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Though reminiscent of a discarded clump of salad greens, this is actually a lettuce sea slug. I spotted it 20 meters below the surface on a coral reef off Playa Kalki, an idyllic beach on the northwestern edge of Curaçao. Bright video lights and a macrophotography camera in an underwater housing let me capture this invertebrate in all its colorful glory.

Named for the undulating waves of its broad, fleshy appendages—called parapodia—lettuce sea slugs appear in a range of colors from pale yellow to deep blue or even lettuce green. This variation is attributed to how the slugs process their food; they are transiently photosynthetic. (Photosynthesis: not just for plants and algae.)

When a lettuce sea slug eats algae, it steals the plastids, the parts responsible for making and storing food in cells, and moves them out to its parapodia. Once there, the plastids continue to harvest sunlight to make sugar, but now for the slug’s benefit. This arrangement is only temporary; the plastids slowly break down, so the lettuce sea slug has to keep eating algae to replace them.