This is Hopkins' Rose, a gorgeous pink nudibranch. When I first learned this species its scientific name was Hopkinsia rosacea. Recently its name was changed to Okenia rosacea. The new name is a hard one for me to adopt. Partly because I've known this species as Hopkinsia for so long (25 years!). And also because Hopkinsia seems nicer to say than Okenia. Hopkinsia also tells a story about Timothy Hopkins, who helped establish Stanford University's Hopkins Marine Station. I'm glad the species name — rosacea — hasn't changed because the color of this nudibranch is unforgettable!
Here's a closer view as the nudibranch explored its environment:
Note the pink bryozoan (Eurystomella bilabiata) to the left of the nudibranch. Hopkins' Rose eats this bryozoan and obtains its pigment from it!
I'm going to show two more pictures. They highlight a few interesting and important features to look for when observing this species.
In the photo above, note that the light pink projections are called dorsal papillae. (It's a longer story, but they are not cerata. Hopkins' Rose is a dorid nudibranch, not an aeolid nudibranch.) If you scan carefully, you'll be able to see that some of the papillae in this individual are forked, like antlers.
Also, you'll probably notice that a few projections are darker in color than the papillae. On the right side, there are two rhinophores (chemosensory structures). And near the center of the back is a circle of gill plumes.
I've zoomed in to make it easier to see the rhinophores and gill plumes:
Hopkins' Rose occurs from about central Oregon to Baja California. They seem to be less common in northern California. I'm still looking for my first one in the Bodega Bay area.