Photo copyright Tim Nicholson, whose underwater photographs and photo books are available from http://www.photoboxgallery.com/timnicholson.
Taken in Sha'ab Gubal, Red Sea.
Spanish dancer (Hexabranchus sanguineus).

This is one of the largest sea slug nudibranchs, it can grow to 0.4m

Back to the Red Sea Photos....

Further Reading:
Coral Reef Guide Red Sea, by Robert Myers and Ewald Lieske
The Red Sea, by Andrea Ghisotti
Underwater Photographer , by Martin Edge