The lettuce sea slug (Elysia crispata) has enlarged fleshy appendages that are folded over one another, with colors ranging from blue to green, with purple and red lining. The green coloring is what gives this mollusk it's common name, resembling a head of leafy green lettuce. The sea slug eats green algae, but not all of the algae they eat is digested. Some of the green algae gets shuttled off to make a home in those fleshy appendages (called parapodia). The algae's chloroplasts, which convert sunlight into energy, can then live in the parapodia for up to four months, giving the slug photosynthetic energy—and their green coloring.