Before we got a close-up view, we could see the tiny black spots, which made it look like the nudibranch had been sprinkled with black pepper. This individual was ~10 mm long with its foot extended.
Although the northern range limit of Black-tipped Spiny Dorids (Acanthodoris rhodoceras) is Cape Arago in Oregon, it appears that this species is uncommon north of San Francisco.
Here's a closer view of the tall rhinophores (sensory organs) and the black-tipped papillae:
Eric captured some very nice video of this wonderful nudibranch. Here are a few "tips" regarding what to watch for: (1) close-ups of those fabulous black-tipped papillae, (2) the heart beating where there's brown coloration in the center of the nudibranch, and (3) the circle of gill plumes near the back end. [Click on the title of the video below the viewer to see a higher resolution version.]