Aphelochaeta magellanica Blake 2018, new species

Aphelochaeta magellanica new species

Figure 14

Material examined. Southern Chile, Straits Of Magellan, Cape Forward, USNS Eltanin Cr. 11, Sta. 962, 6 Feb 1964, 53.93°S, 71.25°W, 256–320 m, holotype (USNM 1490712).

Description. Holotype complete, moderately sized, 9.6 mm long, 0.7 mm wide across anterior segments at widest point and with about 140 setigerous segments. Segments short throughout body, about six times wider than

long, narrowest in anterior and far posterior segments. Anterior or thoracic segments only slightly wider than middle and posterior segments, anterior segments flattened ventrally, with glandular-appearing segments, not pigmented; rest of body round in cross section, with dorsal groove beginning about two-third distance along body, continuing to end. Ventral groove not apparent. Far posterior segments narrow, not expanded as in most related species, somewhat flattened ventrally, grooved dorsally, with crowded segments narrowing to pygidium with terminal anus and simple rounded ventral lobe (Fig. 14B). Color in alcohol: brown.

Prostomium short, triangular, narrowing to blunt-tipped anterior margin (Fig. 14A); eyespots absent; nuchal organs not observed. Peristomium with three distinct annular rings, of similar length, not extending across dorsum; narrow dorsal crest present (Fig. 14A); posterior margin extending posteriorly and on to setiger 1 mid-dorsally; dorsal tentacles arising near posterior margin; (Fig. 14A); first pair of branchiae lateral to dorsal tentacles on anterior margin of setiger 1; second pair of branchiae on posterior margin of setiger 1 dorsal to notosetae (Fig. 14A); subsequent segments with branchiae in same location as second pair; branchial segments continuing well into second half of body.

Parapodia reduced to low mounds; thoracic parapodia forming broad shoulders with smooth dorsum between left and right sides; parapodial shoulders of posterior segments shifted ventrally down body wall (Fig. 14B). Notosetae longer than neurosetae throughout; natatory like setae not present on holotype; all setae thin smooth capillaries with narrow shafts tapering to long, fine tips; limbate margins not evident; marginal fibrils not evident in light microscopy. Setae numbering 7–10 in anterior fascicles, reduced to 4–5 in posterior segments.

Methyl Green stain. Strong staining reaction on prostomium and peristomium, with most of pre-setiger area staining dark green; a clear unstained gap separates prostomium from peristomium; dorsally, stain ends abruptly at margin of setiger 1; ventrally first 5–8 setigers retain stain, mostly concentrated laterally, but also extending across the venter. Stain not retained on middle and posterior segments.

Etymology. The species name, ma g ellanica, is taken from the sample location in the Straits of Magellan.

Remarks. Aphelochaeta magellanica n. sp. belongs to a large group of species in which the dorsal tentacles are shifted posteriorly to the level of setiger 1. However, there are only a few species known with two pairs of branchiae on setiger 1. Aphelochaeta magellanica n. sp. resembles A. dearborni n. sp. in having three incomplete peristomial rings with a dorsal crest and a narrow posterior end but differs significantly in having a prominent MG staining pattern on the prostomium and peristomium, short segments throughout, distinct parapodial shoulders that are shifted ventrally on the posterior segments, and a dorsal groove on the narrow posterior segments.

Distribution. Known only from the Magellan region South America, 200– 300 m.