Underside of a low intertidal cobble, with two of their egg masses. The larger slug here measured about 30 mm long.
6 Thick-horned nudibranchs were converging on the pair.
https://www.youtube.com/watch?v=cgcimuP1oVc&feature=youtu.be
Short video clip:
Observation was picked up using an ekman grab on the wetdeck of the SB Natural History Museum Sea Center, photographed using a 10x or 20x macrolens attached to cellphone.
8.5 mm long, found with its egg masses on a colony of the ctenostome bryozoan Amathia verticillata. 4th image shows a newly hatched veliger larva, right ventro-lateral view, with a shell 110 microns long.