Cool Critter: Sea Cucumber
The ocean is full of fabulously-named critters like slippery dicks and donkey dungs. Of course, fabulous names don’t always match up with fabulously cool critters. Slippery dicks are pretty boring. Donkey dungs are not.
The donkey dung is an aptly-named species of sea cucumber. Sea cucumbers aren’t vegetables.* They’re animals, echinoderms actually (like sea stars and sea urchins) and they breathe out of their butts.
A sea cucumber pulls water in through its anus, removes oxygen from the water using the respiratory trees that branch off its cloaca—which is a hole, kind of—and then expel the water. With all the water flowing in and out of a sea cucumber’s anus, the cloaca (here’s the Wikipedia page) becomes a nutrient-rich hideaway for critters looking for a little extra protection. Pearl fish, some crabs and a few polychaete worms hide in the sea cucumber’s butt until they are big enough to defend themselves in the big bad ocean.
Sea cucumbers don’t hide in anyone’s butt. They have much more creative mechanisms of defense…
All sea cucumbers can change form. A sea cucumber’s body wall is made of a special kind of collagen called “catch collagen” which allows them to stretch and shrink as much as they want to without damaging any tissue. When touched, a sea cucumber may turn itself into a flaccid gooey blob or a tight turd-like glob.
Some species startle their attackers by shooting some of their internal organs out through their anus. Presumably, the sea cucumber’s attacker is so shocked (or disgusted) by this behavior that they abandon any desire to eat such a wretched creature. The sea cucumber, of course, is fine. Its internal organs quickly regenerate and it goes back to doing the things that sea cucumbers do.
Other sea cucumber species attack their attackers using spaghetti-like strings called cuvierian tubules. One end of the tubules is attached to the sea cucumber’s respiratory tree and, when the sea cucumber is chillin’, the other end floats freely in the sea cuke’s body cavity. When the sea cucumber is threatened, the free-floating ends tear a hole through the cloaca and shoot out through the anus. The tubules become sticky when they come in contact with anything so, when they hit the predator, they stick to it, entangle it and eventually immobilize it. The sea cucumber drops the tubules, disconnecting them from its respiratory tree, and goes on its merry way. The tubules grow back in a few weeks. (Some species that discharge cuvierian tubules also discharge a soap-like chemical called holothurin that kills any animal nearby.)
Sea cukes live on—and eat—the ocean floor. Sure, a species that eats dirt and depends on its anus to breathe and protect itself may not sound like a big deal, but it is. Just as earthworms do on land, sea cucumbers** mix the sediment and recycle detritus by eating dead stuff and pooping out the waste, creating fodder for bacteria and enriching the substrate. In other words, these butt-breathers are crucial to the marine ecosystem.
*For the record, a regular cucumber is a fruit, but because of its flavor, it’s recognized as a vegetable in culinary circles.
**Also for the record, sea cucumbers do not eat with their butts. They use their tube feet to put particles in their mouths, which are at the opposite ends of their bodies from their anuses.