Sensiava longiseta

Sensiava longiseta gen. et sp. nov.

(Figs 1–7)

Holotype. Undissected adult male (ZMH Reg. no. K–40772), body length 4.80 mm. Collected Southern Ocean, Weddell Sea (63º44’S 33º48’W), 14–15 March 2002, above the seabed at 4975 m. Paratypes. Dissected male (ZMH Reg. no. K–40773), body length 4.64 mm. Collection data as for holotype. One male (ZMH Reg. no. K–40774) with Mx2 dissected, body length 4.60 mm, collected Southern Ocean, Weddell Sea (64º02’S 39º06’W), 13 March 2002, above the seabed at 4747.5–4742.5 m,

Description. Male: total length 4.60–4.80 mm. Prosome 2.5–2.9 times longer than urosome. Rostrum with two long filaments (Fig. 1 B–C). Somites 4–5 of prosome incompletely fused, suture clearly visible laterally; posterior corners as short rounded lobes (Fig. 1 A–B). Caudal rami with 4 terminal setae, 1 ventral seta (Fig. 1 E) and a small dorsal seta (Fig. 1 B, D–E).

Antennule exceeding caudal rami by distal segment. Right antennule geniculate (Fig. 2 A–C), 23-segmented, armature as follows: I–1 s + 1ae, II– IV–6 s + 4ae, V–2 s + 2ae, VI–2 s + 1ae, VII–2 s + 2ae, VIII–2 s + 1ae, IX–2 s + 2ae, X– XI–4 s + 3ae, XII–1 ae, XIII–1ae, XIV–1s + 1ae + 1 small sensilla, XV–1s + 1ae, XVI–2s + 1ae, XVII–2s + 1ae, XVIII–2s + 1ae, XIX–1s + 1ae + 1 attenuation, XX–1s + 1ae + 1 spine, XXI–1s + 1ae, XXII–XXIII–1s + 1 hook-like attenuation, XXIV–2s + 1ae, XXV–2s + 1ae, XXVI–2s, XXVII–XXVIII–4s + 1ae. Left antennule (Fig. 3 A–C) 24-segmented, differs in setation from right one in the following way: X– XI-3 s + 3ae + 1?, XII–1 small sensilla + 1ae, XIII–1 small sensilla + 1ae, XVII–1s + 1ae, XIX–1s + 1ae, XX–1s + 1ae, XXII–1s, XXIII–1s, 1 seta on segment 13 (ancestral segment XVI) is strongly curved on both sides (Figs 2 A, 3B).

Antenna (Fig. 4 A–B), coxa with 1 seta, basis with 2 setae, exopod 8-segmented with 0, 1, 1, 1, 1, 1, 1 and 3 setae (as 0, 0-0-1,1,1,1,1,1,3; the second proximal segment is a compound segment of 3 fused segments: comma represents an arthrodial membrane, dash represents absence of an arthrodial membrane, see Markhaseva and Ferrari 2005), about 1.5 times as long as endopod; first endopodal segment with 1 seta, second with 6 and 6 + 2 setae (14 setae in total).

Mandible (Fig. 4 C–E), gnathobase with 5 large and 3 small spine-like teeth plus dorsal seta; basis with 2 setae, exopod 5-segmented with 1, 1, 1, 1 and 2 setae; endopod segment 1 with 2 setae, segment 2 with 9 setae.

Maxillule (Fig. 4 F–G), praecoxal endite with 9 terminal spines, 1 anterior and 4 dorsal setae; coxal endite with 3 setae; proximal basal and distal basal endites with 4 setae each; endopod with 9 setae; exopod with 8 setae; coxal epipodite with 9 setae.

Maxilla (Fig. 5 A–C), proximal praecoxal endite with 5 setae; distal praecoxal endite with 3 setae; proximal coxal endite with 3 setae (1 worm-like sensory seta and 1 seta more sclerotized); distal coxal endite with 3 setae, 2 setae more sclerotized; proximal basal endite with 4 setae (2 more sclerotized, 2 worm-like, sensory); distal basal endite plus endopod with 8 sensory setae (3 longer, worm-like, 5 shorter, brush-like).

Maxilliped (Fig. 6A–C), syncoxa with 1 sclerotized seta on proximal praecoxal lobe, 2 setae on middle lobe and 3 setae (2 sclerotized, 1 poorly sclerotized) on distal praecoxal lobe; coxal lobe with 3 setae. Basis with 3 medial and 2 distal setae. Endopod 5-segmented with 4, 4, 3, 3+1 and 4 setae, 2 setae on segment 5 thicker and very long, about 1.6 times longer than basis.

P1 (Fig. 3 D–F), basis with medial distal seta strongly curved; endopod 1-segmented with lateral lobe ornamented with denticles and 4 pores; exopod segments 1, 2 and 3 with lateral spines of similar lengths; spine of exopod segment 1 slightly exceeding base of following spine, spine of exopod segment 2 not reaching base of following spine.

P2–P4 (Fig. 7) endo- and exopods slender; terminal spine of exopods distinctly longer than exopod segment 3.

P2 (Fig. 7 A), basis with row of 8–9 spinules distally on posterior surface; endopod 2- segmented, spinulated on posterior surface; exopod 3-segmented, lateral spines compact, short, of similar lengths; segment 1 with row of spinules on posterior surface; segment 2 with small spinules distally on anterior surface.

P3 (Fig. 7 B), basis with row of small spinules distally; endopod segments all spinulated on posterior surface, segment 2 with small spinules anterodistally; exopod segment 1 with row of spinules on posterior surface distally, segment 2 covered with small spinules anterodistally; segment 3 lateral spines compact, short, all subequal in length.

Basis of P4 (Fig. 7 C) with patch of spinules on posterior surface; endo- and exopod densely spinulated on posterior surface; endopod segment 2 and exopod segment 2 covered with small spinules on anterior surface distally.

P5 (Fig. 3 G–L), asymmetrical, right leg uniramous, left leg biramous; coxae fused, of similar size; left basis slightly longer than right. Right leg exopod 3-segmented; segment 1 with small, rounded projection in proximal half segment 2 with small spinule distally, segment 3 longer than preceding segment and tapering terminally. Left endopod 1- segmented, curved and longer than 2-segmented exopod, with small terminal spine. Left exopod segment 1 with large medial projection and small outer subdistal projection terminating in small spinule; exopod segment 2 structure complex (Fig. 3 I–L), with cavity opening terminally and rows of teeth and spinules along distal half of segment.

Etymology. The species name “ longiseta ” refers to two exceptionally long setae on the terminal segment of maxilliped.

Remarks. The male of S. longiseta has sensory setae on maxillary endopod, as do species in the clausocalanoidean Bradfordian families Diaixidae, Parkiidae (males unknown), Phaennidae, Scolecitrichidae and Tharybidae. It also shares with Bradfordian families the following plesiomorphies: setation patterns (1,2,3 setae) of maxilliped praecoxal endite and of the praecoxal endite of maxilla (five setae). However, S. longiseta is unique among the clausocalanoidean families in possessing another plesiomorphic calanoid character; the well developed geniculate right antennule.

Sensiava longiseta is distinguished from all Bradfordian males by the following combination of characters: i) the geniculated right antennule, ii) P5 with left leg biramous and armed with 2-segmented exopod, iii) two very long terminal setae on maxilliped endopod segment 5, which is an apomorphy for the genus.

The new genus differs from Phaennidae males in the setation of praecoxal endites of the maxilliped (in phaennid males a 1,2,2 setal pattern is observed, or numbers are reduced); in the composition of sensory setae on the maxilla endopod (in phaennids more than 5 brush-like sensory setae are displayed); and in P5, which has the left leg biramous (in phaennids uniramous) (Park 1983b; Bradford-Grieve 2004a).

Sensiava longiseta is distinguished from males of Scolecitrichidae s.s. (Markhaseva & Ferrari 2005): i) in setation of praecoxal endites of the maxilliped (in scolecitrichids there are 1, 2, 1 setae, or setal numbers reduced); ii) by 5 setae on the proximal praecoxal endite of maxilla (in scolecitrichids usually 3–4 setae); iii) a 23-segmented right and 24- segmented left antennule (due to fusion of segments, 18–21 segments in right and 19–21 in left antennule of scolecitrichid males (Grice & Hulsemann, 1970; Roe 1975; Park 1980, 1982, 1983a; Vyshkvartzeva 2001, 2003; Ohtsuka et al. 2002; Schulz 2005)); iv) in P5 bearing a left biramous leg with 2-segmented exopod (found also in Macandrewella A. Scott, 1909, all remaining scolecitrichids share a 3-segmented exopod) (Ohtsuka et al. 2002; Park, 1983a).

The new genus shows resemblance with some genera thought to belong to an ancestral Bradfordian lineage of genera. This group includes the following genera, for which the males are known: Brodskius Markhaseva & Ferrari, 2005, Cenognatha Bradford-Grieve, 2001, Diaixis Sars, 1902, Falsilandrumius Vyshkvartzeva, 2001, Neoscolecithrix Canu, 1896, Tharybis Sars, 1902, and Xantharus Andronov, 1981. Males are still unknown for the primitive genera Byrathis Markhaseva & Ferrari, 2005, Grievella Ferrari & Markhaseva, 2000 and Landrumius Park 1983. The composition of the ancestral Bradfordian group was proposed recently as a result of analysis considered as preliminary because assumptions about the transformations of character states and the order of transformation of different characters have yet to be applied to many Bradfordian genera (Markhaseva & Ferrari 2005: 163).

The new species shares with males of Brodskius, Diaixis, Cenognatha, Neoscolecithrix, and Xantharus, 24 segments on the left and 23 segments on the right antennule and differs in this character from Tharybis (with left antennule 23–24 and right 22–segmented) (Andronov 2002; Fosshagen 1972; Schulz & Kwasniewski 2004; Ferrari & Markhaseva 2005).

Sensiava longiseta differs from Brodskius in the 1,2,3 setation pattern of maxilliped syncoxa (0,2,3, in Brodskius). The setation pattern of maxilliped (1,2,3) is shared with males of Cenognatha, Neoscolecithrix, Tharybis, Xantharus and probably with Falsilandrumius, whose male is only poorly described. With Xantharus it also shares both segmentation and setation of the antennary exopod realized as a setal pattern of 0, 0-0- 1,1,1,1,1,1,3. Both Neoscolecithrix and Cenognatha deviate from this with a pattern of 1,1-1-1,1,1,1,1,1,3, while in Tharybis this formula is 0,0-0-0,1,1,1,1,1,3 (Fosshagen 1972; Ferrari & Markhaseva 2005; Ohtsuka et al., 2003; Schulz 1981; Schulz & Kwasniewski 2004; Vyshkvartzeva 2001; Markhaseva & Ferrari 2005). The new species differs from all these genera, however, in the lack of sensory setae on the praecoxal endites of maxilliped syncoxa, a feature it shares with males of some species of Tharybis, viz. T. pseudomegalodactyla Ferrari & Markhaseva, 2005 and T. juhlae Ferrari & Markhaseva, 2005, as well as with species of Diaixis (Andronov 2002), while the number of setae on praecoxal endites of maxilliped syncoxa in the males of the latter genus is reduced.

Sensiava longiseta has the number and composition of sensory setae on the maxilla endopod (three worm-like plus five brush-like sensory setae) in common with some species of Cenognatha, Tharybis and Xantharus, and shares eight setae on the maxillulary endopod with males of Xantharus.

The new species is distinguished from males of all the above mentioned genera in that the left exopod of P5 is 2-segmented (3-segmented in Brodskius, Cenognatha, Diaixis, Neoscolecithrix, Tharybis, and Xantharus; segmentation not proven for Falsilandrumius); it deviates from Diaixis in the possession of well developed oral parts (rudimentary oral parts in Diaixis), in two setae on the mandibular basis (three setae in Cenognatha, Neoscolecithrix, Tharybis, Xantharus) and in details of the maxillulary setation.

The new genus is preliminarily placed in the family Diaixidae sensu Markhaseva & Ferrari (2005) as this name is considered tentatively available to the lineage including the genera Byrathis, Cenognatha, Diaixis, Falsilandrumius, Landrumius, Neoscolecithrix and Xantharus.