Leptognathia aneristus Bird, 2007, n.sp.

Leptognathia aneristus n.sp.

Figures 9–11

Material examined. Holotype: 1 non-ovigerous female, 1.68 mm (KMNH IvR 500.180), station XR-7, 42°12.87’– 42°12.10’N 145°33.93’– 145°32.05’E, 3853–3858 metres, 17 September 2001. Paratypes: 1 ovigerous female (in poor condition), 2 non-ovigerous females (KMNH IvR 500.181), same location; 1 nonovigerous female (KMNH IvR 500.182), station TD-7, 38°47.93’– 38°45.87’N 144°08.07’– 144°07.89’E, 7340–7433 metres, 30 September 2001.

Diagnosis. Leptognathia with carapace as long as broad. Pleonites with high sternal spurs. Antennule article 2 less than half as long as article 1. Cheliped propodus with one ventral seta. Pereopod 6 with three pectinate dorso-distal setae; uropod exopod shorter than article 1 of endopod.

Etymology. From Latinisation of the Greek ‘ aneristos ’, meaning ‘undisputed’, ‘undoubted’; referring to its unequivocally close relationship to the generic type species.

Description. Non-ovigerous female.

Body (Figs 9 A). Fairly stout, 5.4 times as long as broad, length 1.68–1.84 mm. Carapace as long as broad, just longer than pereonites 1–2.

Pereonites. With slightly rounded or almost parallel lateral margins, pereonite 1 shortest, pereonites 1–6 times as long as broad respectively.

Pleon (Fig. 9 B). 18.5% of body length, pleonites much shorter than wide, with shallow epimera, each with a small seta. Sternites with high, recurved processes.

Pleotelson (Fig. 9 B). As long as two preceding pleonites, with shallow conical apical profile in dorsal view, with two setae and two sensory setae, but with deflected acuminate apex bearing two terminal setae.

Antennule (Fig. 9 C). About 0.8 times as long as carapace. Article 1 just over half length of whole, about twice as long as broad, with one simple and four sensory setae on outer margin. Article 2 1.2 times as long as broad, as long as articles 3–4 together, with outer seta. Article 3 short, annular, with two setae. Article 4 with three simple setae (one blunt-tipped), one aesthetasc and two sensory setae.

Antenna (Fig. 9 D). Six-articled, 0.81 times as long as antennule. Article 1 short, annular. Articles 2–3 shorter than broad, each with dorsal seta. Article 4 without pseudo-articulation or ‘fusion-line’, 3.3 times as long as broad, with two simple and one sensory setae. Article 5 0.4 times as long as article 4, with long terminal seta. Article 6 small, with three terminal setae.

Mouthparts. Labrum (Fig. 9 E) hood-like, broadly conical in ventral or dorsal view. Labium (Fig. 9 F) with acuminate distal processes, lacking setae. Mandibles (Figs 9 G–I) with crenulate incisor, that of right mandible more curved than left, lacinia mobilis of left mandible broad; molar (Fig. 9 I) abruptly acuminate with about three minute terminal denticles. Maxilla (Fig. 9 J) lamelliform, ovate, not setose. Maxillule palp with two unequal terminal setae, endite (Fig. 9 K) with eleven terminal spiniform setae. Epignath (Fig. 9 L) elongate, without setae. Maxilliped (Fig. 9 M) bases fused, cardiform, with seta near each palp insertion; endites unfused, with simple distal margin and distal seta; palp articles 2–4 with three, three and five long setae respectively.

Cheliped (Fig. 10 A). Basis narrow, with free posterior margin about 33% of length. Merus sub-triangular, with one ventral seta. Carpus twice as long as deep, with two ventral setae, one mid-dorsal seta and one distodorsal seta. Chela slender, about as long as carpus, dorsal margin of propodus (Fig. 10 B) about twice as long as dactylus, with long ventral seta, three inner setae and small outer seta near dactylus insertion. Fixed finger (Fig. 10 C) with triangular terminal spine and three or four small teeth on incisive margin, with three outer setae. Dactylus unarmed.

Pereopod 1 (Fig. 11 A). Longer than pereopods 2–3. Coxa annular, with setae. Basis slender, about six times as long as broad. Ischium with seta. Merus about twice as long as broad, with ventral seta. Carpus as long as merus, rectangular, with two short and two long setae. Propodus slender, about 1.6 times as long as carpus, with one ventral seta and two stronger dorso-distal setae. Dactylus and unguis together as long as propodus, dactylus with proximal seta, unguis with spatulate tip.

Pereopod 2 (Fig. 11 B). Similar to pereopod 1 but basis, merus, carpus, propodus and dactylus/unguis slightly shorter. Basis with dorsal seta. Carpus with four unequal setae. Propodus with one dorso-distal seta.

Pereopod 3 (Fig. 11 C). Similar to pereopod 2.

Pereopod 4 (Fig. 11 D). Basis slender, about five times as long as broad, with one or two sensory setae. Ischium with two setae. Merus about twice as long as broad, with two ventral spiniform setae. Carpus longer than merus, with three long spiniform setae and a smaller rod-like seta. Propodus as long as carpus, with two ventro-distal spiniform setae and a finely-pectinate terminal spiniform seta. Dactylus and unguis together longer than propodus.

Pereopod 5 (Fig. 11 E). Similar to pereopod 4, but basis with up to three sensory setae (although none on specimen figured).

Pereopod 6 (Figs 11 F–G). Similar to pereopods 4–5, but propodus with three terminal pectinate setae.

Pleopod (Fig. 10 D). Biramous, well-developed. Exopod elongate-ovate, with one proximal seta and nine distal setae on ventral margin. Endopod smaller, with proximal ventral seta, four distal-terminal setae and one disto-dorsal seta.

Uropod (Fig. 10 E). About two-thirds as long as pleotelson, basal article as long as broad. Exopod as long as basal article, just shorter than proximal article of endopod, with distal seta and long terminal seta. Endopod two-articled, twice as long as basal article, article 1 with distal sensory seta, article 2 with distal seta, three long terminal seta and one sensory seta.

Remarks. This species very closely resembles L. breviremis, the Antarctic L. breviremoides, and L. vitjazi; these four constitute a close group of sibling, even cryptic species together with ‘ Leptognathia cf. breviremis ’ (Dojiri & Sieg 1997), although the Mediterranean taxon L. vitjazi (originally in genus Cryptocope G.O. Sars, 1882) needs further study. Another, as yet undescribed, species of this ‘ breviremis -group’, occurs in the Arctic Ocean (Bird ined.) and may prove to be conspecific with L. aneristus n.sp. From L. breviremis, L. aneristus n.sp. it differs in having a shorter, more rounded cephalothorax, shorter antennule article 2, distally acuminate mandible molars, shorter pereopod 1 dorso-distal propodal setae, and three distinctly pectinate setae on the pereopod 6 propodus. The Antarctic species L. breviremoides is more similar to L. aneristus n.sp. but has more obviously pectinate and shorter carpal setae on pereopods 4–6, lacks the pereopod 6 pectinate propodal setae, has more setiferous pleopods and the uropod exopod is as long as the endopod article 1. From the recently described Japanese L. bamberi, L. aneristus n.sp. is distinguished by a single ventral seta on the cheliped propodus and longer pereopodal setae, especially those of the carpus and merus, but as the former is a shelf/upper slope species it is unlikely that the two species will be encountered in the same samples.

Distribution. Kurile-Kamchatka Trench and Japan Trench, at depths 3853–7433 metres.