Ithyomus conopygus Bird, 2004, sp. nov.

Ithyomus conopygus sp. nov. (Figs. 6–7)

Anarthrura sp.AM#8: Bird, 2001: 27

Material examined. Holotype (NMSZ:1999.238.0273) non­ovigerous female AFEN 1996 Stn 53833#1; Paratypes 53836#2, one neuter, partially dissected; 53838#1, one neuter, (NMSZ:1999.217.1360); AFEN 1998 Stn 54521#1, one neuter; DTI 2000 Stn 55286#1, one early prep. male, (NMSZ:2000.249.0005); 55395#1, one male?; 55447#8, one neuter (NMSZ:2000.249.0007); SEA­4 Stn 57025#1&3, one prep. male.

Description. Neuter/non­ovigerous female. Body (Fig. 6 A) slender, 9.3 times longer than broad. Length 2.22–3.15 mm. Cephalothorax 1.4 times longer than broad, slightly narrower rostrally, and with concave caudal margin; anterolateral margins with small seta. Pereonites 1–6 subrectangular with rounded corners, 0.55, 0.86, 0.86, 1.15, 1.15 and 1 times as long as broad; pereonites 4–6 (Fig. 6 B) with sinuate lateral lower margin of tergites. Pleon (Fig. 6 B) 21% of body length, epimera with a small seta over attachment of pleopod. Pleotelson (Fig. 6 C) conical, as long as one and a half pleonites.

Antennule (Fig. 6 D) about three­quarters length of cephalothorax; article­1 0.43 times total length, 2.3 times longer than broad; articles 2–3 about as long as broad; article­4 three times longer than broad, with six terminal setae and one aesthetasc; other setation as figured. Antenna (Fig. 6 E) 0.8 times as long as antennule, articles 2 and 3 with dorsal seta; article­4 four times longer than broad, with three distal setae and two sensory setae; article­5 with distal seta; article­6 small, with two terminal setae, one long and thickened, possibly a fusion of two or more ‘normal’ setae.

Labrum (Figs. 6 F–G) conical, slightly deeper than wide. Mandibles (Fig. 6 H) (not fully seen) weak, with long and narrow incisor process. Maxillule endite (Fig. 6 J) with seven terminal setae. Maxilla (Fig. 6 K) ovoid or subrectangular. Epignath (Fig. 6 L) falciform. Maxilliped (Fig. 6 M) with small triangular basal part; endite subrectangular, with rounded anterolateral margins, with a long medial seta; palp relatively large, with broad articles; article­2 with three inner setae, article­3 with three long and one small inner setae; article­4 less than one­third width of articles 1–3, curved, with five terminal setae.

Cheliped (Fig. 7 A) typical of family, armoured with many rows of microtrichia; pseudocoxa massive, 1.4 times longer than broad; basis with posterior seta; merus elongate, with one ventral seta; carpus 1.6 times longer than broad, with ventral free margin less than half as long as dorsal, crenulate near insertion of two setae, dorsal margin with proximal and distal setae; propodus (Fig. 7 C) about twice as long as deep, dactylus and fixed finger twisted in opposition, propodus with two anterior spiniform setae; fixed finger (Fig. 7 B) with two ventral setae, one seta near articulation with dactylus and three unequal spiniform setae near the incisive margin, which bears three or four low teeth, terminal spine triangular; dactylus strongly curved, with large spiniform anterior seta.

Pereopods 1–3 similar but decreasing in size from pereopod­1 to pereopod­3; pereopod­1 (Fig. 7 D) basis 4.4 times longer than broad, curved; ischium with small seta; merus twice as long as distal breadth, with two unequal ventral setae; carpus subrectangular, as long as merus, with four distal setae; propodus three­quarters length of carpus, with spiniform terminal seta; dactylus and unguis just longer than propodus, dactylus with accessory seta. Pereopod­2 (Fig. 7 E) 0.9 times length of pereopod­1 and basis more slender; merus stouter than in pereopod­1; carpus, propodus and dactylus/unguis as in pereopod­1 but proportionately smaller. Pereopod­3 (Fig. 7 F) 0.8 times length of pereopod­1; otherwise similar in setation and proportions to pereopod­2.

Pereopod­4 (Fig. 7 G) basis 3.3 times longer than broad, with proximal sensory seta; ischium with small seta; merus strongly curved, with two spiniform setae; carpus subrectangular, 2.25 times longer than broad, with four spiniform setae of unequal length; propodus three­quarters length of carpus, with spinules on dorsal margin, two spiniform ventrodistal setae and a more slender dorsodistal seta; dactylus and unguis together as long as propodus. Pereopod­5 (Fig. 7 H) slightly larger than pereopod­4, basis with two sensory setae. Pereopod­6 (Fig. 7 J) 0.9 times length of pereopod­5, and just smaller than pereopod­ 4, basis with one sensory seta; propodus with additional dorsodistal spiniform seta. Pleopods (Fig. 7 K) well developed; endopod subovate, three­quarters length of exopod, 1.6 times longer than broad, with one distal and eight terminal plumose setae; exopod broader than endopod, with strongly convex dorsal margin and eighteen plumose setae along entire ventral and distal margin.

Uropods (Fig. 6 N) as long as pleotelson, exopod as long as basal article, with one distal and two terminal setae; endopod 2­articled, twice as long as exopod, proximal article twice as long as distal, with distal seta and sensory seta, distal article with one median and four terminal setae.

Preparatory male. Similar to neuter but with slightly thicker antennule. Length 2.75– 3.02 mm.

Type locality. AFEN 1996 Stn 53833#1, Faeroe­Shetland Channel, 61o 36.16’N 2o 26.46’W, 1513 m, coarse silt.

Etymology. From Greek konos, ‘cone’ and pyge, ‘rump’, referring to the pleotelson.

Remarks. It is distinguishable from the partially sympatric Thorkelius latiremis comb. nov. by its longer and more conical pleotelson, sinuous pereonite 4–6 margins and longer uropods. There are eight records from the Faeroe­Shetland Channel, 1014–1655 m. It inhabits a variety of sediment types including ‘coarse silt’, ‘very fine sand’ and ‘gravelly muddy sand’. A putative ‘swimming male’ of this species measures 2.0 mm in length but further study and comparison with other highly dimorphic males is required before details are published.