Byrathis penicillatus Markhaseva & Renz, 2011, sp. nov.

Byrathis penicillatus sp. nov.

(Figs 5–9)

Holotype. Partly dissected adult female, body length 4.05 mm. ZMH Reg. no. K–42157. Collected on 15 March 2005 by the DIVA –II expedition above the sea bed at abyssal depths (5058 m).

Paratypes, 1 partly dissected adult female, body length 3.95 mm. ZMH Reg. no. K–42158. Collected on 15 March 2005 by the DIVA –II expedition above the sea bed at abyssal depths (5050 m) in the South Atlantic (00º08.5’S 02º30.2’W); 1 partly dissected adult female, body length 3.35 mm. ZIN –91103. Collected 23 July 2009 by the DIVA –III expedition in the South Atlantic (26º35’S 35º14’W), above the sea bed at abyssal depths (4482– 4489 m). Additional material: 4 females, body length 3.20–4.90 mm, collected in the South Atlantic from the equator to about 36ºS at depths from 4601 to 5395 m.

Type locality. South Atlantic (00º01.2’S, 02º28.7’W).

Description. Female. Body length 3.35–4.90 mm. Prosome 3.7–4.1 times as long as urosome. Rostrum as a triangular plate with 2 filaments (Fig. 5 C–D). Cephalosome and pedigerous somite 1 and pedigerous somites 4 and 5 partly separate; posterior corners sharply triangular in dorsal and lateral view (Fig. 5 A–B, E–G). Urosome of 4 somites. Caudal rami with 4 terminal setae, 1 ventral seta and 1 small dorsolateral seta (Fig.5 E–F).

Antennule (Fig. 6 A–C) extending to distal margin of pedigerous somites 4–5, of 24 articulating segments; armature as follows: I–3 s, II– IV–6 s + 1ae, V–2 s + 1ae, VI–2 s, VII–2 s + 1ae, VIII–1 s+1?, IX–2 s + 1ae, X– XI–4 s + 1ae, XII–1?, XIII–1?, XIV–2s + 1ae, XV–1?, XVI–2s + 1ae, XVII–1?, XVIII–2?, XIX–1?, XX–2?, XXI–1s + 1ae, XXII–1?, XXIII–1s, XXIV–1s + 1?, XXV–2s, XXVI–2s, XXVII–XXVIII–4s + 1ae, (setation on Fig. 6 A–C is given for segments II–IV, VI, VII, IX–XI, XIV, XVI and XXIII after the holotype and supplemented from the paratypes).

Antenna (Fig. 6 D), coxa with 1 seta, basis with 2 setae; exopod of 7 free segments with 1, 1-1-1-1, 1, 1, 1, 1, and 3 setae, seta on proximal exopod segment rudimentary, following complex segment with 3 rudimentary setae, partly fused with first short segment bearing long seta; first endopodal segment with 2 setae, second with 8 + 7 setae.

Mandible (Fig. 6 E–G), gnathobase with crest, number of teeth on cutting edge of holotype difficult to follow, paratype bearing 4 large and 3 small teeth near dorsal seta, lateral tooth situated apart from the remaining teeth; basis with 3 setae; exopod 5-segmented with 1, 1, 1, 1, and 2 setae; endopod segment 1 with 2 setae, segment 2 with 9 setae and distal rows of small surface spinules.

Maxillule (Fig. 6 H–I), praecoxal arthrite with 9 terminal, 4 posterior, and 1 anterior setae; coxal endite with 2 setae; coxal epipodite with 9 setae; proximal basal endite with 3 setae, distal basal endite with 3 setae; endopod with 9 setae; exopod with 8 setae.

Maxilla (Fig. 7 A–C), praecoxal endite (previously considered as proximal praecoxal endite) with 4 setae and a short attenuation; coxal endite (previously considered as distal praecoxal endite) with 3 setae; basal endites (previously considered as coxal endites) with 3 setae each, 1 seta on the proximal basal endite very short, 1 seta on distal basal endite very strong, spine-like, enditic-like lobe of proximal endopodal segment (previously considered as proximal basal endite) with 4 setae, 1 is thicker and 1 is sensory; all endites with a patch of long spinules at the base of the setae; endopod with 3 worm-like and 5 brush-like sensory setae: 2 setae shorter with large brushes, 3 setae longer with smaller brushes.

Maxilliped (Fig. 7 D), syncoxa with 1 seta and a row of spinules on proximal praecoxal endite, 2 setae (1 sensory in distal part) and a patch of spinules on the middle endite and 2 sclerotized and 1 large brush-like seta on the distal praecoxal endite; coxal endite with 3 setae. Basis with 3 setae, proximal row of long spinules extended to the proximal basal seta and row of small spinules extended from the proximal to the distal-most basal seta. Endopod 6- segmented with 2, 4, 4, 3, 3+1, and 4 setae.

Swimming legs. P1 (Fig. 8 A), coxa with anterior row of spinules along the distal margin; basis with medial distal seta strongly curved with setules; endopod 1-segmented with lateral lobe, its lateral margin with spinules; exopod segments 1 to 3 with 1 lateral spine each, spine of segment 1 reaching base of following spine; spine of exopod segment 2 not reaching the base of distal-most spine. P2–P4 (Fig. 8 B–D), coxa with 1 seta; basis without seta; endopod 2-segmented in P2, second segment on posterior surface with patch of long spinules; exopods 3-segmented. Endopods 3-segmented in P3–P4, segment 2 with a patch of long spinules on the posterior surface, segment 3 posterior surface densely spinulate, although with shorter spinules. Posterior surface of P4 coxa, basis and exopod densely spinulate.

P5 (Fig. 7 E) 3-segmented, coxa and basis of equal length; coxa and basis with a patch of minor spinules distolaterally; exopod ornamented with surface spinules, 4 distal spines, medial terminal spine is curved, about 1.7 times longer than lateral terminal spine.

Etymology. The species name “ penicillatus ” refers to a large brush-like sensory seta on the syncoxa of the maxilliped.

Remarks. B. penicillatus sp. nov. is more closely related to its most geographically distant congener B. laptevorum (Fig. 9) and shares with this species the following distinguishing characters: i) maxilla distal basal endite (previously considered as distal coxal endite) and enditic-like lobe of proximal endopodal segment (previously considered as proximal basal endite), each with 1 very strong, spine-like setal element (vs. these setal elements are neither strong nor spine-like in congeners); ii) each maxilla endite with a patch of long surface spinules (vs. surface spinule patch is absent in the other species of the genus); iii) medial praecoxal endite of maxilliped syncoxa with a patch of spinules (vs. no spinule patch in congeners).

B. penicillatus sp. nov. differs from B. laptevorum in: i) large brush-like sensory seta on distal praecoxal endite (vs. poorly developed brush in B. laptevorum); ii) maxilliped basis with a patch of long spinules proximally (vs.

spinules absent in B. laptevorum), and iii) P5 medial terminal spine curved, about 1.7 times longer than lateral terminal spine (vs. straight spine, nearly twice as long as lateral terminal spine in B. laptevorum). Maxilla endopod of single known specimen of B. laptevorum is described as bearing 1 worm-like sensory seta (vs. 3 brush-like setae in B. penicillatus sp. nov.), but this character should be re-examined for the species when new specimens of B. laptevorum are obtained.