Photo copyright Tim Nicholson, whose underwater photographs and photo books are available from http://www.photoboxgallery.com/timnicholson.
Taken in Ras Samadia, Red Sea.
Sponge.

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