Aphelochaeta clarionensis Blake 2019, new species

Aphelochaeta clarionensis new species

Figure 2

urn:lsid:zoobank.org:act: 10B1C7E1-C673-48C3-B784-1168C5BE2C20

Tharyx sp. C: 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 11257); Sta. H 356, top water from box core, 12 Jun 1983, 14°42.4541′N, 125°24.2664′W, 4518 m, 1 paratype (LACM-AHF Poly 11258).

Description. A moderately sized species, holotype complete, in two parts, collectively measuring 11 mm long, 0.52 mm across anterior segments, 0.48 mm across posterior segments, with 81 setigerous segments. Body more or less cylindrical throughout with dorsum low, rounded and venter weakly flattened along entire body; with midventral longitudinal ridge line evident at ventral midpoint of each segmental ring. All body segments narrow, wider than long; anterior segments crowded, about seven times wider than long (Fig. 2A); posterior segments about three times wider than long (Fig. 2B). Color in alcohol light tan; pigment spots absent.

Prostomium short, semi-circular, broadly rounded on anterior margin; eyespots absent; nuchal grooves narrow slits on posterior lateral margin. Peristomium longer than wide, narrowing anteriorly, divided into two annular rings (Fig. 2A); first anterior ring shorter and narrower than posterior ring; second ring about 2.5 times longer than first ring, with dorsum elevated into an oval-shaped crest (Fig. 2A); dorsal tentacles arising from posterior margin at anterior border with setiger 1 (Fig. 2A). First pair of branchiae arising posterior to dorsal tentacles on setiger 1, dorsal to notosetae; subsequent branchiae occurring in same position on following setigers (Fig. 2A). Branchiae mostly missing, limited to stumps or scars but occurring along entire body, short when present.

Parapodia reduced, with setae arising directly from body wall; anterior parapodia not producing shoulders. Setae all long capillaries with extra-long natatory-like notosetae present along most of body. Anterior notosetae numbering 12–14 per fascicle; neurosetae 8–10 per fascicle.

Pygidium with a short lobe ventral to anal opening (Fig. 2B).

Methyl Green stain. Stain producing pattern in pre-setiger region. Posterior half of prostomium stained, followed by two unstained areas on anterior of first peristomial ring; posterior half of first ring with stained wedge producing a clear area in first half of ring behind stain on prostomium. Second peristomial ring with two diffuse bands extending laterally and ventrally. Rest of body staining lightly with no pattern.

Etymology. The name clarionensis is taken from the first part of the name of the Clarion-Clipperton Facture Zone where the species was collected.

Remarks. By having all body segments narrow, but distinctly wider than long, and with the first branchiae arising from setiger 1, Aphelochaeta clarionensis n. sp. is most similar to A. abyssalis n. sp. found in the same and adjacent samples. Aphelochaeta clarionensis n. sp., however, differs from A. abyssalis n. sp. in having the prostomium short and rounded instead of longer, triangular and pointed apically, and having the peristomium divided into two distinct annular rings with a large prominent dorsal crest instead of the peristomium being smooth with no annular rings or dorsal crest. In addition, A. clarionensis n. sp. has a MG pattern in the pre-setiger region instead of no staining pattern at all.

Globally, most species of Aphelochaeta are known from intertidal zones or shelf depths and have the first pair of branchiae arising from the peristomium lateral to or in close proximity with the dorsal tentacles or, if the first branchiae do arise from setiger 1, there is usually a second pair of branchiae present on the same setiger (Blake 1996, 2018; Dean & Blake 2016; Choi et al. 2018; Blake & Magalhães 2019). Further, for most of these shallow-water species, the posterior end is enlarged or expanded and moniliform segments are often present. Thus, A. abyssalis n. sp. and A. clarionensis n. sp. differ from the majority of known Aphelochaeta species, including other abyssal species described in this study, by the arrangement of the anterior branchiae and by having narrow segments present along the entire body.

Distribution. Abyssal Pacific Ocean, 4516–4518 m.