Nudibranch:- Dendronotus frondosus
Taken off the Isle of Man.
Photo copyright Tim Nicholson.
A large nudibranch, up to 10 cm long. Feathery in apperance, its protruberances act as gills.
Great British Marine Animals, by Paul Naylor, Deltor (2011)
The Underwater Photographer: Digital and Traditional Techniques, by by Martin Edge, Paperback, 536 pages (2009)