Anarthrura simplex G.O. Sars 1882

Anarthrura simplex G.O. Sars, 1882 (Figs. 1–2)

Anarthrura simplex: G.O. Sars, 1882: 54.

Anarthrura simplex: G.O. Sars, 1896: 38 –39, plate XVI figure 2.

Anarthrura simplex: Nierstrasz & Schurmans Stekhoven, 1930: 165, figure 40 (after Sars 1896). Anarthrura simplex: Stephensen, 1948: 178, 180, figures 56.16–56.20 (after Sars). Anarthrura simplex: Lang, 1971: 366, figure 2.

Anarthrura simplex: Holdich & Jones, 1983a: 88 –89, figure 32.

Anarthrura simplex: Howson & Picton, 1997: 209, species code S1149.

Diagnosis. Anarthrura with pleonites fused. Dactylus/unguis of pereopods 1–3 as long as propodus and carpus, or nearly so. Cheliped fixed finger with four teeth. Uropod endopod twice as long as basal article.

Material examined. British shallow­water BT305 (northern North Sea, Brae Oilfield, ca. 58° 40’N 1° 15’E, 110 m.): one manca­II, three neuters, two ovigerous females, two preparatory males; BT304 (Brae Oilfield): one prep. male.

Redescription. (Also see G.O. Sars 1896). Neuter/non­ovigerous female. Body (Fig. 1 A) slender, 7–8 times longer than broad. Length 1.74–2.35 mm. Cephalothorax about as long as broad, 15% of body length, with abruptly tapering rostral third. Pereonites 1–6 of different shapes, subhexagonal or subrectangular with rounded corners, 0.67, 1.0, 1.2, 1.2, 1.2 and 0.64 times as long broad respectively. Pleon (Fig. 1 B) 14% of body length, pleonites indistinct, appearing fused together, not distinct from pleotelson. Pleotelson (Fig. 1 B) short and with rounded posterior margin, bearing two setae.

Antennule (Fig. 1 C) 0.8 times length of cephalothorax; article­1 44% of total length, article­2 1.6 times longer than broad; article­4 with five setae and aesthetasc; other setation as figured. Antenna (Fig. 1 D) 0.8 times length of antennule, article­2 nearly twice as long as broad, without setae; article­3 as long as broad, without seta (?); article­4 about five times longer than broad, with distal setae and sensory setae; article­5 3.5 times longer than broad, with distal setae; article­6 small, with one short and two longer setae, the last possibly fused over basal half.

Mouthparts small, typical of genus. Labrum (Fig. 1 E–F) broadly conical. Mandible (Figs. 1 E,G) incisor process weak and subrectangular. Maxillule endite (Fig. 1 H) with four unequal spiniform setae. Maxilliped (Fig. 1 L) bases small, lacking setae; endites as long as article­1 of palps, with two distal setae; palp article­3 largest.

Cheliped (Fig. 1 J) basis less than half as long as pseudocoxa, with dorsolateral seta; merus triangular, with seta; carpus trapezoidal, 1.2 times longer than broad, with two ventral and two dorsal setae; propodus with anterior spiniform setae, one ventral seta, three spiniform setae near incisive margin and one near articulation with dactylus; fixed finger (Fig. 1 K) with convex incisive margin with four teeth, terminal spine elongate­triangular; dactylus with large anterior spiniform seta.

Pereopods 1–3 similar but progressively smaller, pereopods 2–3 88 % and 80% length of pereopod­1 respectively. Pereopod­1 (Fig. 2 A) basis curved, 4.5 times longer than broad, with dorsal seta; ischium with seta; merus about three times longer than broad; carpus subrectangular, 2.8 times longer than broad, with two distal seta; propodus over 3.5 times longer than broad, with dorsodistal seta; dactylus and unguis together nearly as long as propodus and carpus. Pereopod­2 (Fig. 2 B) with shorter articles; merus with distal seta; carpus with one distal seta; dactylus and unguis as long as propodus and two­thirds of carpus. Pereopod­3 (Fig. 2 C) as pereopod­2 but merus, propodus and dactylus/unguis shorter.

Pereopods 4–6 (Figs. 2 D–F) similar, smaller than pereopods 1–3, with more tumid basis bearing one or two sensory setae; ischium with two setae; merus curved, with two ventral spiniform setae; carpus subrectangular, three times longer than broad; propodus as long as carpus, with one long and two shorter ventrodistal seta; pereopod­6 (Fig. 2 F) propodus with additional long spiniform seta.

Uropods (Fig. 2 G) longer than pleotelson, exopod as long as basal article, with two distal setae; endopod twice as long as basal article, weakly 2­articled, with two sensory setae from article­1 and article­2 with five distal and terminal setae.

Manca­II. Similar to neuter but pereonites shorter and pereopods­6 absent. Length 0.83 mm.

Ovigerous female. Similar to neuter, but pleon more dorsoventrally compressed and broader, with thin oostegites on coxae of pereopods 1–4. Length 1.71–1.9 mm.

Preparatory male. Body (Fig. 2 H) fairly slender, length 1.47–1.63 mm. Cephalothorax relatively small, as long as broad, abruptly truncate behind antennules. All pereonites shorter than wide, subrectangular. Pleon well developed, 20% of body length. Antennule very stout, 4­ or 5­articled depending on state of maturation; articles 1–3 about 1.5 times wider than in female. Pleopod (Fig. 2 K) rami elongate, endopod smaller than exopod, with about nine and eleven terminal setae respectively; setae as long as rami.

Remarks. The description and drawings given by G.O. Sars are accurate. The very long dactylus­unguis on pereopods 1–3 is diagnostic of this species.

It is a small and delicate shelf species, distributed in the Boreal province of Northeast Atlantic from western France to Tromso in northern Norway 30–270 m (Dollfus 1898, Hansen 1909, Greve 1965a, b, 1966, 1968,). Published British records are sparse: only from the northern North Sea in the vicinity of the Fladen Ground and two stations in the Brae Oilfield, 110–140 m (Holdich & Jones 1983a, b), i.e. British & North East Atlantic Marine Census Areas North Sea [8] and Viking [4] respectively. The record from the Sound of Kerrera (Holdich & Bird 1986) has since proved to be erroneous and actually refers to Anarthrurella clairae gen. et sp. nov. (see below).