I took a picture and then zoomed in to see more detail. This is where digital cameras come in handy! Here's what I saw in the field:
The distinct head and tentacles (rolled rhinophores) told me this wasn't a flatworm but rather a gastropod or sea slug. I knew I hadn't seen this species yet, but I had a guess about what it could be. I'd been wanting to see one for years!
I brought it into the lab for a few microscope pictures for documentation. Here's one of the first photographs under magnification:
This is a Hedgpeth's Sea Hare (Elysia hedgpethi). Note the beautiful blue spots scattered across the lateral flaps on either side of the body. There were tiny orange spots, too, but they're harder to see because of their smaller size.
Hedgpeth's Sea Hares are known to feed on two species of green algae: Codium fragile and Bryopsis corticulans. The sea hare is able to retain the algal chloroplasts which continue to photosynthesize for at least 10 days after they've been ingested!
Although this individual was quite active under the microscope, I managed a few more pictures:
I wish I had more time to write about Joel Hedgpeth, for whom this sea hare was named. I'll have to do that another night. For now, if you'd like to read more about him, I think you can still access a short NY Times obituary here. And there's a much more complete article in the Journal of Crustacean Biology here. Among many (many!) other things, Joel was a marine biologist, a local Santa Rosa resident, editor of Between Pacific Tides, and well known for protesting the PG&E attempt to build a nuclear power plant on Bodega Head.
Joel also wrote an Introduction to Seashore Life of the San Francisco Bay Region and the Coast of Northern California. I laughed when I read this passage about Hedgpeth's Sea Hare in that book: "...at first glance may be confused with a large flatworm, but the rolled structure of the head tentacles clearly separates it from flatworms..." As mentioned in the very beginning of this post, this was my experience exactly!