To peer into the soul of a sea cucumber, don’t look to its face; it doesn’t have one. Gently turn that blobby body around, and gaze deep into its marvelous, multifunctional anus.
The sea cucumber's posterior is so much more than an exit hole for digestive waste. It is also a makeshift mouth that gobbles up bits of algae; a faux lung, latticed with tubes that exchange gas with the surrounding water; and a weapon that, in the presence of danger, can launch a sticky, stringy web of internal organs to entangle predators. It can even, on occasion, be a home for shimmering pearlfish, which wriggle inside the bum when it billows open to breathe. It would not be inaccurate to describe a sea cucumber as an extraordinary anus that just so happens to have a body around it. As Rebecca Helm, a jellyfish biologist at the University of North Carolina at Asheville, told me, “It is just a really great butt.”
But the sea cucumber’s anus does not receive the recognition it deserves. “The moment you say ‘anus,’ you can hear a pin drop in the room,” Helm said. Bodily taboos have turned anuses across the tree of life into cultural underdogs, and scientific ones too: Not many researchers vocally count themselves among the world’s anus enthusiasts, which, according to the proud few, creates a bit of a blind spot—one that keeps us from understanding a fundamental aspect of our own biology.