Aphelochaeta bimaculata Blake 2018, new species

Aphelochaeta bimaculata new species

Figures 4–5

Chaetozone sp.: Hartman 1978: 166 (in part).

Material examined. Weddell Sea, E of Antarctic Peninsula. Glacier Sta. 69-11, 05 Mar 1969, 77.15°S, 38.97°W, 851 m holotype (USNM 1490704) and 12 paratypes (USNM 1490705).

Description. An elongate, threadlike species, with narrow pre-setiger region (Figs. 4 A–B, 5C–D), expanded anterior segmental region, becoming narrow again in posterior segments (Fig. 5A, D). All specimens incomplete; holotype narrow pre-setiger region becoming wider by setiger 3, then narrowing again by setiger 12 (Figs. 4B, 5 C–D), 6.25 mm long, 0.45 mm wide across peristomium, 0.87 mm wide across expanded anterior segments, with 31 setigers.

Body with distinctive pigmentation; all specimens with black spots on posterior lateral margin of peristomium (Fig. 4 A–B), one paratype with black pigment on lower lip of mouth; all specimens with black pigment spots on middle and anterior margins of parapodia (Figs. 4 A–B; 5A–B), variable between specimens, best developed on holotype on all segments, other specimens with these spots reduced or limited to middle body segments. All specimens with distinctive paired black spots within ventral groove on each segment (Fig. 5 A–B); sometimes with additional spots in groove (Fig. 5B). Overall color in alcohol, light tan, with black pigment spots conspicuous or faint.

Anterior expanded segments narrow, about 10 times wider than long (Fig. 4 A–B); posterior segments becoming longer (Fig. 5A, D). One posterior fragment with last setigers about as long as wide. Dorsum without groove, weakly rounded on posterior portion of each segment; venter with narrow shallow groove extending along entire body.

Pre-setiger region very long, narrow, about 2.2 times as long as wide (Figs. 4 A–B; 5C–D), relatively smooth. Prostomium conical to triangular, narrowing to rounded tip; eyespots absent; nuchal organs not observed. Peristomium elongate, weakly divided into three annular rings, with grooves observed only laterally, not crossing dorsum; with dorsal surface slightly elevated, but without distinct dorsal crest (Fig. 4A). Dorsal tentacles arising medially on posterior margin of peristomium (Fig. 4B). First pair of branchiae lateral to dorsal tentacles on peristomium; second pair of branchiae on setiger 1 in line with first pair and medial to notosetae (Fig. 4A). Branchial scars only evident on first 12–15 setigers.

Anterior parapodia with elevated longitudinal ridge or lamella posterior to origin of noto- and neurosetae (Fig. 4 A–B); from about setigers 12–15, parapodia reduced to low mounds from which setae arise. Setae all capillaries, numbering 6–8 per fascicle through about 15 segments, then reduced to 3–4 per fascicle to end of fragments. Noto- and neurosetae similar in number and length. All setae simple capillaries.

One posterior fragment narrowing posteriorly, but pygidium not present.

Methyl Green stain. Distinctive MG pattern: holotype with heaviest stain on prostomium and ventral lip of the mouth (Fig. 5C). Entire peristomium retaining stain with each annular ring denoted by grooves that do not stain; first two rings stain more intensely than third ring, which has a lighter speckled pattern. Anterior parapodia with strong staining reaction, extending across venter as distinct bands, variable in width and intensity (Fig. 5C); dorsal bands also present across anterior segments, but not as pronounced.

Etymology. The epithet, bimaculata, is derived from the Latin bis, for two, and macula, for spot; referring to the distinctive paired pigment spots along the venter.

Remarks. Aphelochaeta bimaculata n. sp. is unique in having a distinctive pattern of black pigment spots in various locations, but consistently with paired spots in the mid-ventral channel. In addition, the species has a prominent and distinctive MG staining pattern that, together with the pigmentation and the long, relatively smooth pre-setiger region, makes this species easy to recognize.

Aphelochaeta bimaculata n. sp. is similar to A. antelonga from Costa Rica and Chile (see above) in having a long, smooth, narrow pre-setiger region and having dorsal tentacles arising from the posterior margin of the peristomium and the first pair branchiae lateral to them. In A. bimaculata n. sp., however, the peristomium has three distinct annular rings with a narrow ring following the prostomium and a long second ring continuing to setiger 1; in contrast, A. antelonga has one or two thin lines that cross the peristomium, but no distinct grooves that divide the peristomium into separate rings. In addition, A. bimaculata n. sp. has a prominent MG staining pattern, whereas A. antelonga has only a weak staining response or none at all.

Distribution. Weddell Sea, east of the Antarctic Peninsula, 851 m.