I recently went on a collecting trip up the coast, and there were hundreds of these sea slugs wandering around on the mud flats we were at. They are really small (my hand is the background in the picture), and the trip leader could not identify them. I took a quick picture and they took a egg mass back with them, to try and ID these slugs. I asked a friend who works with sea slugs and she tentatively identified them as an Aplysiopsis species. Interestingly, she had gone on a trip farther north, and seen hundreds of these as well. So they seemed to be blooming all along the central/northern part of the coast.
Alas, we won't truly know what species it is unless we look at its radula and penis, two of the main features used to distinguish different sea slugs. But this little adventure has allowed me to get some fun footage of mystery slug veliger larvae! This larval type is characteristic of gastropods, and if you look at them you will notice that as larvae, they all have shells. The shell is lost in the adult sea slug.
Without further ado...