This is a Fringed Back Nudibranch. This animal preys on hydroids and cnidarians(animals that have stinging cells). The nematocysts (stinging cells) are transported to the cerata (fringes on the backof the nudibranch) and provide protection agaisnt other predators. This picture was taken on a patch reef, 125 yards from shore, at a depth of fifteen feet in south east Florida.