Sternaspis affinis Stimpson 1864

Neotype (RBCM 005-138-001), with body cream to light tan, sometimes greyish (Fig. 5A, B). First six segments smooth with a few minute cuticular papillae widely and evenly spaced. Remaining segments more papillate and opaque in appearance. Segments seven and eight slightly more opaque and dense than preceding ones, with stout cuticular papillae especially near genital papillae, some cuticular papillae with small grains of sediment adhered to bases. Body 15.5 mm long, 5.0 mm wide (other specimens up to 22 mm long, 7 mm wide), about 29 segments. Prostomium hemispherical, opalescent, translucent, sometimes with crescent shaped red eyespots laterally on smaller individuals (Fig. 5C, insert). Peristomium round, without papillae. Mouth oval, covered by papillae, extending from base of prostomium to anterior edge of second segment. First three chaetigers with 8–14 light bronze, widely separated, slightly falcate introvert hooks per bundle, each with subdistal dark areas (Fig. 5C). Genital papillae protrude ventrolaterally from intersegmental groove between segments 7 and 8. Pre-shield region with 7 segments, with papillae evenly spaced, slightly denser than on anterior segments, although less so ventrally, and in single rows of clusters of short filaments closer to ventro-caudal shield, especially on dorsal surface, rarely showing delicate short capillary chaetae protruding laterally from body wall. Ventro-caudal shield with concentric lines, slightly ribbed; suture extended throughout shield (restricted to the anterior region in larger specimens). Anterior margins rounded; anterior depression deep; anterior keels not exposed (Figs 1B, 2, 5B, D). Lateral margins gently rounded (straighter in larger specimens), not expanding posteriorly. Fan truncate, almost straight in juveniles, sometimes with median notch, becoming crenulated in larger specimens. Marginal chaetal fascicles include 10 lateral ones (Fig. 5E), chaetae ovally arranged, and five posterior fascicles, chaetae in a linear arrangement. Peg chaetae on conical extensions emerging under most prominent oblique rib of the shield. Peg chaetae with stout base in cross section; a small fascicle of delicate capillary chaetae (peg-associated capillary chaetae) between peg chaetae and first fascicle of posterior chaetae. Branchiae numerous, thick, coiled, slender, long, protruding from two oval plates, separated by a wide angle, on either side of anus. Additional fine, long filamentous papillae extending to lateral and posterior margins of shield.

Alaska, USA (in the Gulf of Alaska) south along the coast and inland waters to Monterey, California, USA, and into the Gulf of California. This species, identified as Sternaspis fossor, has been regarded as one of the most abundant ones along the coast in the East Sound of the San Juan Islands (Weese and Macnab 1930), and along the Washington coast in 95–154 m with sediment having 50–68% mud (Lie and Kisker 1970). Moore (1923: 218) reported two species from Southern California, based upon the number of chaetal fascicles along the shield margins; one with 16 total bundles found in 441–492 m, and the other, smaller in size, with 15 total bundles and collected in sediments at 92–1190 m.