Original: This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly-colored hydroids, sponges and seaweeds on which many of the species are found.
Simplified: Sea slugs are often flamboyantly coloured, either as a warning if they are poisonous or to camouflage them on the corals and seaweeds on which many of the species are found.