We could tell that it was snail veliger (larva), but we weren't certain about the species.
In between broad wings, it had a very pretty shell:
A tapering foot was visible below:
The veliger had large velar lobes — and the lobes were edged with bright orange polka dots. With active bundles of cilia, it was using the velar lobes to swim. (This is best viewed in the video at the end of this post.)
As mentioned, we weren't familiar with this snail, so we started sending pictures around for help. No one responded right away, so Eric decided to keep it in the lab for a few days.
A couple of days later, the veliger had undergone metamorphosis into a juvenile snail! This made it much easier to identify.
Note that the ciliated velar lobes are gone; the orange and gold speckles have expanded; and there are two siphons — the anterior one (to the left) is formed by a flap near the front end, and the back one (to the right) is formed by the folding of the wings. (In this case, the wings are extensions of the foot.) Water is pulled into the front siphon, passes over the gill, and then is expelled out the back siphon.
As the snail glided along and explored its environment, we occasionally caught a glimpse of its two dark eyes:
Pacific Stomach Wings (Gastropteron pacificum) are capable of crawling and swimming.
Eric was fortunate to have filmed this snail when it was still a larva (swimming with its velar lobes) and after it metamorphosed into a juvenile.
Check out his video below! (Click on the title of the video to see a high resolution version.)
Many thanks to Kate and her team for sharing this wonderful find from the plankton off Bodega Head!
P.S. If any gastropod enthusiasts out there are wondering — Pacific Stomach Wings are cephalaspideans, related to headshield slugs and bubble snails.