Levinsenia longobranchiata n. sp.

(Figures 15–18)

Levinsenia longobranchiata n. sp.: urn:lsid:zoobank.org:act: F2E7D547-2182-455B-84B6-4ACD19250C3

Material examined. Holotype: ESFM-POL/2013-1030, 16 June 2013, Y27, 40°23’30’’N, 29°03’23’’E, 50 m, black mud, 1 specimen; Paratypes: ESFM-POL/2013-1033, 16 June 2013, Y27, 40°23’30’’N, 29°03’23’’E, 50 m, black mud, 3 specimens; ESFM-POL/2013-1043, 17 June 2013, Y28, 40°29’15’’N, 28°51’09’’E, 50 m, muddy sand with pebbles, 5 specimens; ESFM-POL/2013-1054, 20 June 2013, Y41, 40°43’18’’N, 29°24’58’’E, 100 m, mud, 2 specimens; ESFM-POL/2013-1054, 20 June 2013, Y42, 40°45’43’’N, 29°29’39’’E, 50 m, muddy sand with pebbles, 1 specimen .

Description. Holotype incomplete, 5.5 mm long, 0.21 mm wide for 50 chaetigers; all paratypes incomplete, 3.34–12.15 mm long, 0.10–0.21 mm wide for 25–83 chaetigers. Color in alcohol dull white (Fig. 16A). Body slender, fragile; anterior part of body slightly wider and anterio-dorsal side slightly swollen in prebranchial chaetigers; prebranchial and branchial chaetigers nearly of same width; posterior part of body filiform and spiral (Figs 15A; 16A; 18A).

Prostomium conical, longer than wide (ratio length / width: 1.27); anterior part of prostomium somewhat truncated, with an eversible palpode, without eyes (Figs 15B; 16B; 17A). A pair of nuchal organs as narrow, deep short slits, vertically placed on dorso-lateral sides of posterior part of prostomium; with scarce internal cilia; without pigmentation. (Figs 15 A–B; 16A–B; 17A). Cilia patches and lateral organ absent on lateral sides of prostomium. Cheek organ present, located on lateral sides of prostomium, retractile, rounded in shape (Figs 15 A–C; 16B; 17A).

Peristomium indistinct on dorsal and lateral sides, only discernable under SEM, partly fused with anterior margin of chaetiger 1 on dorsal side. Mouth with two buccal lips; one placed anteriorly and one placed posteriorly, extending to anterior margin of chaetiger 1 (Fig. 15C).

Short dorsal ciliary bands (sdcb) present, located just ventral to each branchial base, arranged as a transversal line (Fig. 17C); ciliary bands absent on ventral side of body.

Branchiae 12 pairs in holotype, 10–13 pairs in paratypes, starting on chaetiger 6 (in holotype and paratypes); very long (two and a half times the segment width), flattened dorso-ventrally, with a rounded tip; relatively short on chaetigers 6–7, getting longer to middle and posterior parts of branchial region, shortest on last branchiferous chaetiger; dense ciliary bands present on both sides of outer margin of branchiae to pre-tip region (Figs 15A; 16A, C–D; 17C–D); 285 µm long in anterior part, ca. 385 µm long in middle part, ca. 292 µm long in posterior part of branchial region; varying between 258–441 μm long in paratypes. Notopodia fused with branchiae.

Interramal lobes and notopodial papilla absent. Notopodial postchaetal lobes absent on prebranchial chaetigers (Figs 15B; 17A, B); starting on chaetiger 6 (branchial region), emerging on branchiae, very short, bottle shaped; in postbranchial region similarly shaped, (Figs 15 A–B; 16C, E; 17A–D; 18B). Neuropodial postchaetal lobes absent.

Lateral sense organs present on all chaetigers, located between notopodia and neuropodia, just posterior to notopodial postchaetal lobes; more or less elliptical with irregular clustered pores from prebranchial region to end of body (Figs 17 B–C; 18B); with 4–5 pores in prebranchial region (long axis of organ: 2.5–3 μm), with 5–6 pores (long axis: 2–3.5 μm) in branchial region, with 11–12 pores (long axis: 5–5.5 μm) in postbranchial region; with flexible cilia distinctly protruding from opening or embedded into pore.

Three types of chaetae present on chaetigers: limbate, capillary and modified neurochaeta (Figs 17 A–B, D; 18B).

Limbate chaetae of one type; present on both notopodia and neuropodia of chaetigers 1–5, long, thin and straight with fibrils along edge (hirsute), positioning straight, colorless; in notopodia, numbering 3–8, arranged in two rows, ca. 86 µm long; in neuropodia, numbering 3–10, arranged in two rows, ca. 83 µm long.

Capillary chaetae starting from noto- and neuropodia of chaetiger 6 to posterior-most chaetiger; in middle notopodia, numbering 3–6 in each ramus, arranged in two rows and ca. 90 μm long; in posterior notopodia, numbering 2–3 in each ramus, arranged in one row and ca. 30 μm long; in middle neuropodia, numbering 5–9 in each ramus, arranged in two rows and ca. 160 μm long; in posterior neuropodia, numbering 3–5, arranged in one row and ca. 44 μm long.

Modified neuropodial chaetae present from chaetiger 23 to end of body (from 20–25 in paratypes), numbering 4–8 per fascicle, arranged in one row, superior chaetae much longer (about 45 µm), inferior chaetae much shorter and curved; hook- shaped with longitudinal stripes, slightly curved, with distinct, fibrillary hood on concave side, tip naked and abruptly tapered (Figs 15D; 16 E–G; 18B–E)

Pygidium absent in holotype (also in all paratypes).

Parasite. Unidentified parasites were observed on the branchiae and body surface (Fig. 16D).

Reproduction. Holotype had sperm packages in its coelomic cavity, which first appeared on chaetiger 31 and were present in all subsequent chaetigers.

Remarks. Levinsenia longobranchiata n. sp. is mainly characterized by having very long branchiae (two and a half times longer than the segment width), by the absence of notopodial postchaetal lobes on the prebranchial chaetigers, and by the notopodia being completely fused with the branchiae.

Within the genus Levinsenia, only L. multibranchiata, which was originally described from the Santa Barbara Channel Basin (California, Pacific Ocean) at 475–557 m depth by Hartman (1957) and subsequently reported by Strelzov (1979), Blake (1996) and Lovell & Fitzhugh (2020), shares the presence of branchiae at least twice longer than the body width. Levinsenia longobranchiata n. sp. is also similar to L. gracilis japonica (Imajima, 1973) and L. kosswigi Çinar, Dağlı & Açik, 2011, but these species differ from each other in the following characters; (1) Notopodial postchaetal lobes: absent in the prebranchial region, and short, bottle-shaped in the branchial and posterior regions in L. longobranchiata n. sp.; short, digitiform in the branchial region in L. gracilis japonica; present in all body regions, short and rounded in the prebranchial region, long and digitiform with a rounded joint on tip in the branchial region, short, conical in the posterior region and filiform in the preanal region in L. kosswigi; present in all body regions, short and conical in the prebranchial region, digitiform in the branchial region in L. multibranchiata (2) Branchiae: elongate, two and a half times longer than the segment width, 10–12 pairs and the last pair always shorter than the others in L. longobranchiata n. sp.; elongate, lanceolate, as long as the segment width with an acute tip, 22 pairs and the last pairs always shorter than the others in L. gracilis japonica; conical, distinctly shorter than the segment width, 17–21 pairs, the last pairs shorter than the others in L. kosswigi; filiform, almost twice as long as segment width, 20–38 pairs, and the last pair not shorter than the others in L. multibranchiata .

Etymology. The species name refers to its long branchiae.

Habitat and Distribution. This species was found in soft substrata between 50 and 100 m from the Sea of Marmara.