Arctonoe pulchra (Johnson 1897)

How to Distinguish from Similar Species: Arctonoe vittata has a band of dark pigment extending across segments 7 and 8. Arctonoe fragilis has ruffled or folded margins on the elytra.

Symbiotic with several invertebrates including sea stars such as Solaster stimpsoni, Pteraster tesselatus, Luidia foliata, and Dermasterias imbricata, sea cucumbers such as Parastichopus californicus and P. parvimensis, and other animals such as Cryptochiton stelleri, Diodora aspera, Megathura crenulata and terebellid polychaetes such as Loimia montagui.

Geographical Range: Gulf of Alaska to Baja California, Mexico; uncommon in California

Depth Range: Lower intertidal to 275 m

As with all Polynoids, this species is mostly benthic, few if any of the segments are longer than wide when contracted, the dorsal surface has clearly visible elytra, and all of the neurosetae are simple. Arctonoe pulchra has at least 20 pairs of elytra, elytra are on segments 2, 4, 5, then every other segment to 23, 26, 29, then every other segment to the end of the body. The edges of the elytra are smooth, and they nearly meet along the animal's dorsal midline. It has no prominent nuchal fold and the lateral prostomial antennae are inserted slightly ventral to the edge of the prostomium, may have few or sometimes no notosetae. The neurosetae are falcate, with pointed tips, and hooked. Most individuals are brick red, and the species has no dark band across segments 7 and 8. Up to 7 cm long.

Biology/Natural History: This species is attracted by the smell of its host, but can learn to live with a new species. Its body color is usually close to that of its host. It eats detritus. Adults are territorial and will try to drive other individuals off their host. They may injure or kill one another in fights over a host. Members of Family Polynoidae, unlike most other errant polychaetes, have parapodia specialized for walking rather than as paddles. Their longitudinal muscles, which caused lateral undulations in other polychaetes, are poorly developed and they don't undulate much. As a result, although they can walk efficiently they are poor swimmers.