Come to think of it, there definitely is not that many sea slugs in aquariums as far as I know, why is that? They are a beautiful and diverse range of gems, probably one of the prettiest things on the reef.
@CheeseChameleon2007 the reason nudibranchs are rare in captivity is because of their eating habits. They eat marine sponges, but can be extremely picky and will only eat one or two species, even if starving, and each species has it's own preference in sponges. There isn't much information about which species eat which sponges, so most aquariums just put a lot of live rock in the nudibranch's tank and hope that it has enough of the right type of sponge to sustain the nudibranchs. The vast majority of nudibranchs in aquariums end up starving to death. Chromodoris kunei is less picky than other nudibranchs and have been kept with some success, but are still difficult. Some non-nudibranch sea slugs are much less picky, like the Sea Hare, which aren't too difficult to keep but lack the colours of nudibranchs and are as such less desirable for display. I think that aquariums should probably avoid keeping nudibranchs until more is known about which types of sponges each species eats.
