THR Blog   /   June 23, 2020

Cousin Starfish

Spare a thought for the sea star, fellow pandemic sufferer.

David Egan

( Specimens of the sea star wasting disease; photo by Michael Kyte/UC Santa Cruz; seastarwasting.org.)

I was ambling along the rocky sea shore when I came upon the scene of a massacre. A tidal pool about two feet in diameter was littered with the disarticulated shells of dozens of mussels. The perpetrators hadn’t gone far. Two plump starfish clung to one side of the pool, taking a rest after gorging themselves. 

Starfish aren’t known for their swiftness but if you can’t move, they can be fearsome predators. Thousands of tube feet sprout from their undersides, which clamp on to a mussel shell and pry it open. The mouth at the center of the starfish is too small to take in a mussel whole and has no teeth to chew its food into smaller chunks. What the starfish does instead isn’t pretty. It turns its guts inside out, extruding its stomach through its mouth and into the mussel shell—a crack of just a millimeter is enough. The stomach then gets to work digesting the mussel in situ before drawing the half-digested chowder back into the starfish body to finish the job. Dine in or take out—why choose?

What if I told you that these graceless eaters are among our closest non-vertebrate cousins? You might have thought we’re more closely related to the industrious and sociable ant or the fiendishly clever octopus, or really anything that has a discernible face. But those qualities we take most pride in are peripheral from an evolutionary perspective. Starfish belong to phylum Echinodermata—most notable for their radial symmetry, usually five-pointed—which also includes sea urchins, sea cucumbers, and sand dollars. The echinoderms and the chordates—that’s (mostly) backboned creatures like us—are neighboring twigs on the tree of life, both emerging from a branch that split off from the rest of the animal kingdom more than 500 million years ago.

It’s strange to think that we have this close connection to starfish because starfish themselves are very strange. They have no brains, just a ring of nerve cells encircling the central mouth and extending up the arms. At the tip of each arm, bunchings of light-sensitive cells serve as rudimentary eyes—starfish effectively see with their fingertips. The individual arms are surprisingly autonomous. The body follows whichever arm is most excited by what it sees or senses with its tube feet—in addition to serving as fingers and feet, these hydraulically-powered wonders are exquisitely sensitive organs of touch, taste, and smell, as well as accessory gills. This decentralized organization makes starfish remarkably regenerative. Not only can individuals regrow severed arms, but in some species, severed arms can regrow new bodies.

Our connection to starfish is most evident in the way the gut is formed. When you’re designing a body, the most important question is where you get your energy from. For most animals the answer is a digestive tract: you take in matter through one orifice, pass it through a gut that extracts the nutrients it needs, and expel the remaining waste through another orifice. The digestive system begins to take shape in an early stage of embryonic development called gastrulation. What starts out as a simple ball of cells forms a cavity at one end that folds in upon itself making a bowl shape. That bowl-like cavity deepens until it goes all the way through, turning the embryonic bowl into an embryonic donut. The donut hole becomes the digestive tract. That first hole—the one that makes the bowl-like cavity—is a different orifice for different animals. Most animals—ants, octopuses, and the rest—form the mouth first. These are the protostomes. That alternative evolutionary pathway that starfish and humans followed makes us deuterostomes: We’re first and foremost assholes.

Contemplating your cousinhood with starfish, you can see yourself the way Mother Nature sees you: not as a soul shimmering with intelligence but as one solution to the problem of metabolism. Different animals have found different ways of passing food through their guts. Starfish developed their eversible stomach and tube feet loosely coordinated by a decentralized nervous system. Chordates evolved a more linear body plan around a central spinal column or notochord. Some of those chordates grew four extra appendages that helped them to move about in search of food and central brains to coordinate that activity. But from an evolutionary perspective, those legs and arms and brains are all latecomer accessories to the digestive system. You might think you eat to feed your body and mind but Mother Nature sees it the other way around: the body and mind are there to help you feed.

Starfish are like us in another respect: They’re lousy at social distancing. In my patch of the Pacific Northwest, clumps of purple sea stars (Pisaster ochraceus) are visible when the tide recedes. (Biologists prefer “sea star” since starfish aren’t fish—but then they aren’t stars, either.) They range in color from deep purple to an orange ochre and the biggest ones can stretch a foot and a half from tip to tip. Squeezed together in crevices that are safe from gulls and desiccating sunlight, they form a brightly colored squishy mass.

It’s both reassuring and concerning to see them gathered in such numbers. Like us, starfish have felt the bite of a deadly pandemic in which their cuddling instincts have cost them dearly. “Sea star wasting syndrome” begins like melancholia and ends like a zombie apocalypse. At first, an afflicted starfish grows listless and loses interest in food. Then, white lesions appear on its body as the body tissues decay. Its tube feet lose their grip and the hydraulic system that pumps water through the body stops working so that the starfish collapses in on itself. Individual arms fall off one by one as the body disintegrates. Some of the arms keep crawling for a while after they’ve fallen from the body—literally the walking dead, infecting the healthy starfish they come into contact with. Eventually the arms, too, gradually dissolve into a white mush.

No one is sure what causes starfish to fall apart like this. The best guess is a virus, called sea star-associated densovirus (SSaDV). The last major wave of this plague started in Washington and British Columbia in 2013 and spread down the Pacific coast into 2014, causing massive die-offs. Seven years later, the curve has flattened somewhat, but a group at University of California–Santa Cruz continues to monitor periodic flare-ups of the disease. In the meantime, like the deer and coyote populations recently seen roaming emptied-out urban spaces, mussel beds are thriving.