Akanthophoreus crassicauda Bird, 2007, n.sp.

Akanthophoreus crassicauda n.sp.

Figures 1–3

Material examined. Holotype, 1 non-ovigerous female (KMNH IvR 500.194), station XR-12, 41º37.67’– 41º37.08’N 146º54.19’– 146º52.72’E, 5473–5484 metres, 4 m ORE beam trawl, 23 September 2001.

Paratypes: 1 ovigerous female, 1 post-ovigerous male (partially dissected) and 1 preparatory male (KMNH IvR 500.195), same locality.

Diagnosis. Akanthophoreus with all pereonites shorter than broad. Pleotelson without lateral spurs. Cheliped carpus with weak shield; propodus and dactylus without crenulations. Dactylus/unguis of pereopods 1–3 short, half as long as propodus. Uropod stout, endopod articles three or four times as long as broad.

Etymology. From combination of Latin crassus ‘thick’ or ‘stout’ and cauda, ‘tail’, referring to the diagnostic uropods.

Description, non-ovigerous female.

Body (Fig. 1 A). Fairly slender, 6.9 times as long as broad (Holotype), length 2.61 mm.

Carapace. Just shorter than pereonites 1–2 together (excluding pereonal gap), 1.3 times as long as broad, with parallel margins for posterior two-thirds portion; rostrum a slight sub-triangular process.

Pereonites. With weakly convex lateral margins, pereonite 1 slightly narrower posteriorly, pereonites 1–6 0.55, 0.59, 0.68, 0.69, 0.67, 0.62 times as long as broad, respectively.

Pleon (Fig. 1 B). Shorter than preceding two pereonites, 14.6% of body length, all pleonites much wider than long, with long seta on epimeral margin; each pleonite sternum rounded in lateral profile.

Pleotelson (Figs 1 C). Longer than two preceding pleonites, shorter than broad, with rounded posterior margin in dorsal view obscuring deflected apex; four setae (two sensory) on posterior margin.

Antennule (Fig. 1 D). Slender, about 0.75 times as long as carapace. Article 1 just over half of total length, 3.1 times as long as broad (mid-length), with disto-outer seta and sensory setae and more proximal group of three sensory setae. Article 2 sub-rectangular, 1.24 times as long as broad, with a long disto-outer seta and one smaller inner seta. Article 3 half as long as article 2, with two setae. Article 4 twice as long as article 3, 2.8 times as long as broad, with four long, quite thick setae, one small seta and one aesthetasc.

Antenna (Fig. 1 E). Slender, about 0.74 times as long as antennule. Articles 1–3 short, but article 2 larger than article 3, both with dorsal seta, two on article 2. Article 4 almost half as long as antenna, about five times as long as broad, with pseudo-articulation near mid-point, accompanied by a sensory seta, terminally with two long setae and at least three sensory setae. Article 5 slender, three times as long as broad, with terminal seta. Article 6 small, with four unequal setae.

Mouthparts. Labrum (Fig. 2 A) of sub-conical or sub-triangular lateral profile. Labium (Fig. 2 B) typical of genus. Maxilla not recovered. Mandibles (Figs 2 C–D) strong, with dentate incisor and molar with acuminate shape, terminally with about eight unequal spines, longer apically; left mandible with at least bicuspid lacinia mobilis. Maxillule (Fig. 2 E) palp with two setae and endite with nine spiniform setae, two of which are pectinate. Maxilliped (Figs 2 F–G) basis fused and longer than endites, with long seta at base of each palp; endites unfused, each with weakly lobed distal margin, minutely setose lateral processes and distal seta; palp with stout articles 1–3, article 2 with three inner and one outer seta; article 3 with two long and two smaller setae; article 4 with four long and one smaller setae.

Cheliped (Figs 2 H–I). With distinct pre-basal sclerite attachment. Basis with rounded free posterior margin and outer seta. Merus sub-triangular, with ventral seta. Carpus 1.9 times as long as broad, with very weak shield, two ventral setae and two dorsal setae. Propodus and terminal spine longer than carpus, 0.47 times as deep as long, dorsal crest with two processes (Fig. 2 I) with four inner spiniform setae and one outer seta near articulation with dactylus. Fixed finger with two ventral setae, and three unequal setae near incisive margin, the latter with three unequal teeth, central one bifid. Dactylus strong, without crenulations.

Pereopod 1 (Fig. 3 A). Coxa annular, with seta. Basis about 3.8 times as long as broad, with dorsal plumose setae. Ischium with seta. Merus with inner slender seta and outer spiniform seta. Carpus as long as merus, rectangular, 2.2 times as long as broad, with one dorsal and one ventral terminal spiniform seta. Propodus slender, largely devoid of small spinules, 1.5 times as long as carpus, with disto-ventral spiniform seta and terminal fringe of small setae. Dactylus and unguis together just over half as long as propodus, unguis as long as dactylus, with spatulate tip.

Pereopod 2 (Fig. 3 B). Similar to pereopod 1 but carpus with three spiniform setae. Propodus with a few ventral spinules.

Pereopod 3 (Fig. 3 C). Similar to pereopod 2 but basis without sensory seta. Dactylus with accessory seta (possibly present on pereopods 1–2).

Pereopod 4 (Fig. 3 D). With no discrete coxa. Basis slightly longer than in pereopods 1–3. Ischium with two setae. Merus with two unequal spiniform setae. Carpus rectangular, longer than merus, with three long spiniform setae and one smaller rod-like dorsal seta. Propodus longer than carpus, with two ventro-distal pectinate setae and one dorso-distal spiniform seta. Dactylus and unguis as long as propodus, dactylus with double row of spinules on inferior margin.

Pereopod 5 (Fig. 3 E). Similar to pereopod 4.

Pereopod 6 (Fig. 3 F). Similar to pereopods 4–5 but propodus with two additional dorso-distal spiniform setae.

Pleopod (Fig. 2 J). Relatively small compared to pereopods, both rami sub-ovate. Endopod with five terminal setae. Exopod two-articled, with five terminal setae and one on small article 1.

Uropod (Fig. 2 K). Relatively stout (for genus), only as long as pleotelson and half of preceding pleonite. Basal article about twice as long as broad. Exopod two-articled, barely half as long as article 1 of endopod, with one seta on article 1 and one long seta on article 2. Endopod two-articled, article 1 just over half total length of endopod, as long as basal article, just over three times as long as broad, with one long distal seta and two distal sensory setae, article 2 as long as article 1, about four times as long as broad, with one distal seta and four unequal terminal setae, with two sensory setae.

Ovigerous female. Generally as non-ovigerous female, but pereon more dorso-ventrally compressed, with four pairs of oostegites; length 2.24 mm.

Post-ovigerous female. As ovigerous female but lacking oostegites, length 2.22 mm. Preparatory male.

Body (Fig. 1 F) generally as non-ovigerous female, but pleon (Figs 1 F–G) slightly larger, 17% of body length, length 2.12 mm.

Pleotelson similar, but specimen shows full profile of triangular apex (Fig. 1 H).

Antennule (Fig. 1 I) stouter, five-articled, i.e. with additional short article.

Pleopod (Fig. 2 L) proportionately larger.

Remarks. This species is easily distinguished from other sympatric Akanthophoreus taxa by its relatively stout uropods, ‘ordinary’ chelipeds, short dactylus/unguis in pereopods 1–3 and the latter with a relatively ‘smooth’ carpus and propodus. A very similar, or possibly conspecific, species remains undescribed from the Iceland-Greenland region, some specimens of which were identified by Hansen (1913) as Leptognathia hanseni (i.e. the type material is a mixture of two species – Bird ined.).

Distribution. Kurile-Kamchatka Trench, 5473–5484 metres.