Sea cucumbers are more than fat blobs

There is a photo from one of our dive trips that confused the shit out of me. A fuzzy structure that looked somewhat coral-like but wasn’t. When my husband said it was a cucumber, I was sure he was joking. Months later, I found a similar photo online and realized he’d not only meant it but been right. And yes, I’m aware I studied this shit.

Today, we’ll talk about fat globs with weird parasites: sea cucumbers.

I’ve often encountered them while diving, as they are very common sea floor inhabitants worldwide. Sea floor means marine, and yes, they are exclusively marine. But, as all echinoderms are exclusively marine, that’s not really a surprise, right?

Holothuroidea is the scientific name for these sea cucumbers, but the pronunciation alone makes it hard to remember.

Sea cucumbers? More like zucchini.

Sea cucumbers look more like very fat zucchini than cucumbers, because of their mouth end, but I guess they didn’t want to fight over whether they are courgette or zucchini.

By François Michonneau – d2008-Kosrae-0084.jpg, CC BY 3.0,

Most Holothuroidea are soft blobs much like slugs, which is probably why a lot of people think they are related. Well, they aren’t slugs. They are echinoderms, not mollusks.

Like our sea stars, the sea cucumbers have plates called ossicles. You can’t see them because they are right underneath the animal’s skin. In sea cucumbers, those plates can be fused to form a kind of armor, but there are also cucumbers where this endoskeleton has been lost to further evolution.

One of those examples is especially cool. Pelagothuria natatrix, a free-swimming cucumber that looks essentially like an umbrella or a sea jelly, has no more endoskeleton. It’s also one of the few cucumbers that isn’t benthic, so doesn’t live on the ocean floor. But honestly, it looks like someone put a wig on a funnel. It’s pretty, but also pretty weird.

By NOAA Okeanos Explorer –, Public Domain,

There’s another one that looks like an alien. Enypniastes eximia is a deep-sea cucumber that’s also bentho-pelagic, so free-swimming but with a lot of time on the bottom. And that one’s straight out of some sci-fi movie. They have swimming fins and can move pretty well, sometimes traveling 1,000 meters in the water column. Oh, and it’s bioluminescent, too. A glow-in-the-dark alien. Cool!

By Image courtesy of Expedition to the Deep Slope 2007. –, Public Domain,

The basic body plan.

But let’s return to a more representative holothuroid. They have elongated bodies shaped like a, well, zucchini. There are ossicles all over the body which can be fused or remain movable. They can even look like warts, form flappy protrusions, or even be spiky like an urchin.

Graphic by Kate Hildenbrand based on classroom materials while studying (original source thus unknown; thanks teachers…)

Sea cucumbers come in all kinds of patterns and colors, too.

They are typically between 10 and 30 centimeters, so a couple inches to a foot. But they can reach up to 3 meters, so 10 fucking feet!

One end has a mouth surrounded by tentacles, the other end has an anus. Basic division of in and out, check. They use the tentacles to stuff food pieces into their mouth.

Tube feet along their body allow the cucumbers to move and a respiratory tree that looks much like their tentacles but inside their body lets them breathe.

To reproduce, they usually throw eggs and sperm into the water and hope for the best. Their reproductive system, much like their nervous and digestive systems, is pretty simple in most species. There are a few exceptions of sea cucumbers who can do internal fertilization and keep the eggs in a pouch until they are more developed, though. There is a lot we do not know.

Though I’m not surprised they broadcast spawn usually, as most cucumbers are pretty much without sensory organs. They have some nerves on their skin for a bit of touch and light awareness.

Cucumber weapons

The sea cucumber has a trick up it’s sleeve that’s so weird one of my favorite German punk bands has sung a song about it. They puke up their intestines as a weapon. Yes, weaponized guts.

They can regrow them, but that takes a shitton of energy and effort, so it’s still not cool that dive instructors think it’s totally okay to impress students by being unkind to marine life. But if you want to see this without being a dick to nature, watch my video. I’ll show you. Or google it, there are plenty of videos out there.

Some cucumbers also have teeth on their butt as a special defense. Why would their butt need defending, you wonder? We’ll see in a moment.

Fucking weird parasites and friends

I can’t get through an episode on sea cucumbers without talking about their symbiotic relationships. There are quite a few species that live inside the body cavity of sea cucumbers. Because of their super simple body plan, there’s a bit of room inside there. Not all species are well suited as hotels, but some apparently are first-rate lodgings.

Especially considering they don’t seem to provide any benefit to the cucumber. Free rent, yay! Most of the pearlfish who inhabit cucumbers live in their cloaca, so the empty space behind their asshole. Some live in their guts, some in their respiratory tree.

Luckily for the cucumbers, most of the pearlfish species are commensalists, so they don’t damage their host. But some pearlfish species like Encheliophis pearlfishes are real dicks and eat their hosts, gonads first. Going for the balls isn’t fair, is it now. Can’t blame some species for developing anal teeth now, can you?

Usually, there’s only one or two fish inside a cucumber, and if it’s more than one, it’s usually a breeding pair. But sometimes, there are plenty of fish in there, and we don’t know why yet. Some think they’re having real orgies in there. Well, we all know most scientists are children.

That these pearlfish are totally fine inside the cucumber’s butt is due to a resistance against the cucumbers toxin, by the way. Cucumbers produce saponins, so soap-like usually toxic stuff-things that the pearlfish have a very high tolerance for.

Ecosystem engineers

Lucky for us, sea cucumbers are pretty resilient things. They are very common in all kinds of climates all over the planet. Nonetheless, there are fewer and fewer of them out there.

Sea cucumbers are essentially like earthworms. They gobble everything up and poop it out in a more digestible form. They are considered ecosystem engineers because of this. Their digging also influences oxygen levels higher up. Breathing is kinda important. In short: They make ecosystems possible. Cool, right?

Well, if we could stop overexploiting them, they might help keep the planet’s lungs alive. Due to poaching and overexploitation, some regions are really struggling.
But, as long as these things remain a delicacy in China and other parts of Asia, people will sell them. As so often, they don’t even taste good. They are supposedly pretty bland, but the gelatinous but solid texture appeals to some Asian cuisines. Ah well.

Sea cucumbers, in general, breathe through their butt, expanding and contracting their, well, asshole to cycle oxygen-rich water through their system and past their respiratory tree. So, essentially they have butt-lungs. Yeah, nope, they are pretty fucking weird.



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