actually, Elysia crispata aka the lettuce nudibranch is photosynthetic and lives kinda the same way as coral, has algae living in its back lives off of the nutrients it gets from it. higher light is preferred but not required as they will sit at the top of the tank during lit hours and will eat at night. they mostly like hair algae but will eat different kinds of brown algea as well.
we have two of them in our 25 gal
and they are doing great. they eat a lot of the nuscence algae in our overflow and clean the rocks of mostly all algae. they are pretty much the only nudibranchs (besides the elysia diodomeda, still a lettuce nudi) that i know of that can live in aquaria without starving to death. i have even heard of them spawning in captivity where the guy had to sell the young just to keep the numbers low. HTH