Nudibranchs are found in all areas of reefs, on seaweeds, on the ocean floor and even swimming in the ocean currents. Some feed on other nudibranchs, others bryozoans, jellyfish, corals, sponges, ascideans, fish mucous, and seaweed.
Those that feed on animals with stinging cells, can absorb the stinging cells without triggering the firing mechnism. Any predator is not so protected and in biting into the nudibranch, causes all the stinging cells to fire their poisonous darts. Sponge eaters absorb and concentrate the sponge toxins in their bodies.
Those with stinging cells and toxins are often brightly coloured to warn of their danger. Those living on seaweeds are often extremely cryptic. Sediment living species often live within the sediment and only come out at night to feed.
The classification followed here is that in "Nudibranch & Sea Slug Identification", by T. M. Gosliner, A. Valdés & D. W. Behrens (2nd edition, 2018). Images with a species number are detailed in this guide. A review of the guide can be found on the reviews page on this web-site. Many details of sea slugs and nudibranchs can be found there and online in the Slug Site.