Chaetozone akaina, Blake, 2019

Blake, James A., 2019, New species of Cirratulidae (Annelida, Polychaeta) from abyssal depths of the Clarion-Clipperton Fracture Zone, North Equatorial Pacific Ocean, Zootaxa 4629 (2), pp. 151-187 : 171-174

Chaetozone akaina new species

Figures 13–14

urn:lsid:zoobank.org:act:ADCA83C0-FA5B-4957-BCC3-5EAE8EE292B9

Chaetozone sp. B: Wilson & Hessler 1987: 66 Appendix E.

Material examined. North Equatorial Pacific Ocean, abyssal plain, Clarion-Clipperton Fracture Zone.

NOAA BIE Project site, coll. D.D. Trueblood, Sandia box corer, Sta. DDT-5-94, veg. 20, 2– 5 cm fraction, 27 Jul 1994, 12°55.798′W, 128°35.769′N, 4879 m, holotype ( USNM 1557544 ).— ECHO I, DOMES Site C, R/V Melville cruise, coll. R. Hessler, Sandia box corer , Sta. H349C, 1–5 cm fraction, 14 Jun 1983, 14°38.3925ʹN, 125°27.1791ʹW, 4517 m, 1 paratype ( LACM-AHF Poly 11277) ; Sta. H350C, 0–1 cm fraction, 14 Jun 1983, 14°38.1226ʹN, 125°26.8208ʹW, 4506 m, 1 paratype ( LACM-AHF Poly 11278) ; Sta. H356T, 1–5 cm fraction, 21 Jun 1983, 14°42.4541ʹN, 125°24.2664ʹW, 4518 m, 1 paratype ( LACM-AHF Poly 11279) ; Sta. H357, 0–1 cm fraction, 23 Jun 1983, 14°42.1404ʹN, 125°24.2787ʹW, 4480 m, 1 juvenile ( LACM-AHF Poly 11280) ; Sta. H362C, 0–1 cm fraction, 18 Jun 1983, 14.7013°N, 125.4309°W, 4480 m, 1 paratype ( LACM-AHF Poly 11281).

Description. An elongate, slender species with narrow pre-setiger region, gradually widening to about setiger 15 ( Figs. 13A , 14A ), then narrowing. Holotype complete in two parts, 9.67 mm long, widest 0.2 mm at about setiger 10–12, far posterior segments narrowing to 0.07 mm wide, with 50 setigerous segments. Complete paratype (LACM-AHF Poly 11279) 4.5 mm long with 35 setigers; incomplete paratype (LACM-AHF Poly 12281) 5.8 mm long with 27 setigers. Body generally cylindrical in cross section, posterior third of body with prominent armature consisting of heavy spines and capillaries forming partial cinctures ( Fig. 13B ). Large heart body visible in anterior segments ( Fig. 14A ). Color in alcohol light tan.

Pre-setiger region elongate, narrow, about as long as first five setigers ( Figs. 13A , 14 A–B). Prostomium short, conical with rounded apex; eyespots absent; nuchal organs not observed. Peristomium divided into two weakly developed annular rings, grooves not prominent, best seen laterally; both rings equivalent in length, first ring widest, merging with narrow second ring ( Figs. 13A , 14B ); dorsal surface smooth, dorsal crest absent; dorsal tentacles arising about mid-way along second ring ( Figs. 13A , 14B ); first pair of branchiae located immediately posterior to dorsal tentacles near boundary with setiger 1. Setiger 1 weakly set off from peristomium; second pair of branchiae arising in line with first pair, dorsal to notosetae; subsequent branchiae in same location ( Figs. 13A , 14B ). Branchiae or their stubs evident for only 8–10 setigers, not observed along rest of body.

Parapodia of anterior and middle segments reduced, with setae arising from low mounds or directly from body wall ( Fig. 13A ); no parapodial shoulders present in anterior segments. Anterior segments with noto- and neuropodia close to one another, then becoming farther apart in middle segments and closing up again posteriorly. Posterior segments with parapodia becoming elevated, forming prominent cinctures of spines ( Fig. 13B ). Anterior segments with prominent fascicles of long capillaries, a few natatory-like notosetae in anteriormost segments. Acicular spines first present in neuropodia from setiger 3 on all five type specimens; initially a single spine accompanied by 4–8 capillaries ( Fig. 14D ), increasing to two and then three spines in middle segments and 3–4 spines in far posterior segments; acicular spines first present in notopodia from setiger 6–7, following similar pattern of numbers as neuroacicular spines. Posterior cinctures each with 3–4 spines in noto- and neuropodia ( Figs. 13B , 14E ), with up to 6–8 spines on a side; fewer spines on smallest specimens. Spines accompanied by a variable number of long, thin capillaries, usually alternating with spines. Individual spines thick, with weakly curved to pointed apex; core of spine relatively clear ( Fig. 14F ); spines unusually large and conspicuous for such a thin, elongate worm ( Figs. 13 B–E, 14C–E).

Posterior end terminating in pygidial segment with a flattened dorsal lobe and broad ventral disk below anal opening ( Figs. 13B , 14C ).

Methyl Green stain. No obvious pattern, stain absent from pre-setiger region and retained more or less uniformly on anterior and middle segments. Some stain appearing concentrated within intersegmental grooves of some segments and across venter as a band on a few segments, but pattern not prominent.

Etymology. The epithet is from the Greek, akaina , f., for thorn or spine, referring to the large acicular spines found on this small species.

Remarks. Like other bitentaculate cirratulids discovered in these abyssal samples, Chaetozone akaina n. sp. has an elongate, narrow body; however, this species does have a slight widening of anterior segments. Unique characteristics include the long pre-setiger region, which is as long as the first five setigers and about 2.5 times as long as wide, and an unusual pygidium with a narrow dorsal lobe and a large broad ventral disk. The most obvious character however is the large size of the acicular spines. These begin in setigers 3 and 6–7 in the neuro- and noto- podia, respectively; other species of Chaetozone at the CCFZ have acicular spines beginning in middle or posterior segments. The posterior cinctures of these spines are far more conspicuous than in other species encountered and occur over many more segments.

The parapodia of some anterior and middle parapodia are widely separated, reminiscent of species of Caulleriella . However, none of the spines show any evidence of being bidentate and the posterior spines are clearly arranged into reduced cinctures. Chaetozone armata Hartman, 1963 from California shelf depths has widely separated parapodia throughout, but in that species the neuropodial hooks number 3–4 per setiger in anterior setigers, then are reduced to a single spine in posterior segments ( Blake 1996). This species was recently transferred to Caulleriella because some bidentate hooks were observed in small specimens and appear to be abraded in larger specimens ( Blake & Magalhães 2019).

Distribution. Abyssal Pacific Ocean, 4480–4880 m.

Figures

FIGURE 13. Chaetozone akaina n. sp. Holotype (USNM 1557544): A, Anterior end, dorsal view; B, posterior end, dorsal view; C, notopodial acicular spine from posterior setiger; D–E, neuropodial acicular spines from posterior setiger.

FIGURE 14. Chaetozone akaina n. sp. Holotype (USNM 1557544): A, anterior one-third of body, dorsal view; B, anterior end, dorsal view; C, posterior end, dorsal view. Paratype (LACM-AHF Poly 11279): D, neuropodial acicular spines from anterior segments; E, posterior noto- and neuropodial acicular spines; F, neuropodial acicular spine. Stained with Shirlastain A.