The Berghia Nudibranch Berghia verrucicomis is a very specialized animal, its sole diet is Aptasia anemones.

They grow to be only one inch long, they are reef friendly and can be raised in aquaria. If you have Aptasia then you can't go wrong with these little guys. Please be aware though that when they have finished eating all of the Aptasia in your tank they will starve to death so pass them on to someone else who has a need for them or give/sell them to your LFS.