Thorkelius latiremis Hansen (1913)

Bird, Graham, 2004, Tanaidacea (Crustacea) of the Northeast Atlantic: non­filiform species of Anarthruridae Lang from the Atlantic Margin, Zootaxa 471, pp. 1-44 : 21-26

Thorkelius latiremis Hansen (1913) comb. nov. ( Figs. 8–10 )

Leptognathia latiremis: Hansen, 1913: 100 –103, plate X figure 2a–n. Leptognathia latiremis: Lang, 1968: 159 , table II, figure 98 g –h. Leptognathia latiremis: Bird, 2001: 28 .

Diagnosis. Slender Thorkelius with pereonites 3–5 almost as long as broad; pleotelson with almost truncate apex; cheliped fixed finger broadly triangular, with distinctly convex incisive margin.

Material examined. Syntype from Ingolf Stn. 58 (Zoologisk Museum, Copenhagen); AFEN 1996 Stn 53713#2, one early prep. male?; 53758#1, one neuter; 53766#2, one manca­II, one neuter; 53770#2. one prep. male; 53821#1, one manca­III; 53821#2, one manca­II, one early prep. male?; 53838#1, one manca­III; 53899#1, one manca­II, one early prep. male?; AFEN 1998 Stn 54549#1, one manca­II, ( NMSZ:1999.218.1758); Stn 54550#3, one manca­III, one prep. male; DTI 2000 Stn 55265#1, one manca­II; 55282#2, one prep. male; 55283#1, one manca­II; 55286#1, one early prep. male?; 55292#1,2, one manca­III; 55302#1, two mancae­III; 55308#1, one manca­III, one neuter; 55324#1, one manca­II, one neuter, one early prep. male?; 55336#2, one manca­III; 55338#1, one early prep. male?; 55339#1, one manca­II, one manca­III, one neuter; 55342#2, one manca­III; 55344#1, one early prep. male?; 55344#2, one manca­III; 55349#1, one manca­II; 55350#1, one neuter; 55350#2, one neuter; 55372#2, one manca­II; 55358#2, one manca­ II; 55383#1, one manca­III; 55386#1, one manca­II, one neuter?; 55396#1,2, one neuter; 55402#1,2, one manca­II; 55429#1, one early prep. male?; 55431#2,3, one manca­III; 55433#1,2, one neuter; 55443#1,2, one manca­II; 55444#3,4, one manca­III; 55450#1, one neuter; 55452#1, one manca­II; 55452#2, one manca­II, one manca­III; 55453#1, two mancae­III, one prep. male; 55454#1, one manca­II; (accession nos. NMSZ:2000.249.0009­0033 for DTI 2000); SEA­4 Stn 57004#1&2, one manca­III ( NMSZ:2003.114.0037); 57006#1&2, one manca­III, one neuter ( NMSZ:2003.106.0180); 57008#1­3, one neuter, two prep. males, ( NMSZ:2003.106.0181); BIOICE Stn 2087, one early prep. male?; 2117, one manca­II; 2124, one manca­II; 2136, one prep. male; 2315, two mancae­II, one manca­III, one neuter, one prep. male; 2317, two neuters; 2318, one early prep. male?; 2323, one prep. male; 2324, one manca­II, two mancae­III, one neuter, one early prep. male?; 2364, one manca­II, one prep. male; 2591, one manca­II; 2655, two mancae­II, one manca­III, one neuter.

Also seen BIOFAR Stns 172, two specimens; 722, one specimen.

Redescription. Neuter/non­ovigerous female. Body ( Fig. 8 A) slender, 8.5 times longer than broad. Length 2.20–2.88 mm. Cephalothorax comparatively small (13% of body length) and round, 0.9 times as long as broad; with concave caudal margin. Pereon over half of body length; pereonite­1 trapezoidal, 0.44 times as long as broad, narrower caudally, with anterolateral setae; pereonites 2–6 weakly subhexagonal or subrectangular, 0.83, 0.9, 0.93, 0.96 and 0.78 times as long as broad respectively. Pleon ( Fig. 8 B) 19–23% of body length, with low rounded sternal processes, pleonite­5 with one lateral seta each side. Pleotelson short, not much longer than preceding pleonite, round caudal margin with slightly concave apex, bearing several setae.

Antennule ( Fig. 8 C) as long as cephalothorax, article­1 40–45% of total length, twice as long as broad; article­2 almost square in plan view; article­3 two­thirds length of article­ 2, almost square; article­4 twice as long as article­3, with five terminal setae and one aesthetasc; setation of antennule otherwise as figured. Antenna ( Fig. 8 D) more than threequarters length of antennule; articles 1–3 similar in length, about as long as deep, articles 2 and 3 with dorsal seta; article­4 more than four times longer than broad, with distal setae and sensory setae; article­5 about half length of article­4, with long distal seta; article­6 small, with six terminal setae.

Labrum ( Fig. 8 E) broadly conical with setose distal margin. Mandible ( Figs. 8 F–G) incisor process a thin plate supported by a thickened ridge. Maxillule endite ( Fig.8 H) with five (or six?) unequal terminal setae, palp with two terminal setae. Maxilla ( Fig. 8 J) small and ovoid. Maxilliped ( Fig. 9 A) bases and endites together cardiform, bases each with a long seta near articulation with palp; endite with distomedial seta and distolateral rounded process; palps relatively large, article­1 without setae, article­2 broad, with two inner setae, article­3 longer than broad, with inner row of four long setae, with smaller seta inserted on dorsal aspect; article­4 subrectangular, three times longer than broad, with four long setae and a weaker, shorter seta.

Cheliped ( Fig. 8 K) typical of family, stout and with surface ornamentation of microtrichia; basis about half length of large pseudocoxa, with long dorsolateral seta; merus 1.3 times longer than broad, with ventral seta; carpus very broad, with dorsal margin about three times longer than ventral, these each with two setae; propodus larger than carpus, 1.75 times longer than broad, with one large seta on anterior surface distal to a shallow ridge ( Fig. 9 B); fixed finger with two ventral setae, one seta near articulation with dactylus, incisive margin strongly convex with about six teeth and three associated setae; distal spine strong and triangular; dactylus crescentic, with strong anterior seta and small rounded process bearing a seta on incisive margin.

Pereopods 1–3 similar, but pereopod­1 largest. Pereopod­1 ( Fig. 9 C) basis curved; ischium small, with seta; merus longer than carpus, with two unequal ventral setae; carpus rectangular, 1.7 times longer than broad, with three distal setae; propodus subrectangular, 3.3 times longer than broad and 1.5 times longer than carpus, with strong distoventral seta; dactylus and unguis together longer than propodus, dactylus with distal seta. Pereopod­2 ( Fig. 9 D) with shorter basis; merus shorter, as long as carpus; propodus 2.6 times longer than broad; dactylus and unguis as long as propodus; setation as for pereopod­1. Pereopod­ 3 ( Fig. 9 E) as pereopod­2 but with proportionately shorter merus, carpus and propodus.

Pereopods 4–5 ( Figs. 9 F–G) basis broader than pereopods 1–3, 2.7–2.8 times longer than broad, with sensory seta; ischium with one seta; merus curved, shorter than carpus, with two strong ventral setae; carpus subrectangular, 1.8 times longer than broad, with three spiniform distal setae and one dorsodistal seta; propodus 1.2 times longer than carpus, subrectangular, with three strong distal setae; dactylus and unguis about half length of propodus, unguis short, with minutely hooked tip. Pereopod­6 ( Fig. 9 H) as pereopod­5 but basis stouter, 2.5 times longer than broad, propodus with additional distal seta and dactylus/unguis only just over one third as long as propodus.

Pleopod ( Fig. 9 J) with small article preceding main basal article, the latter relatively large in comparison to the rami; endopod 2.5 times longer than broad, with five terminal plumose setae; exopod longer, with seven terminal setae.

Uropod ( Fig. 9 K) often largely beneath pleotelson, but as long. Basal article half length of endopod, with short exopod bearing three terminal setae; endopod about four times longer than broad, with four long terminal setae and one weak seta.

Manca­II. Body ( Fig. 10 A) generally similar to neuter but with shorter and more angular pereonites 2–5 and very short pereonite­6; pleon ( Fig. 10 B) only 14.2–18.0% of body length; chela slightly more gracile and with only one ventral seta; pereopod­6 and pleopods absent. Length 1.43–2.11 mm.

Manca­III. Body ( Fig. 10 C) generally similar to manca­II but with longer pereonites; pleon ( Fig. 10 D) 17.6–21.7% of body length; pereopods­6 and pleopods present but rudimentary, lacking setae. Length 2.05–2.47 mm.

Early preparatory male (?). As neuter but antennules slightly thicker; pleon 20.5– 24.2% of body length. Length 2.15–3.15 mm.

Preparatory male. Body ( Fig. 10 E) generally similar to neuter but antennules ( Fig. 10 F) thicker and pleon better developed (23.3–26.4% of body length); pleopod ( Fig. 10 G) rami larger than in neuter, with more numerous terminal setae, endopod and exopod with about eight and ten respectively. Length 2.15–2.85 mm.

Remarks. Including those of Hansen (1913), there are over sixty records from the Denmark Strait, Iceland Plateau, Kolbeinsey Ridge, Iceland­Faeroe Rise (northern flank), Wyville Thomson Ridge (northern flank), Faeroe Bank Channel, and Faeroe­Shetland Channel, at depths 333–1355 m; the shallowest locality on the eastern margin of the last area (=West Shetland Slope) is at 434 m. Available bottom temperature data indicate a range from ­1o C to 2o C and sediment types ‘gravelly muddy sand’ through to ‘very fine sand’ and ‘silt’ (‘ooze’). It is a relatively common bathyal or upper slope species, largely characterising the Norwegian Sea Deep Water but has not been recorded from abyssal depths (> 2000 m) during the ‘NORBI’ expedition (own observations). It is sympatric with the scarcer Ithyomus conopygus gen. et sp. nov. at deeper levels, at least in the Faeroe­ Shetland Channel.

A large proportion (55%) of material is of mancae. The predominance of samples from May­September suggests that breeding, or at least release of the brood from the marsupium occurs a short time previously, i.e. early spring or late winter. Growth in this species is primarily through elongation of the pereonites and pleon, with the cephalothorax of mancae no smaller than neuters and preparatory males.

Figures

FIGURE 8. Thorkelius latiremis. Non­ovigerous female A, dorsal view; B, pleon, lateral view; C, antennule; D, antenna; E, labrum, ventral view; F – G, mandibles; H, maxillule; J, maxilla; K, cheliped. Scale bar 1 mm (A – B), 0.27 mm (C – D, K), 0.2 mm (E – J).

FIGURE 9. Thorkelius latiremis. Non­ovigerous female A, maxilliped; B, anterior of cheliped propodus and dactylus; C – H, pereopods 1 – 6 respectively; J, pleopod; K, uropod. Scale bar 0.27 mm (A – K).

FIGURE 10. Thorkelius latiremis. Manca­II A, dorsal; B, posterior abdomen, lateral; manca­III C, dorsal; D, posterior abdomen, lateral; preparatory male E, dorsal; F, antennule; G, pleopod. Scale bar 1 mm (A – E), 0.27 mm (F – G).