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

Byrathis divae sp. nov.

(Figs 1–4, 9)

Holotype. Dissected adult female, body length 2.39 mm. ZMH Reg. no. K–41630. Collected on 28 July 2000 by the DIVA –I expedition above the sea bed at abyssal depths (5390 m).

Paratypes, body length 2.23–2.47 mm. ZMH Reg. no. K–41631, 9 adult females, the same label data as for the holotype; ZIN –91092, 8 adult females. Collected 25 July 2000 by the DIVA –I expedition in the South Atlantic (17º06.2’S 04º41.7’E), above the sea bed at abyssal depth (5415 m). Additional material: 9 females, body length 2.10–2.30 mm, collected in the South Atlantic from the equator to about 41ºS between 05ºW–09ºE at depths from 4469 to 5395 m and 6 females, body length 2.00– 2.28 mm, found in the Southern Ocean between about 58º–65ºS and 24º–51ºW at depths from 2965 to 4748 m.

Type locality. South Atlantic (16º18’S 05º27’E).

Description. Female. Body length 2.10–2.47 mm. Prosome 2.8–3.3 times as long as urosome (Fig. 1 A–B). Rostrum absent (Fig. 1 C–D). Cephalosome and pedigerous somite 1 and pedigerous somites 4 and 5 separate; posterior corners as short rounded lobes in lateral view, obtuse triangular in dorsal view (Fig. 1 F–G). Urosome of 4 somites (Fig. 1 E). Caudal rami with 1 dorsal seta (later broken in studied specimens), 1 ventral seta, and 4 terminal setae.

Antennule (Fig. 2 A–C) extending to distal margin of urosomal somite 3–4, 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–2 s, IX–2 s + 1ae, X– XI–4 s + 1ae, XII–1 s, XIII–1s, XIV–2s + 1ae, XV–1s, XVI–2s + 1ae, XVII–1s + 1ae (or 2s), XVIII–2s, XIX–2s, XX–2s, XXI–1s + 1ae, XXII–1s, XXIII–1s, XXIV–2s, XXV–2s, XXVI–2s, XXVII–XXVIII–5s + 1ae (setation on Fig. 2 A–C is based on the holotype and supplemented considering the paratypes and additional females).

Antenna (Fig. 3 A–B), coxa with 1 seta, basis with 2 setae; exopod of 7 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, and is partly fused with first short segment bearing long seta; first endopodal segment with 2 setae, second with 8 + 6 setae.

Mandible (Fig. 3 C–E), gnathobase with crest, cutting edge with 5 large and 3 small teeth near thick dorsal seta; basis with 3 setae; exopod 5-segmented with 1, 1, 1, 1, and 2 setae; endopod segment 1 with 2 setae, segment 2 with 8 long and 1 short setae.

Maxillule (Fig. 3 F), praecoxal arthrite with 9 marginal, 4 posterior and 1 anterior setae; coxal endite with 2 setae; coxal epipodite with 6 long and 3 short setae; proximal basal endite with 3 setae, distal basal endite with 2 setae; endopod with 7 setae; exopod with 6–7 setae.

Maxilla (Fig. 3 G–J), 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 of distal basal endite thicker, spine-like; enditic-like lobe of proximal endopodal segment (previously considered as proximal basal endite) with 4 setae, 1 is thicker and 1 is partly sensory; endopod with 3 worm-like and 5 poorly developed brush-like sensory setae.

Maxilliped (Fig. 2 D–E), syncoxa with 1 seta on proximal praecoxal lobe, 2 setae on middle praecoxal endite and 2 sclerotized setae and 1 poorly developed brush-like seta on distal praecoxal endite; coxal lobe with 3 setae and small attenuation. Basis with 3 setae. Endopod 6-segmented with 2, 4, 4, 3, 3+1, and 4 setae.

Swimming legs. P1 (Fig. 2 F–G), coxa with anterior row of spinules along distolateral margin; basis with medial distal seta 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 the shortest, exceeding base of following spine; spine of exopod segment 2 exceeding base of distal-most spine. P2–P4 (Fig. 4 A–D), coxa with 1 seta; basis without seta; endopod 2-segmented in P2, 3-segmented in P3–P4; exopods 3-segmented. Posterior surfaces of P2–P3 poorly spinulate, of P4 densely spinulate.

P5 (Fig. 2 H) 3-segmented, coxa and basis of equal length; coxa and basis with distolateral patch of spinules, exopod ornamented with rare surface spinules, 3 distal spines and terminal attenuation.

Etymology. The species name “ divae ” refers to the name of the DIVA expeditions where the studied specimens were obtained.

Remarks. Some specimens from the Antarctic samples differ from those of the type series in setation details of antennule and oral parts: i) distal segment of antennule (XXVII–XXVIII) bears 5s +1 ae (vs. 4s + 1ae in type specimens); ii) maxillule distal basal endite with 3 setae (vs. 2 setae in type specimens), and iii) seta of the medial praecoxal endite of maxilliped syncoxa exceeding coxal endite (seta is shorter in type specimens).

Byrathis divae sp. nov. shares with B. penicillatus sp. nov. and B. laptevorum the following feautres: i) size usually more than then 2.10 mm (vs. size less than 1.40 mm in the other congeners); ii) surface of genital double somite without denticles (vs. surface ornamented with denticles in the other congeners); iii) maxillule proximal basal endite with 3 setae (vs. 2 setae in the other congeners); iv) maxillule exopod with 6 setae or more (vs. with 5 setae or less in other congeners), and v) maxilla endopod with 3 worm-like (1 worm-like in B. laptevorum) and 5 brush-like sensory setae (vs. 5 worm-like and 3 brush-like setae in the other congeners).

Byrathis divae sp. nov. differs from B. penicillatus sp. nov. and B. laptevorum in: i) smaller size (2.10–2.47 mm vs. 2.70 mm in B. laptevorum and 3.20–4.90 in B. penicillatus sp. nov.); ii) rounded posterior corners of prosome (vs. sharp triangular in B. laptevorum and B. penicillatus; iii) rostrum absent (vs. rostral filaments present in B. penicillatus sp. nov. and B. laptevorum), and iv) poorly developed brush-like seta in the distal praecoxal setal group of the maxilliped syncoxa (vs. well developed brush-like seta in B. penicillatus sp. nov. and B. laptevorum).