SOMEWHERE IN THE reefy waters of the Indo-Pacific, a daisy parrotfish (Chlorurus sordidus) is feasting on coral. She crunches up the hard knobs of calcium carbonate with her tough beaklike teeth, digesting the algae that once covered it and expelling the rest as tidy piles of fresh sand in her wake. It’s relentless work, and by the day’s end, having chewed through enough coral to create several handfuls of sand, our fish is exhausted. But she has more work to do before she can rest. Little by little, she begins to excrete a gelatinous substance that balloons from her mouth, forming a roomy spit sack large enough to envelop her entire body from tip to Technicolor tail. She’s Glinda the Good Witch, snug in her pink bubble: a small shimmering world unto itself. Only, this one’s made of mucus.

Scientists have puzzled over the purpose of these slimy sleeping bags. Are they a lubricant against nighttime scratches and scrapes? A tripwire warning against marauding eels? Your basic dust cover? Evidence suggests they protect against smaller foes, providing a physical barrier against tiny vampiric crustaceans. Dr. Alexandra Grutter of the University of Queensland likens them to gauzy mosquito nets draped over beds to ensure a long uninterrupted night’s sleep.

We may not yet know the meaning of these bubbles, but every night we see her refusing to rest until she can slip inside one of her own making. Only then does she allow herself to settle onto a new bed of undigested sand and dream, if in fact fish can dream (another thing we don’t yet know). But if—as Blake once wrote—an entire world can be seen in a single grain of sand, then these bright fish are the architects of countless worlds.