The word nudibranch (pronounced nooda-brank) comes from the Latin word nudus (naked) and Greek brankhia (gills), in reference to the gills or gill-like appendages obviously sticking out from the backs of many nudibranchs. They have poor vision and their sense is obtained through through their rhinophores (on top of the head) and oral tentacles (near the 'mouth'). They also have a foot that leaves slimy trails.
There are over 3,000 species of nudibranchs, and new species are still being discovered. They range in size from a few millimeters to 12" long, and can weigh up to just over 3 pounds. Two main types of nudibranchs are dorid nudibranchs and eolid nudibranchs.All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.
Nudibranchs eat colorful food, which gives them their brilliant color.
Nudibranchs eat using a radula. They are carnivorous - their prey includes sponges, coral, anemones, hydroids, barnacles, fish eggs, sea slugs, and other nudibranchs. Nudibranchs are picky eaters - individual species or families of nudibranchs may eat only one kind of prey.
Nudibranchs get their bright colors from the food they eat. These colors may be used for camouflage or to warn predators of the poison that lies within.
Nudibranchs may be toxic (to their prey, but not usually to humans)
Eolid nudibranchs can use their cerata for defense. When they eat prey with nematocysts (such as Portuguese man-of-wars), the nematocysts are eaten but not discharged,and instead are stored in the nudibranch's cerata where they can be used to sting predators.
Dorid nudibranchs make their own toxins or abosorb toxins them from their food and release those into the water when needed. Despite the unsavory or toxic taste they can present to their predators, most nudibranchs are harmless to humans.