Aphelochaeta wilsoni Blake 2019, new species

Aphelochaeta wilsoni new species

Figures 9–10

urn:lsid:zoobank.org:act: BA8C564D-1003-4DD6-A26B-CC8CBB8F2CC3

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

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

Chaetozone sp. A: Wilson & Hessler 1987: A: 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 354, 0–1 cm fraction, 18 June 1983, 14°41.8091′N, 125°24.2202′W, 4514 m, holotype (LACM-AHF Poly 11268); Sta. H 348, 0-1 cm fraction, 13 June 1983, 14°38.0720′N, 125°38.0720′W, 4504 m, 1 specimen (LACM-AHF Poly 11269); Sta. H 349, 0–1 fraction, 14 June 1983, 14°38.3925′N, 125°27.1791′W, 4517 m, 1 paratype (LACM-AHF Poly 11270); Sta. H 353, 0–1 fraction, 18 Jun 1983, 14°42.0879ʹN, 125°24.2679ʹW, 4516 m, 1 paratype (LACM-AHF Poly 11271); Sta. H 354, 0–1 cm fraction, 18 Jun 1983, 14°41.8091ʹN, 125°24.2202ʹW, 4514 m, 1 specimen (LACM-AHF Poly 11272); Sta. H-360, nodule wash, 17 Jun 1983, 14°40.7987′N, 125°22.0379′W, 4500 m, 1 specimen (LACM-AHF Poly 11273); Sta. H 361, 0–1 cm fraction, 18 Jun 1983, 14.7013°N, 125.4309°W, 4567 m, 1 specimen (LACM-AHF Poly 11274).— NOAA BIE Project site, coll. D.D. Trueblood, 0.25 m 2 Sandia box corer, Sta. DDT-7-93, veg. 12, 5– 10 cm, 02 Sep 1993, 122° 56.303′N, 128°35.311′W, 4844 m, 1 specimen (USNM 1557540); Sta. DDT-9-93, 03 Sep 1993, 12°56.280′N, 128°35.440′W, 4860 m, 1 specimen (USNM 1557541); Sta. DDT-10-93, veg. 7, 2– 5 cm, 03 Sep 1993, 12°56.127′N, 128°35.854′W, 4840 m, 1 specimen (USNM 1557542); Sta. DDT-3-94, veg. 8, 0–2 cm, 26 Jul 1994, 12°56.104′N, 128°35.523′W, 4854 m, 1 specimen (USNM 1557543).

Description. An elongate threadlike species, holotype (LACM-AHF Poly 11268) complete, 6.2 mm long, 0.18 mm wide across anterior segments, reduced to 80–100 µm wide across middle and posterior segments; with 31 setigers; paratype (LACM-AHF Poly 11270), complete, 3.9 mm long, ca. 100 µm across anterior segments. Both specimens with beaded or moniliform segments, with anterior and far posterior segments rounded, about as long as wide (Figs. 9A, 10A, C–D); middle segments elongate and narrow, three to four times as long as wide with parapodia and setae located in posterior one-fourth of these segments (Figs. 9 B–C, 10B). Anterior segments weakly expanded in holotype (Fig. 10A), with rest of body consistently narrow along length; smaller paratype with pre-setiger region slightly wider than following setigers (Fig. 9A). Dorsal and ventral grooves absent. Color in alcohol, opaque white.

Pre-setiger region elongate, relatively smooth with no evidence of annular rings, about three times as long as wide (Figs. 9A, 10A). Prostomium short, conical, rounded on anterior margin; eyespots absent; nuchal organs not observed. Peristomium lacking annular rings, merging seamlessly with setiger 1 (Fig. 9A); dorsal tentacles arising from posterior margin of peristomium, first pair of branchiae immediately posterior to dorsal tentacles (Fig. 9A). Second pair of branchiae arising dorsal to notosetae on setiger 1; subsequent branchiae in same position on following segments (Fig. 9A); branchiae present on middle and some posterior segments.

Noto- and neuropodia close together along entire body, individual parapodia simple mounds on posterior margin of anterior and posterior segments from which setae arise; parapodia of middle body more elongate (Fig. 9B), sometimes pointed, extending laterally from posterior end of each segment from which setae arise. All setae capillaries, with those of anterior and middle segments long, but relatively simple, with narrow shaft gradually tapering to fine tip (Fig. 9D); with about 6–7 capillaries per fascicle, many notopodia having 1–2 long natatory-like capillaries (Fig. 10 A–B). Far posterior setigers with 2–3 short, bent neurosetae, those of posterior-most segments shorter, with thick, curved shafts, appearing spinous (Figs. 9 E–F, 10D) but tapering to short, finely pointed tip; notosetae of same segments including 2–3 longer, thin capillaries.

Pygidium a simple disk-like lobe ventral to anal opening (Fig. 10 C–D).

Methyl Green stain. Stain somewhat concentrated over first 4–5 anterior segments, but no stain retained over rest of body; no pattern.

Etymology. This species is named for Dr. George D.F. (Buz) Wilson, prominent deep-sea biologist and isopod specialist who has written extensively on the benthic biology of the Clarion-Clipperton Fracture zone.

Remarks. Aphelochaeta wilsoni n. sp., like most of the abyssal cirratulids encountered in this study, has an elongate, threadlike body. Although a few of the anterior segments are crowded in some specimens, the individual segments are generally moniliform in shape along the entire body, with those of middle segments becoming long and pellet-shaped before rounding up again in the far posterior segments. This is in contrast to the type-species, A. monilaris (Hartman, 1960), from off California and A. hormosa Blake, 2018 from Antarctica that have distinct moniliform segments limited to middle body segments, and the anterior and posterior segments forming expanded regions of narrow, crowded segments.

Unlike other species of Aphelochaeta, A. wilsoni n. sp. has a few short, heavy-shafted capillary neurosetae in the posterior-most, pre-pygidial segments. These heavier shafted capillaries suggest a modification of posterior segments similar to species of Chaetozone and Tharyx where acicular spines of various kinds and size are present. However, these thicker curved capillaries in A. wilsoni n. sp. exhibit no other modification that would suggest they belong to the other genera.

Distribution. Abyssal Pacific Ocean, Clarion-Clipperton Fracture Zone, 4500–4860 m.