the carpus). Additional records of S. biscayensis from off Cape Point ( South Africa) by K.H. Barnard (1916) and Schellenberg (1926) also lack illustrations. K.H. Barnard (1916) wrote a description of a single specimen of S. biscayensis collected from 38 miles off Cape Point. This specimen is unlikely to be S. arabiensis notably by the shape of the apex of the palp of maxilla 1 (rounded in this specimen and straight in S. arabiensis ) and the convex palm of gnathopod 2 (concave in S. arabiensis ). The South African specimen has 8 stout setae on the outer plate and 5 stout setae on the palp while S. arabiensis has 11 stout setae on the outer plate and 7 stout setae on the palp. The carpus of gnathopod 2 is twice as long as the propodus in S. biscayensis sensu K.H. Barnard, 1916 and 2.5 times longer in S. arabiensis .

Schellenberg (1926) described a specimen from SW Cape Agulhas, 564m, with a telson similar to both S. normani and S. biscayensis ; the telson is described as ‘spiny on the edges’ and is said to have a narrow form like S. biscayensis but shows “the greatest similarity to Stebbing’s picture” ( S. normani ). The outer plate of the maxilliped reaches the second article of the maxilliped palp, the flagellum of antenna 1 consists of 25 articles (with 10 articles in the accessory flagellum) and 45 articles in antenna 2. Schellenberg comments that pereopods 3 and 4 have stout setae similar to those described by K.H. Barnard (1916): “anterior margin with 5 groups of 3 stout setae each and a group on the apex [of the merus]”. The maxilliped and the telson are similar to S. arabiensis (the telson of S. arabiensis has stout apical setae and the outer plate of the maxilliped almost reaches the tip of the second palp article), but the description of the stout setae on pereopods 3 and 4 does not fit with those found on S. arabiensis , which has up to 2 pairs of 3 stout setae with a group on the apex on pereopods 3 and 4; it has 26 flagellar articles in antenna 1 (with 10 articles in the accessory flagellum) and 40 flagellar articles in antenna 2. Schellenberg’s S. biscayensis is found in a different ocean basin to S. arabiensis ( S. arabiensis is found in the Indian Ocean and Schellenberg’s S. biscayensis was found in the Atlantic Ocean). Schellenberg’s S. biscayensis was also found at much shallower depths (564 m) compared with S. arabiensis (1864 m) .

Lowry & Stoddart (1989) suggested that these specimens were probably not S. biscayensis and our work further supports their conclusions. These specimens are also unlikely to be S. arabiensis and are probably another, as yet undescribed species of Stephonyx .

A total of 67 specimens of Stephonyx arabiensis were collected in the trap. The mature adult females (with large setose or pre-setose oostegites with setal buds) were the largest organisms with a total body length of between 19.8mm and 26.7mm (holotype) (n=7). The males have a total body length of between 13.1mm and 24.9mm (allotype) (n=32). The juveniles (with no discernible oostegite buds or penes) have a total body length of between 10.5mm and 13.0mm (n=14) and the immature females (with immature buds or short oostegites) have a total body length of between 13.6mm and 17.4mm (n=14).

Figures

FIGURE 1: Stephonyx arabiensis sp. nov. X — Holotype female, 26.7 mm; Y — Allotype: male, 24.9 mm; Z — immature female paratype, 18.3 mm.

FIGURE 2: Stephonyx arabiensis sp. nov. X — Holotype female, 26.7 mm; Y — Allotype: male, 24.9 mm.

FIGURE 3: Stephonyx arabiensis sp. nov. X — Holotype female, 26.7 mm; Y — Allotype: male, 24.9 mm.

FIGURE 4: Stephonyx arabiensis sp. nov. X — Holotype female, 26.7 mm; Y — Allotype: male, 24.9 mm.