These three nudibranchs are the same species, Polycera atra. Some individuals of this species are primarily black, while others are primarily white, as shown here. Photograph taken in Princeton Harbor, California, by Robin Agarwal. Shared via Flickr Creative Commons License.
Three Polycera atra nudibranchs face left. Two larger individuals are mostly white with black stripes and orange dotted coloring. A smaller individual below and to the left of these individuals is mostly black with orange stripes and some grey/white streaks.