@ganyujapan Look at Malacoceros and Rhynchospio, two genera with distinct T-shaped prostomium. The main difference is whether the branchiae start on chaetiger 1 or 2
@Kelporama There's been a lot of confusion over the years about which species is in the Northeast Pacific. A recent revision cleared it up. 2 big species are on the coast here, A williami and A plenidentata. A virens is Arctic and northern Europe, A brandti is northwest Pacific
@ALIENHARUCHAN It's a family Nereididae, and it's in the reproductive life stage called epitoky. The rapid swimming and circling is normal behavior for an epitoke