otranto:

A nudibranch is any one of the 3,000 species found within the clade Nudibranchia. Their name comes from the Latin ‘nudus’, meaning naked, and the Greek ‘brankhia’ meaning gills - this is especially applicable to dorids, whose naked gills are visible in the form of a rosette situated on their back.

Nudibranchs are hermaphroditic (though incapable of asexual reproduction) and carnivorous, with prey ranging from sea sponges (Cuthona) to the Portuguese Man O’ War (Glaucus atlantica). Amongst the most aesthetically varied and colourful of all the organisms on earth, it is thought that their appearance is either in order to camouflage, or an example of aposematism - an antipredator adaptation appearing as warning colouration that is beneficial to both potential predator and prey; the former is forewarned that the slug is unpalatable or poisonous, and the latter avoids being eaten.

Species of Nudibranch are found at almost all depths of salt water, and occur in seas worldwide.