Lettuce Nudibranch (Elysia crispata)

Elysia crispata pulls off one of the stranger tricks in the animal kingdom: it eats specific algae, digests everything except the chloroplasts, and then stores those chloroplasts intact in its own tissue where they keep photosynthesizing for weeks — a process called kleptoplasty that lets this sea slug run partly on solar power. That biology ties it directly to a narrow diet of the right algae species, which is why it's a short-lived, advanced-hobbyist animal rather than a beginner pickup.

Care level: Difficult · Temperament: Peaceful · Max size: ~3" · Min tank: 30 gal, mature · Reef safe: Yes · Diet: Herbivore, specific algae types (bryopsis)