The lettuce sea slug, Elysia crispata, has a ruffled, lettuce-like appearance. It obtains some of its energy from kleptoplasty, or stealing chloroplasts. As the sea slug digests algae, it absorbs the chloroplasts into its blood stream and can then produce its own food through photosynthesis. This gives the sea slug a competitive advantage when the algae supply is scarce.
Sea hares, however, aren’t “solar-powered” and rely only on algae for food. When they run out of algae, they start to shrink and can die from starvation. They’re heterotrophic (can’t make their own food), unlike lettuce sea slugs which can also be autotrophic (can make their own food).