Anarthrura edentatus Bird, 2004, sp. nov.

Anarthrura edentatus sp. nov. (Figs. 3–5)

Anarthrura sp.5: Holdich & Bird, 1989: tables III, IV and appendices as taxon ‘29’. Anarthrura sp.AM#5: Bird, 2001: 26 –27.

Diagnosis. Anarthrura with distinct pleonites. Cheliped fixed finger with smooth incisive margin. Uropod endopod three times as long as basal article.

Material examined. Holotype neuter (NMSZ:1999.238.0275), AFEN 1998 Stn 54587#1. Allotype early preparatory male (NMSZ:1999.218.2092), AFEN Stn 54592#2. Paratypes AFEN 1998 Stn 54587#1, one neuter (NMSZ:1999.238.0276); SMBA Stn ES18, one manca­II, two neuters, one prep. male; SBC64, two mancae­II, one manca­III, two neuters, one prep. male; SBC65, two mancae­II, one neuter; SBC168, two mancae­II, three mancae­III, three neuters, one ovigerous female; SBC279, eight neuters, two prep. males, three incomplete specimens (NHM 1988:551–557 and 1989:912:13 for SMBA material).

Also seen: IOS Stn 51403#2, one specimen; ‘Chain 106’ Stn 313, one specimen.

Description. Neuter/non­ovigerous female. Body (Fig. 3 A) fairly slender, 6.6 times longer than broad. Length 1.40–1.98 mm (holotype 1.85 mm). Cephalothorax short, 17% of body length, with convex caudal margin and tapering rostral third; rostrum (Fig. 3 B) small, and area behind attachment of antennules with lateral creases. Pereonites 1–6 weakly subhexagonal or subrectangular, 0.63, 0.78, 0.89, 1.0, 0.82, and 0.67 times as long as broad respectively. Pleon (Fig. 3 C) short and weak, only 11% of body length, pleonites with low rounded sternites, that of pleonite­1 most prominent. Pleotelson (Fig. 3 D) half as long as pleon, wider than long, and with round apex.

Antennule (Fig. 3 E) just longer than cephalothorax; article­1 0.44 times total length, 2.7 times longer than broad, article­2 half as long as article­1, article­3 half length of article­2, article­4 just longer than article­2, setation as figured. Antenna (Fig. 3 F) 84% length of antennule, article­2 1.5 times longer than broad, article­3 smaller, both without dorsal setae; article­4 about six times longer than broad, with two sensory and two long setae; article­5 0.6 times as long as article­4, with distal seta; article­6 small, with one thickened (fused?) and two small setae.

Labrum (Fig. 3 G) small, conical, not readily visible in whole specimens. Mandibles (Fig. 3 H) small, ovoid. Maxillule endite (Fig. 3 J) with (at least) four terminal spiniform setae. Maxilla (Fig. 3 J) elongate ovoid. Maxilliped (Fig. 3 K) bases small, without setae; endites simple, each with a long medial seta and shorter lateral seta.

Cheliped (Fig. 4 A) pseudocoxa 1.7 times longer than broad, with dorsodistal seta; basis subcylindrical, with dorsal seta; merus with ventral seta; carpus broadly oval, 1.5 times longer than broad, with two dorsal setae and two ventral setae; propodus longer than carpus, 2.4 times longer than broad, with long anterior spiniform seta; fixed finger with seta at articulation with dactylus, one ventral seta and three spiniform setae associated with incisive margin, terminal spine tapering, incisive margin convex without dentition; dactylus with strong anterior spiniform seta.

Pereopods 1–3 similar, coxa with seta; other setation sparse. Pereopod­1 (Fig. 4 B) basis slender, 4.6 times longer than broad; ischium with seta; merus 2.75 times longer than broad, with one seta; carpus about twice as long as broad, with curved dorsodistal seta and small ventrodistal seta; propodus longer than carpus, with long distal seta; dactylus and unguis 1.8 times longer than propodus, dactylus with accessory seta and unguis tip slightly expanded, blunt (possibly with spinning pore?). Pereopod­2 (Fig. 4 C) basis with sensory seta, merus and carpus shorter than in pereopod­1; dactylus and unguis shorter, 1.3 times longer than propodus; overall 86% of pereopod­1 length. Pereopod­3 (Fig. 4 D) basis, merus, carpus and propodus shorter than pereopod­2, overall 72% of pereopod­1 length.

Pereopod­4 (Fig. 4 E) basis 4.2 times longer than broad; ischium with one seta; merus twice as long as broad, with two ventral spiniform setae; carpus subrectangular, as long as merus, with three spiniform distal seta and a smaller dorsodistal seta; propodus as long as carpus but only half as broad, with two spiniform ventrodistal setae and a longer dorsodistal spiniform seta; dactylus and unguis about two­thirds length of propodus. Pereopods 5– 6 (Figs. 5 A–B) as above but with long sensory seta on basis and pereopod­6 propodus with four distal spiniform seta.

Uropod (Fig. 5 C) about twice as long as pleotelson, clearly visible from dorsal view; exopod twice as long as broad and half length of basal article, with three unequal distal and terminal setae; endopod 3.2 times longer than basal article and eight times longer than broad, one­articled although with appearance of a weak articulation near insertion of two large sensory setae, with four long distal and terminal setae and two weaker terminal setae.

Manca­II. Generally as above but with shorter pereonites and pereopods­6 absent. Length 0.98–1.20 mm.

Manca­III. Similar to manca­II but with larger pereonite­6 and with rudimentary pereopods­6. Length 1.20 mm.

Preparatory male. Generally similar to neuter/non­ovigerous female. Body fairly slen­ der, 6.5 times longer than broad. Pleon (Fig. 5 E) 20% of body length. Antennule (Fig. 5 F) broader than in neuter/non­ovigerous female. Pleopods (Fig. 5 G) present, elongate­ovoid, without setae in early moult stage, later with terminal setae. Length 1.41–1.75 mm.

Type locality. AFEN Stn 54587#1, Hebrides Slope, 57o 05.50’N 9o 25.84’W, 1498m, sandy mud.

Etymology. Latin edentatus, ‘toothless’, referring to the smooth incisive margin of the cheliped fixed finger.

Remarks. It is unusual for tanaids to have a cheliped fixed finger devoid of teeth. It is a bathyal Anarthrura species from the North Feni Ridge, northern Rockall Trough and the Hebrides, Malin and Porcupine Slopes, 1206–1832m; two of the records describe the sediment as ‘sandy mud’.