Chaetozone adusta, Blake, 2023

Blake, James A., 2023, New Species of Cirratulidae (Annelida) from Continental Slope and Abyssal Depths off Eastern Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (3), pp. 249-270 : 262-265

Chaetozone adusta sp. nov.

urn:lsid:zoobank.org:act:ED16634E-8008-4EC0-9029-6109605A7754

Figs 9–10

Holotype: Abyssal plain off southeastern Victoria, eastern Australia, East Gippsland Marine Park, RV Investigator, Sta. 033, coll. 24 May 2017, Brenke sledge, 38.521°S 153.213°E to 38.498°S 150.207°E, 4107 m ( AM W.52709).

Description. A moderately-sized species, holotype only specimen, incomplete, broken in two pieces, with 39 setigers, 8.4 mm long, 0.38 mm wide across setiger 1 and 0.80 mm across widest thoracic segments. Body with narrow pre-setiger region and about first eight setigers then expanding ( Fig. 10A ) and becoming widest over setigers 14–20, then narrowing posteriorly ( Fig. 10A ). All anterior and middle setigers short; anterior setigers about 4.5 times as wide as long; widest middle setigers about ten times wider than long; posterior setigers narrowing ( Fig. 10A ), becoming moniliform, about as wide as long ( Fig. 10D ). Simple ventral groove present from anterior border of setiger 1 ( Fig. 9B ); in middle setigers, groove joined by a conspicuous mid-ventral swelling at junction of each segment on venter. Colour in alcohol light tan; body with areas of dark pigment along most of body, mostly concentrated in intersegmental areas and as parapodial bands ( Fig. 10A–C arrows); some branchiae pigmented.

Pre-setiger region about as long as first five setigers, consisting of a long, narrow prostomium, followed by a bulbous first peristomial ring and smooth second ring ( Fig. 9A ). Prostomium tapering anteriorly to a narrow tip ( Figs 9A–B , 10A–B ); eyespots absent; nuchal organs visible in a pit on dorsolateral margin ( Fig. 10A ). Peristomium with first bulbous ring merging with prostomium dorsally ( Figs 9A , 10A–B ) and forming anterior lip of mouth ventrally ( Fig. 9B ); second peristomial ring rectangular, merging seamlessly with setiger 1 dorsally ( Fig. 9A ), forming posterior lip of mouth ventrally, then merging with setiger 1 posteriorly ( Fig. 9B ). Three ciliated oral lobes observed within mouth opening ( Fig. 9B ). Dorsal tentacles arising from middle of second peristomial ring with first pair of branchiae arising posterior to tentacles on posterior margin ( Fig. 9A ). Second pair of branchiae on posterior border of setiger 1 dorsal to notosetae; subsequent branchiae in similar position; most branchiae missing with location observed as scars or stubs along most of body.

Parapodia of anterior setigers reduced to low mounds from which setae arise; middle expanded setigers with parapodia more prominent, narrow, forming lateral lobes; posterior segments again with parapodia reduced.Anterior setae all long capillaries numbering about 7–9 per fascicle, including long natatory-like setae in most notopodia of anterior and middle segments. Acicular spines from setiger 24 in neuropodia and setiger 31 in notopodia. Spines numbering five in neuropodia and three in notopodia in setiger 38 or with eight spines on a side, but not as cinctures; however, nature of far posterior segments and spines unknown. Spines accompanied by 1–2 capillaries in neuropodia and 3–4 in notopodia. Individual neuropodial spines with basal manubrium near emergence from podial lobes, thick, weakly curved, tapering to narrow pointed tip ( Fig. 9D ); notopodial spines longer and straighter than neuropodial spines ( Fig. 9C ).

Pygidium unknown.

Methyl Green staining. No pattern.

Remarks. Chaetozone adusta sp. nov. is easily recognized by the nature of the two peristomial rings of which the first is enlarged and bulbous and the second is smooth and merges with setiger 1 as well as the narrow anterior segments that transition to a distinctly enlarged middle section that then narrow to moniliform posterior segments. In addition, the body is distinctly pigmented with brown areas along most of its length, including distinct bands on the anterior borders of individual anterior and middle body segments.

Chaetozone adusta sp. nov. is most similar morphologically to two other deep-water species: Chaetozone grasslei Blake, 2019 from the abyssal Pacific Ocean and Chaetozone lophia Blake, 2022 from the U.S. Atlantic slope in bathyal depths. Both of these species also have two peristomial rings of which first is bulbous and the second is smooth. Chaetozone grasslei differs from C. adusta sp. nov. in having a long, narrow, threadlike body consisting of segments that are mostly rounded or moniliform instead of moniliform segments being limited to posterior setigers. Chaetozone lophia species differs from C. adusta sp. nov. by having instead of lacking a distinct dorsal crest on the bulbous first peristomial ring. In addition, while the new Atlantic species has an enlarged middle body region, this is the result of the intestine having intestinal loops that form a “stomach” packed with sediment.

The only other deep-water species with extensive body pigment is Chaetozone brunnea Blake, 2006 from the U.S. Pacific coast off northern California. The California species however is an entirely different kind of Chaetozone , in having two peristomial rings, but with the first tightly merged with the prostomium and the second likely being an achaetous segment where the dorsal tentacles arise anteriorly on that segment instead of at the posterior margin where the branchiae occur. In addition, C. brunnea has full cinctures with an armature of up to 16 acicular spines on a side. Another unusual characteristic of C. brunnea is the presence of a prominently enlarged “stomach” at the juncture of the anterior and abdominal segments, which in some specimens results in a distinct twist to the body.

Etymology. The epithet is from the Latin, adustus, for brown or swarthy, in reference to the brown pigment that characterizes this species.

Distribution. Abyssal plain off southeastern Victoria, eastern Australia, 4107 m.

Figures

Figure 9. Chaetozone adusta sp. nov. (A–D) holotype (AM W.52709). (A) anterior end, dorsal view; (B) pre-setiger region, ventral view; (C) notoacicular spine; (D) neuroacicular spines.

Figure 10. Chaetozone adusta sp. nov.Photomicrographs.(A–D) holotype (AM W.52709).(A) anterior and middle body segments, dorsal view, brown pigment emphasized; (B) anterior end, dorsal view, arrows denoting brown pigment; (C) middle body segments showing brown pigment; (D) posterior moniliform segments, lateral view.All stained with Shirlastain A; arrows indicate pigment.