Chaetocirratulus epitocus Blake 2018, new combination

Chaetocirratulus epitocus (Monro, 1930), new combination

Figures 31–32

Tharyx epitoca Monro, 1930: 157–158, fig. 62; Hartman 1953: 47–48; 1966: 31–32, Plate VIII, fig 4.

Not: Tharyx epitoca: Lowry 1975: 9–10, 12; Richardson & Hedgpeth 1977: Table 2; Cantone & Sanfilippo 1992: Table 1; Siciński & Janowska 1993: 161–166; Cantone 1994: 40; Gambi et al. 1997: Table 1; Cantone et al. 2000: 554; Pabis & Siciński 2010: Table 2.

Not Aphelochaeta epitoca: Cantone & DiPietro 2001: Table 1; Parapar et al. 2011: 728.

Not Aphelochaeta cf. epitoca: Hilbig 2001: 540; Montiel et al. 2005: 197, 2016: Appendix 2 (in part).

Material examined. Antarctic Peninsula. Bismarck Strait, R / V Discovery Sta. 190, coll. 24 Mar 1927, 64.933°S, 65.583°W, dredged 93–126 m, stones, mud and rocks, lectotype (BMNH 1930.10.8.2116).— Schollaert Channel, R / V Discovery Sta. 181, coll. 12 Mar 1927, 64.333°S, 63.333°W, Otter trawl, 160–335 m, mud, paralectotype (BMNH 1930.10.8.2089).

Description. Both specimens elongate, with thick, robust bodies, weakly fusiform, thickest in middle segments, narrower anteriorly and posteriorly (Fig. 32A). Lectotype complete, 19 mm long, 1 mm wide across anterior setigers, 1.5 mm wide across middle segments, with 50 setigers; paralectotype also complete, but prostomium to end of setiger 2 separated from rest of body; in total measuring 24.5 mm long, with 50 setigers. Body generally cylindrical in cross section; dorsal groove absent; weakly developed mid-ventral line extending along most of body. Individual segments best defined laterally; segmental grooves not crossing dorsum or venter.

Pre-setigerous region elongate, narrow, relatively smooth, as long as first four setigers (Figs. 31A, 32 A–C). Prostomium broadly triangular, narrowing to rounded tip; eyespots absent; nuchal organs prominently everted oval mounds in lectotype at posterior lateral margin (Fig. 31A). Peristomium elongate, about as long as first three setigers; dorsally with long smooth anterior section followed by narrow section with narrow elongate grooves bearing pair of widely separated dorsal tentacles (Fig. 31A); ventrally, peristomium forming upper and lower lips of mouth (Fig. 32C). First pair of branchial scars lateral to dorsal tentacles on posterior section of peristomium (Fig. 31A). Second pair of branchiae on setiger 1 dorsal to notosetae (Fig. 31A); branchiae of subsequent segments in same location.

Parapodia low mounds or tori from which setae emerge. Noto- and neurosetae of type specimens with capillaries of normal length and long, natatory capillaries along most of body. Last 4–6 setigers with 1–2 unidentate spines in both noto- and neuropodia; each spine simple, weakly curved at tip, colorless (Fig. 31C).

Pygidium with curved rounded lobe ventral to anal opening bordered with small papillae (Figs. 31B, 32D).

Methyl Green stain. Not retaining stain.

Remarks. The syntype from the Bismarck Strait is here designated the lectotype; the second syntype from the nearby Schollaert Channel is designated the paralectype. In the original description, Monro (1930) stated that spines or hooks were absent in this species. However, 1–2 short, straight unidentate spines are present in the last 4–6 noto- and neuropodia of both specimens. These spines are small and cannot be seen clearly among the capillaries unless the specimens are mounted under a coverslip with glycerin. Both type specimens are mature females swollen with eggs that measure 135–150 µm in diameter.

In recent benthic surveys, specimens having all capillaries and believed to be this species have been identified in the genus Aphelochaeta. Because this species has been found to have acicular spines in posterior setigers and a different morphology to the peristomial region, the concept of the species is changed from what was originally described by Monro (1930) and as restated by Hartman (1966). Because of this, recent identifications of Aphelochaeta epitoca (or as Tharyx) most certainly refer to other species.

By having the spines limited to posterior setigers, Chaetocirratulus epitocus is similar to C. andersenensis. However, the two species differ in the nature of the pre-setigerous region. In C. epitocus, the pre-setigerous region is long and narrow and consists of a smooth anterior section and a narrow lobate posterior section that may be a distinct achaetous segment, and which bears both the dorsal tentacles and first pair of branchiae. In C. andersenensis, the pre-setigerous region is short, about as wide as long, and has three distinct annular peristomial rings. The dorsal tentacles and first pair of branchiae are located at the posterior margin of the last peristomial ring. In addition, most specimens of C. andersenensis have distinctly fusiform bodies with obvious segmental grooves. In contrast, C. epitocus, while somewhat expanded in the middle segments, has a thick elongate body where the segmental grooves are either lacking or obscure. Further, C. andersenensis has a distinct MG staining pattern in the pre-setigerous region that is entirely absent in C. epitocus.

Distribution. Antarctic Peninsula, 93– 335 m.