Sea Cucumber: Animal or Vegetable?

You probably have a favorite sea cucumber and don't even know it. There are over 1,200 species in the world's oceans — so many that scientists are even confused about how many there are —- so there's probably one you'd like a lot: some of them are decked out with the colors of your favorite sports team, others look like somebody crocheted an oversized zucchini out of brown acrylic yarn. Even if you weren't particularly interested in a live sea cucumber, you might want to eat a dead one, or even take it as medicine.

But if you had to guess, would you bet a sea cucumber is an animal or vegetable? Go ahead — have a real good look at the beauty in the photo again.

It's an animal, but it looks vegetable-ish, no? It's okay, they get that a lot — that we refer to them as "cucumbers" doesn't help. But they're echinoderms, a phylum of marine animals that also includes starfish and sea urchins. Everybody in this phylum has what's called pentameral symmetry — they have five arms arranged around a center point. Of course, the kindest thing you can say about a sea cucumber's physique is that it looks very much like a large hoagie bun you wouldn't want to eat. But though it more closely resembles a slug than a star, the five rows of tube feet that extend from its mouth to its anus give it the same basic layout as a very elongated sea urchin that fell over on its side.

Holothurians (that's the biological name for sea cucumbers) have a different approach to reproduction than we do — they are usually born either male or female, but they can change from one to another during their lives. They communicate with each other by releasing hormones into the water, which is very similar to their style of reproduction: they expel their gametes — those are egg and sperm cells — out into the water column and just assume they'll find each other, just like they assume their friend got the hormone message they left them. Holothurians eat tiny animals, particles of algae and general sea junk out of the water using frilly tentacles they keep hidden in their mouths between meals. What they have in the way of a brain is a ring of nerve cells around their mouth that tells them when it's time to eat, burrow into the sand, walk somewhere else or attack. And even though they look like tube socks filled with modeling clay, a sea cucumber's defense mechanism is pretty terrifying.

The Human-Sea Cucumber Connection

Imagine approaching your younger brother with the intention of wrestling him to the ground, sitting on his chest and making him admit to listening to JoJo Siwa as he goes to sleep every night. But what if, instead of running away, your brother shot some of his respiratory organs at you out of his butt? That's what sea cucumber combat looks like — they have the amazing ability to regenerate the organs they lost. Some species spit out sticky string like spiderwebs to trip up or ensnare their enemies, while others have a special toxin that kills or stuns small animals. Don't mess with sea cucumbers.

But we DO mess with sea cucumbers! All the time. Some species are at risk of extinction because in some Asian countries, sea cucumbers are the ultimate luxury snack. Some tropical holothurian species can go for between $10 and $600 per kilogram (dried) in mainland China, and one type retails for $3,000 per kilo. Sea cucumbers are also used in Chinese medicine to treat a variety of ailments like muscle aging, compromised immune system, fatigue and arthritis — some even suggest they could be used in cancer treatments. The varieties that are used in food or medicine are becoming increasingly rare.

According to the authors of a 2014 study published in 2014 in the Proceedings of the Royal Society B, human interest in these specific species is what is driving their numbers down:

"For some marine life, such as Bluefin Tuna and Sturgeon, increasing rarity makes them valuable and drives them towards extinction," wrote study authors Steven Purcell of Southern Cross University and Beth Polidoro of Arizona State University in The Conversation. "But this is not true for sea cucumbers: their high value drives intense exploitation eventually making common species rare."

As a result, there are around 377 species of sea cucumbers on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, seven of which are classified as endangered and nine as vulnerable.

But we need holothurians! Not only are they economically important as a seafood, they are essential to ocean ecosystems — they're important filtration systems for oceans all over the world. Plus, some species provide vital services for other marine life: the pearlfish, for instance, takes refuge in a sea cucumber's anus — sometimes entire schools of pearlfish can find shelter in the rear end of a single holothurian. They usually don't mind much — unless they're needing their anus to breathe at the moment.

Oh, yeah — sea cucumbers breathe through their anuses. Forgot to mention that part.