Figures 1–3
Material examined. Station 36, holotype (MNCN 16.01/17768) and 11 paratypes (LBUV), plus 2 specimens for SEM (MNCN 16.01/17768). Station 59, 1 paratype (LBUV).
Description. Holotype longest complete specimen, a female with developing embryos attached to nephridial pores. Embryos from chaetigers 15 to 40, with developed antennae and anal cirri (Fig. 2D), attached by anus to nephridial pores. Body long, slender, 5.2 mm long, 0.21 mm wide, 44 chaetigers. Prostomium semicircular to subpentagonal, about 2.5 times wider than long; with four large eyes in trapezoidal arrangement, anterior pair slightly larger than posterior pair; antennae located close to each other, between anterior eyes (Fig. 1A); antennae all similar, short, ovoid, with median one slightly longer than lateral ones (Figs 1A, 2B, C). Palps broad, fused along their length with distal notch, slightly longer than prostomium (Figs 1A, 2A, C). Peristomium distinct; tentacular cirri small, papilliform (Figs 1A, 2A, C). Dorsal cirri longer than tentacular cirri, similar in shape and length to antennae, somewhat smaller on most anterior segments (Figs 1A, 2A–C), absent on chaetiger 2. Compound chaetae all short falcigers, with proximal tooth distinctly long and distal tooth small (Figs 1C, G, H, 2E, 3 A-D) and short, with straight spines on margin. On median and posterior parapodia, most dorsal compound chaetae with slightly elongated blade and longer spines along margin (Figs 1F, 3D). Anterior parapodia each with ca. 8–9 compound chaetae; number of compound chaetae diminishing posteriorly to 4–5 on mid-body and 2–3 on posterior parapodia. Blades of compound chaetae 10 mm long, shorter on posterior parapodia. Dorsalmost compound chaetae of mid-body and posterior parapodia slightly longer, about 12–13 mm long. Dorsal simple chaetae from chaetiger 1, with rounded tips and finely spinulose subterminally (Figs 1B, 3D), thicker posteriorly (Figs 1E, I, 3E). Ventral simple chaetae on posterior parapodia, sigmoid, smooth or with minute subdistal spines, with small distal tooth and longer subdistal tooth (Figs 1K, 3F). Acicula solitary, distally blunt, becoming larger more posteriorly (Figs 1D, H, L). Pharynx long, through about 5–6 segments; pharyngeal tooth large, located on anterior rim (Fig. 1A), surrounded by ten soft papillae. Proventricle similar in length to pharynx, through about 3– 4 segments, with about 20 muscle cell rows. Pygidium with two anal cirri, relatively short in comparison with other related species (Fig. 2A).
Distribution. Only known from Chilean Patagonia. Puerto Engaño in Ballenero channel; Magellan Strait to the south. Also Beagle channel southwest arm.
Habitat. Inside tubes of Chaetopterus cf. variopedatus in fjords and channels from Patagonia. Salinity: from 20 to 35 PSU, temperature: from 8.01 to 8.7°C. Shallow subtidal, 30 m.
Type locality. Ballenero channel. Magellan Strait south.
Remarks. Exogone yagan n. sp. is characterized by having all the antennae short, ovoid, the lack of dorsal cirri on chaetiger 2, and especially the lack of spiniger-like compound chaetae, a typical character of the genus Exogone, although some other species also lack them, totally or partially.
The general aspect of body is quite similar to Exogone verugera (Claparède, 1868) from the NE Atlantic and Mediterranean, also reported from other areas, but this species has distinct spiniger-like compound chaetae on each parapodium (see San Martín, 2003). Other similar species are E. breviantennata Hartmann-Schröder, 1959 and E. africana Hartmann-Schröder, 1974, which however have dorsal cirri on chaetiger 2 and spiniger-like chaetae (San Martín, 2005; Paresque et al., 2014). The Australian species E. goorapuranga San Martín, 2005 also lacks spiniger-like compound chaetae on anterior segments and develops a short spiniger-like chaeta in the mid-body (San Martín, 2005); however, this species has the median antenna distinctly longer than the lateral ones, dorsal cirri on chaetiger 2, a much shorter proventricle, and the shafts of compound chaetae have long, filiform distal spines. The most similar species is E. anomalochaeta Benham, 1921, because it also lacks spiniger-like compound chaetae, but the median antenna is slightly longer than the lateral ones, the antennae are more slender than those of E. yagan n. sp. and are inserted in front of the anterior eyes, the dorsal cirri are papilliform, and the compound chaetae are more strongly spinose, with even shorter blades than those of E. yagan, n. sp. (see re-description of this species, below).
Etymology. This species is named in honor of the “Yaganes” aboriginals that inhabited the southern Chilean Patagonia.