Polaruschakov limaae Bonifácio & Menot 2019, SP. NOV.

POLARUSCHAKOV LIMAAE SP. NOV.

(FIG. 19A–G; TABLE 1, 2, 5)

Type material: Holotype, MNHN-IA-TYPE 1840 (IFR639- 1), complete, length 3.98 mm, width 0.60 mm, 18 segments (including tentacular segment), Equatorial Eastern Pacific Ocean, Clarion-Clipperton Fracture Zone, Ifremer license area, station 158, collected 15 April 2015, epibenthic sledge epi-net, start 14°3.411′N, 130°7.989′W, end 14°3.813′N, 130°6.481′W, 4946–4978 m depth, 3789 m trawling distance.

Description (based on holotype): Holotype complete, 3.98 mm long and 0.60 mm wide for 18 segments (including tentacular segment), dorsoventrally flattened, posteriorly slightly tapering; colour of live animal not known; ethanol-preserved specimen pale white, slightly translucent.

Prostomium bilobed, wider than long, anteriorly lobes not developed, conical; frontal filaments absent; median notch between prostomial lobes moderately wide and deep (Fig. 19A); eyes absent. Median and lateral antennae absent. Palps smooth, tapering into thin tips, short (reaching segment 3); palpophores not enlarged. Facial tubercle absent.

Tentacular segment fused to prostomium, with a pair of short lobes, inserted laterally and slightly below prostomium; without acicula or chaetae; tentaculophores distinct, bulbous, equal sized; styles smooth, tapering into thin tips, short (reaching segment 4), dorsal tentacular style slightly longer than ventral tentacular style (Fig.19A). Pharynx not everted. Second segment with elytrophores, subbiramous parapodia, chaetae and ventral cirri; lower lip and ventral tentacular segment ventrally lower in relationship to segment 3, which is inflated (Fig. 19B).

Nine pairs of distinct, bulbous to knob-like elytrophores present on segments 2, 4, 5, 7, 9, 11, 13, 15 and 17 (all elytra missing).

Cirrigerous segments with distinct, small dorsal cirrophores, inserted subdistally on notopodia; styles smooth, tapering into thin tips, long (slightly longer than tip of neuroacicular lobe); dorsal tubercles absent.

Segment 6 with very large (covering half of neuropodia in length), swollen dorsal structure (Fig. 19A), located between the dorsum and the base of cirrophores, interiorly whitish, of similar size.

Ventral cirri smooth, tapering into thin tips, present from segment 2 to last segment; inserted basally on neuropodia of segment 2, style long (about as long as tip of neuroacicular lobe); in subsequent segments inserted medially on neuropodia, style short (shorter than tip of neuroacicular lobe).

Parapodia subbiramous; notopodia reduced, much shorter than neuropodia (Fig. 19C). Notopodia narrow, subtriangular, tapering into very short acicular lobe, tip of notoacicula not penetrating epidermis. Neuropodia large, subtriangular, tapering into long acicular lobe, tip of neuroacicula not penetrating epidermis. Notochaetae very few (one or two observed), short, slender, slightly curved with distinct, faint spinous rows in convex side, with blunt tips (Fig. 19E); notochaetae more slender than neurochaetae. Neurochaetae of two types: (1) moderate in number (24 observed), short to long, distally flattened to concave, coarsely serrated along both margins, with abrupt pointed tips (Fig. 19F); and (2) middle group on segments 3–7 modified, few (four to six observed), slightly stouter, long, distally flattened to concave, serrated along both margins, with blunt tips (Fig. 19G).

Nephridial papillae present on segments 10–14, small, bulbous; internally whitish, subtriangular, originating from anterior margin of segment, extending onto posterior margin. Ventrally, on last segment, a pointed structure is present (Fig. 19D). Pygidium rounded, slightly enclosed by last segment; with terminal anus. Anal cirri lost, scars not seen.

Remarks: Only two species of Polaruschakov have been described until now: Polaruschakov polaris from the Polar Basin and north of the Canadian arctic islands (730–2245 m depth); and Polaruschakov reyssi Pettibone, 1976 from the Mediterranean Sea (750 m depth). Polaruschakov reyssi is the only species having notochaetae with capillary tips. Polaruschakov limaae sp. nov. is very close to Polaruschakov omnesae sp. nov. and Polaruschakov polaris in having notochaetae with blunt tips (Table 5). However, in Polaruschakov limaae sp. nov. there are two types of neurochaetae having abrupt pointed tips and blunt tips, whereas Polaruschakov omnesae sp. nov. has one type of neurochaetae with pointed tips, and Polaruschakov polaris has one type of neurochaetae with rounded tips. Moreover, the the average K2P distance among Polaruschakov lamellae sp. nov., Polaruschakov limaae sp. nov. and Polaruschakov omnesae sp. nov. was high (24.4–24.8% for 16S). The pointed structure on the last segment resembles a minute keel, but it is not clear whether this is an artefact of fixation or a natural structure.

Etymology: This species is dedicated to Dr Maria Lima for her friendship and for initiating P.B. into the systematics of polychaetes.

Genetic data: DNA sequencing for this species was successful for 16S and 18S but not for COI.

Distribution: Only one specimen was sampled at a single station within the Clarion-Clipperton Fracture Zone in Ifremer license area (type locality).