Aphelochaeta abyssalis Blake 2019, new species

Aphelochaeta abyssalis new species

Figure 1

urn:lsid:zoobank.org:act: DF88048E-3671-4189-B604-0429DC57FBBC

Tharyx sp. B: Wilson & Hessler 1987: Appendix E (in part).

Material examined. North Equatorial Pacific Ocean, abyssal plain, Clarion-Clipperton Fracture Zone, ECHO I, DOMES Site C, R/ V Melville cruise, coll. R. Hessler, 0.25 m 2 Sandia box core, Sta. H 358, 0–1 cm fraction, 23 Jun 1983, 14°42.1930′N, 125°24.2556′W, 4516 m, holotype (LACM-AHF Poly 11255); Sta. H 354, 1– 5 cm fraction, 18 Jun 1983, 14°41.8091′N, 125°24.2202′W, 4514 m, 1 paratype (LACM-AHF Poly 11256).— NOAA BIE Project site, Sta. DDT-4-93, veg. 13, 0–2 cm fraction, 11 Aug 1993, 12°55.595′N, 128°35.943′W, 4861 m, 1 specimen (USNM 1557529).

Description. A small, elongate, slender species, with body cylindrical in cross section and lacking dorsal and ventral grooves; all segments narrow, wider than long throughout with no obvious separation of body into defined regions except for anterior 10–12 segments bearing weakly developed parapodial shoulders (Fig. 1A); posterior segments not expanded (Fig. 1B). Holotype complete, 4.0 mm long, 0.275 mm across anterior segments, with about 73 setigerous segments. Color in alcohol opaque white; black peristomial pigment spots extending across venter posterior to ventral lip of mouth with a few spots extending dorsally to lateral margin of peristomium (Fig. 1A). Holotype with body wall of mid-body segments ruptured, intestinal lobes with fine silt particles emergent.

Prostomium triangular, merging with peristomium; narrowing anteriorly to pointed apex (Fig. 1A); eyespots absent; nuchal organs not observed. Peristomium as wide as long, with smooth dorsal and lateral surface; merging seamlessly with dorsal margin of setiger 1 (Fig. 1A); annular grooves not apparent except on ventral surface; dorsal crest absent. Dorsal tentacles arising from posterior margin of peristomium (Fig. 1A); first pair of branchiae on setiger 1 dorsal to notosetae; subsequent branchiae in similar position (Fig. 1A). Branchiae or their stubs and scars most apparent in anterior 10–15 setigers; a few observed in middle and posterior segments.

Parapodia reduced, with setae appearing to arise directly from body wall. All setae long capillaries providing a bristled appearance to body (Fig. 1 A–B). Notosetae numbering 6–10 capillaries including 1–2 long, natatory-like setae along most of body; neurosetae numbering 4–6 capillaries per fascicle along most of body.

Posterior 2–3 segments narrowing to conical pygidial lobe (Fig. 1B).

Methyl Green stain. No pattern, de-stains rapidly.

Etymology. The epithet is from the Latin abyssus, for bottomless pit or the deep sea and represents the occurrence of this species in abyssal depths of the ocean.

Remarks. Aphelochaeta abyssalis n. sp. differs from other congeners at the CCFZ in having a long, thin, body with no expanded segments and with all segments narrow but wider than long throughout. The peristomium is relatively short, smooth, and with no evidence of annular rings, grooves, or dorsal crest.

The presence of rows of transverse black pigment spots on the venter and lateral margin of the peristomium posterior to the lower lip of the mouth on A. abyssalis n. sp. is similar that found in A. dearborni Blake, 2018, a widespread Antarctic shelf species that ranges to a depth of about 1510 m and Aphelochaeta sp. 1, an undescribed deep-water continental slope species that occurs off northern California (Blake, unpublished). However, both of these latter species are larger, more robust cirratulids with expanded anterior segments, prominent peristomial rings or annuli, and an extra pair of branchiae on either the peristomium or the anterior margin of setiger 1 (Blake 2018; unpublished observations). See additional comparative comments with A. clarionensis n. sp. (below).

Distribution. Abyssal Pacific Ocean, 4514–4861 m.