Siphonolabrum tenebrosus, Bird, Graham J., 2007

Bird, Graham J., 2007, Leptognathiidae Sieg, 1976 *, Zootaxa 1599, pp. 61-85 : 65-68

Siphonolabrum tenebrosus n.sp.

Figures 1–3

Material examined. Holotype: 1 non-ovigerous female, 3.03 mm ( KMNH IvR 500.173), station TD-4, 39°27.08’– 39°29.15’N 143°37.79’– 143°38.52’E, 3272–3146 metres, 26 September 2001. Paratypes: 1 nonovigerous female, 1 preparatory male (?) same location; 1 non-ovigerous female ( KMNH IvR 500.174), station XR-7, 42°12.87’– 42°12.10’N 145°33.93’– 145°32.05’E, 3853–3858 metres, 17 September 2002; 1 nonovigerous female (partially dissected; KMNH IvR 500.175), station TD-7, 38°47.93’– 38°45.87’N 144°08.07’– 144°07.89’E, 7340–7433 metres, 30 September 2001.

Diagnosis. Siphonolabrum with labrum elongate but almost as broad as deep. Cheliped merus without triangular process, propodus inner comb with seven setae. Uropod exopod as long as proximal article of endopod.

Etymology. From the Latin adjective tenebrosus , ‘dark’ or ‘gloomy’, referring to the blackness of the abysso-hadal environment.

Description non-ovigerous female.

Body ( Figs 1 A–B). Slender, 6.75 times as long as broad (contracted holotype) to 7.4 times as long as broad (relaxed paratype specimen from station XR-7); length 2.89 mm (range 2.13–3.16 mm).

Carapace. As long as broad, with parallel lateral margins and weak rostrum, just shorter than pereonites 1–2 combined (i.e. 0.9 times as long), 14.2% of body length (range 14.2–16.3%); with small seta behind antennule insertion.

Pereonites. With slightly rounded or sub-parallel lateral margins. Pereonite 1 shortest. Only pereonite 5 as long as or slightly longer than broad. Pereonites 1–6 0.49, 0.65, 0.75, 0.90, 1.10, and 0.69 times as long as broad respectively; seta on lateral margins of each pereonite; inter-pereonal gaps greater in extended/stretched specimens.

Pleon. 22.2% of body length (range 20.7–23.6%). Pleonites much shorter than wide, with weakly expanded epimera in dorsal view, each with a seta; sternal processes weakly developed.

Pleotelson ( Fig. 1 D). About as long as preceding two pleonites (foreshortened in dorsal view), with broadly rounded apex; one seta near each uropod basis and two simple and two sensory setae dorsally.

Antennule ( Fig. 2 B). Stout, about as long as carapace. Article 1 broad at base, about 2.5 times as long as broad, and not quite half as long as the whole appendage, with one disto-outer seta and three outer sensory setae. Article 2 stout, 1.3 times as long as broad, with unequal inner and outer setae. Article 3 about as long as broad, with inner seta and sensory seta. Article 4 about as long as articles 2–3, narrow, with about four terminal setae and one aesthetasc.

Antenna ( Fig. 2 C). About three-quarters as long as antennule. Article 1 short and annular. Article 2 longer than article 3, the latter with a dorsal seta. Article 4 about 3.5 times as long as broad, with terminal simple and sensory setae. Article 5 about 3.6 times as long as broad, with terminal seta. Article 6 small, with one distal and four terminal setae.

Mouthparts. Labrum ( Figs 2 D–E) clearly visible in whole animal, protruding with sub-conical lateral profile and slightly narrower cross-section. Labium not recovered. Mandibles ( Fig. 2 F) (only one recovered), delicate, elongate, without molar or lacinia mobilis, incisor with denticulate tip. Maxilla ( Fig. 2 G) sub-ovate, without setae. Maxillule ( Fig. 2 H) endite with group of eight unequal terminal spiniform setae, the longest three closely grouped, almost fused. Maxilliped ( Figs 2 I–J) typical of genus, bases small; endites unfused, with simple distal margin, each with a long inner seta; palps relatively large, article 2 with three inner setae; article 3 with one small and three large setae; article 4 with at least four long terminal setae.

Cheliped ( Fig. 1 E). Attached to carapace via a large sub-rectangular pre-basal sclerite or ‘pseudocoxa’ ( Fig. 1 C). Basis short and almost annular, with seta. Merus lying along wide proximal part of carpus, with strong seta. Carpus broad, about 1.3 times as long as broad, narrower distally, with proximal and distal setae on dorsal margin and two unequal setae on ventral margin. Chela about as large as carpus. Propodus deep, without fold, but with outer longitudinal ridge on fixed finger, with seven setae on inner face ( Fig. 1 F) and one outer setae near dactylus insertion. Fixed finger ( Fig. 2 A) about as long as propodus, with long narrow terminal spine and two sub-triangular teeth on almost straight cutting margin, the distal tooth bifid, with strong ventral seta and three setae near cutting edge. Dactylus as broad as fixed finger, dorsal margin strongly curved, with long inner spiniform seta.

Pereopod 1 ( Fig. 3 A). Longer than pereopods 2–3; annular coxa with seta. Basis about four times as long as broad. Ischium with seta. Merus just longer than carpus, with ventral seta. Carpus sub-rectangular, about twice as long as broad and as long as propodus, with one dorsal and two ventral setae. Propodus tapering distally, with ventral seta nearly as long as dactylus. Dactylus and unguis about 1.4 times as long as propodus, dactylus with long accessory setae, tip minutely bifid, unguis tip slightly spatulate.

Pereopod 2 ( Fig. 3 B). Similar to pereopod 1 but slightly smaller, with merus and carpus proportionately stouter. Dactylus and unguis shorter than that of pereopod 1 but still longer than propodus.

Pereopod 3 ( Fig. 3 C). Similar to pereopod 2 but shorter still, especially merus and carpus.

Pereopod 4 ( Fig. 3 D). Without distinct coxa. Basis broader than in pereopods 1–3, about as long as succeeding articles together, with two ventral plumose setae. Ischium with two unequal setae. Merus shorter than carpus, with two ventral spiniform setae. Carpus sub-rectangular, about as long as propodus and about twice as long as broad, with two ventral spiniform setae, one long outer spiniform seta, and one small rod-like dorsal seta. Propodus about 2.8 times as long as broad, with three long terminal spiniform setae. Dactylus longer than unguis, together about as long as propodus.

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

Pereopod 6 ( Fig. 3 F) similar to pereopods 4–5 but with one basal plumose seta. Propodus with four terminal spiniform setae.

Pleopod (not figured). Basal article without setae. Exopod sub-ovate, just over twice as long as greatest width, as long as basis of pereopods 4–6, with at least 23 plumose setae, graduated in length from short near basal article to long near apex. Endopod slightly smaller than exopod, with one dorso-distal seta and about 21 on ventral margin, one near to basal article separated from remainder.

Uropod ( Figs 1 D, 2K). Just shorter than pleotelson, basal article about twice as long as broad. Fused exopod as long as proximal article of endopod, with one inner and two strong terminal setae. Endopod five times as long as broad, two-articled, proximal article longer than distal, with two sensory setae, distal article with four long setae.

Remarks. The abysso-hadal S. tenebrosus n.sp. shows a close morphological affinity to the shallow-shelf S. californiensis , in an interesting parallel to the relationship between Akanthophoreus phillipsi Sieg & Dojiri, 1991 and A. undulatus n.sp. (Bird, elsewhere in this publication) Both are characterized by a long uropodal exopod but S. tenebrosus n.sp. appears to have a proportionately shorter carapace, a longer pleon, a greater size-disparity in antennule articles 2–3, a longer pereopod 1 merus, and lacks a prominent triangular process on the cheliped merus.

Siphonolabrum tenebrosus n.sp. can be distinguished from Anarthruropsis langi at least by its longer uropod exopod (a very short process in A. langi ), single ventral seta on cheliped fixed finger (two in A. langi ), double-toothed incisive margin of fixed finger (more numerous and less obvious dentition in A. langi ) and a narrower labrum. In the unlikely circumstance of direct sympatry, similar chelipedal and uropodal criteria separate S. tenebrosus from the shallower-occurring A. longa . As its specific epithet might imply, A. longa is also more slender than S. tenebrosus n.sp., being about 8.7 times as long as broad, its pereonites 4–5 as long as or slightly longer than broad.

Distribution. The three records of S. tenebrosus cover a remarkable bathymetric range, from 3272–7433 metres in the Japan Trench and the Kurile-Kamchatka Trench.

Figures

FIGURE 1. Siphonolabrum tenebrosus n. sp. A, holotype non-ovigerous female (contracted state); B, paratype nonovigerous female (relaxed-extended state); C, cephalothorax, lateral view; D, pleotelson, lateral view; E, right cheliped; F, inner setal group on cheliped propodus, adjacent to dactylus. Scale bar = 1 mm for A-D, 0.25 mm for E-F.

FIGURE 2. Siphonolabrum tenebrosus n. sp. Non-ovigerous female. A, cheliped fixed finger detail; B, antennule, C, antenna (lateral view); D – E, labrum dorsal and lateral profile respectively; F, mandible; G, maxilla; H, maxillule endite, I, maxilliped palp, one omitted; J, maxilliped endite (distal); K, uropod. Scale bar = 0.25 mm for A-E, G, I-K, 0.125 mm for F and H.

FIGURE 3. Siphonolabrum tenebrosus n. sp. Non-ovigerous female. A – F, pereopods 1 – 6 respectively. Scale bar = 0.25 mm for A-F.