White's Hypselodoris (Hypselodoris whitei)

Madidir III, Lembeh.
It is the same bunch as in previous pic. Link to previous image to see them mating:

This time pay attention to the one in the center; it only has one rhinophore. Sometimes they retract them but it has also been noticed by diver's that nudis are sometimes simply missing one of the rhinophores. I have read that one reason is because of encounters with predators like crabs or shrimps which remove them with their pincers. On rarer cases it could be the result of mutations as well. Rhinophores are very important organs for the nudi so no wonder why they can retract them when threatened (at least some of the species can do).
More info on this important organ here:
http://www.seaslugforum.net/showall/rhinonud