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.

Each summer, thousands of bluebottles (also known as Portuguese man-of-wars) wash up along the shores of Sydney, Australia. These remarkable relatives of jellyfish are actually colonial organisms: each is made up of four kinds of specialized, interdependent bodies called zooids. One zooid provides the gas-filled sail; when the wind catches, it whisks the bluebottle across the ocean’s surface. The other zooids take care of hunting, digestion, and reproduction.

Some of the bluebottles that arrive in Sydney get trapped in shallow rock pools, which is where I like to photograph them, because they’re more protected there from currents and waves. With their electric-blue bodies and yellow-tipped tentacles, bluebottles contrast beautifully against a dark-water background. I have to be careful, though: the tentacles pack powerful venom for catching and killing prey, such as larval fish. Although not usually deadly to humans, the sting can be nasty.

For this particular image, I went out at sunrise so I could use backlighting to isolate the bluebottle from its background. I also added a touch of artificial strobe light to illuminate its lower portion.