Gastropods from several different families are called sea slugs. They are often flamboyantly coloured. This may be either as a warning if they are poisonous or to camouflage them on the corals and seaweeds where many of them live. Their gills are often feathery plumes on their backs. This is what gives the name to the nudibranchs. Nudibranchs with smooth or warty backs have no visible gill mechanisms and respiration may take place directly through the skin. A few of the sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in association with certain species.