orange spiky sea cucumber in the water
Sea Cucumber. Greg McFall/NOAA

The following is an excerpt from ADAPT by Amina Khan.

The sea cucumber is not exactly the beauty queen of the seas. Named for the long, tubular vegetable with a clean smell and a crisp flavor, the animal looks more like a pickle with an unmentionable skin disease. It crawls along the bottom of sandy floors; depending on the species, it can look like a ridged, knobbly, or even spiked giant sea slug. The animal has no brain—just a ring of nerves around its oral cavity that extend both to the tentacles around its mouth and also down the length of its body. It gobbles up the detritus that falls to the seafloor, extending long tendrils that are modified versions of the tube feet it shares with starfish. Depending on the species, these feeding tube feet can be gorgeous and ornate, like the branches in a tree or the delicate ends of a neuron.

These animals hold their feeding arms up to catch particles in the water, or they plow them into the sand and chow down, pooping out the clean sand that covers the seafloor. This trashy diet is probably what gives them that strong briny flavor that makes them a sought-after delicacy in places like China and Korea.

adapt book cover
ADAPT by Amina Khan is on shelves now. St. Martin’s Press

Sea cucumbers are bottom-feeders, a term that’s used unflatteringly to describe certain types of people: ambulance chasers, paparazzi, payday lenders. But that’s an insult to literal bottom-feeders everywhere. It’s true that sea cucumbers eat dead and discarded matter, from carcasses to excrement. But that’s not a bad thing—in fact, it’s a crucial cleaning service for the world’s oceans. Sea cucumbers clean all that crap out of the water and off of the substrate, and then poop out nice, “clean” sandy substrate. They’re the earthworms of the sea in that way, recycling decomposing matter and aerating the seafloor.

As the demand for sea cucumbers has grown in recent years, many populations are shrinking, which means more undigested nutrients in the ocean. This reduces the water’s clarity—which, for the sea creatures who have to swim through this murky liquid, is probably about as healthy as breathing in the smog of Shanghai. All those extra nutrients can trigger algal blooms, which suck up all the oxygen in the water and cause a mass die-off as fish and other sea life suffocate. Without sea cucumbers tilling the seafloor, it hardens, making it impossible for other benthic organisms to survive there.

Sea cucumbers are indeed the janitors of the ocean. But janitors are often called custodians, and sea cucumbers do perform that function, caring for the ocean that they live in.

Sadly, there’s no such thing as gratitude in the wild. While the sea cucumber performs a service that benefits its fellow ocean dwellers, many of those inhabitants see the soft, slow animal as easy fast food. The sea cucumber has a few defenses against predators; some species can shoot their respiratory organs out of their anuses and let them wave around, because the sticky tubes are covered with a soap-like chemical that is toxic to other animals. But it can’t hurl its lungs out every time it feels threatened—those organs can take weeks to grow back. Some species burrow into the sand to hide from predators—but that’s a time-consuming process, and they can’t stay buried forever.

Unlike its cousins, the starfish and the sea urchin, the sea cucumber seems woefully under-armored for the fish-eat-fish world it lives in. Starfish have bony plates made of calcium carbonate called ossicles to protect them, which is why they feel so tough. In sea urchins, those plates have fused together, and its bristling array of sharp spines further warn predators to keep out. But in sea cucumbers, those ossicles seem to have shrunk to near uselessness. This is great for the sea cucumber if it wants to, say, squeeze itself into a safe little nook in a rock or cranny in some coral—it can practically liquefy its body as it pushes into the hole. But that particular quality is not so useful for fending off a razortoothed attack.

Luckily, the sea cucumber has a secret superpower, one that isn’t apparent when it’s happily snagging detritus out of the water or pooping its way across a reef. When threatened—and when farting its lungs out doesn’t work—the sea cucumber can go rigid, turning from the consistency of play dough to hard plastic. If you’re wondering how a soft-bodied creature manages this feat with its shrunken, vestigial calcite plates, you wouldn’t be alone. It’s a question that dogged a handful of researchers for decades, and that’s because the sea cucumber uses a totally different adaptation from those of its well-known echinoderm cousins.

Instead of relying on its now-shrunken calcite plates, the sea cucumber calls to arms a network of tiny collagen fibers—known as fibrils—embedded under its skin. These fibrils link together, creating a scaffolding through the body that acts like protective dermal chain mail. When the animal is in its soft state, any stresses that travel through a random fibril quickly pass into the soft matrix, easily penetrated by pointy objects (like teeth). But when they’re connected, the skin has structure and strength. It’s kind of like the beams of a building—you shake one beam, you shake them all, because the force is traveling from one connected post to the other. But even though it’s all getting shaken, the structure doesn’t fall down. When these hard elements are connected, the stress can pass through and be safely channeled away.

The same thing is happening in the sea cucumber skin. Joined together, the collagen fibrils in its skin make a sort of structure through which stress can be transferred safely, without bending or breaking.

This excerpt has been adapted from ADAPT. Copyright (c) 2017 by Amina Khan. All rights reserved. Published by St. Martin’s Press.

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