Hypselodoris picta

Nudibranchs breathe through the branchial plumes in a rosette, the circular arrangement, on their backs.

This particular nudibranch was one of the biggest that I have seen.

It measured between 13-15cm. 

Here you can see the dark blue rhinophores on its head.

The rhinophores are chemosensory organs which function as scent or taste receptors.