Found on a piece of kelp growing on a boat. Honestly, my first through was some kind of larval spiny or slipper lobster, but then I realized I had the animal backwards. I'm fairly confident it's in Sphaeromatidae, and it certainly looks a lot like the pics of male Paracereis, but I mean, Paracereis sculpta has 3 male morphs! What the hell, Nature!

The animal was about 2 cm long, observed in Pillar Point Harbor, near El Granada, CA.