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When diving, I always scan the ocean floor for sea stars, whose intricate body surfaces are often reminiscent of the gaudiest, most psychedelic 1960s wallpapers.

If you look closely at sea stars’ skin, you might spot the tiny crabs and shrimp that make their homes there. These crustaceans spend their whole postlarval lives as guests on the body surfaces of larger animals, such as sea stars and sea slugs. It would be like a human living on top of a giant elephant.

Like so much about minuscule marine animals’ behavior, the commensal relationships that tiny crustaceans participate in are not well studied. The advantage for the shrimp is clear: the crevices in sea stars’ skin provide safety and camouflage. It’s unclear if the sea stars gain anything in return, though the crustaceans might remove parasites from their bodies.

In this image, a shrimp belonging to the genus Periclimenes—probably a juvenile emperor shrimp—huddles next to a brown mesh sea star. The shrimp measure only about five millimeters long, blend in well, and move cautiously. It takes years of studying field guides and closely inspecting the bodies of larger marine invertebrates for an underwater photographer to be able to locate these shrimp.

I took this shot off Tulamben, a small village on the east coast of Bali, Indonesia, near the site of the USAT Liberty wreckage—a ship used in both world wars and one of Indonesia’s most popular dive sites.