Are there stranger animals than sea cucumbers? Probably. But as a whole, sea cucumbers are bizarre and silly animals that definitely deserve a blog post. They are so oddly shaped, and look like they shouldn’t be a successful group of animals. But there’s a lot going on in these sea vegetables, and they are definitely successful.

A massive and neat-looking sea cucumber. Image by Leonard Low from Australia, CC BY 2.0, via Wikimedia Commons

Sea cucumbers make up the class Holothuroidea, which contains over 1,700 species. They are found worldwide, but are particularly abundant in the Asian Pacific Ocean. Sea cucumbers vary in their habitat preferences, but most species are benthic, or bottom-dwelling. They tend to be particularly abundant in deep waters, and make up 90% of the macrofauna at depths greater than 8.9 km.

The size of sea cucumbers ranges from a paltry 3 mm to a massive 3 meters in length. Most sea cucumbers are somewhat cucumber shaped, though others can be nearly spherical and still more can be long and snake-like. There are some odder varieties as well, but I’ll get to them later. As members of the phylum echinodermata (which includes starfish and sea urchins), sea cucumbers have five-fold symmetry, though this is not nearly as obvious as it is in starfish. The radial symmetry in sea cucumbers occurs lengthwise, so if you were looking at one head on (if you assume these guys have heads), you’d be able to see it.

The mouths of sea cucumbers are usually pretty easy to spot, as most species have bunches of tentacles coming out of them. The shape and size of the tentacles are dependent on the cucumber’s diet, but most species can retract the tentacles rapidly in the case of any threat. Sea cucumbers are usually scavengers, feeding on detritus and particles that fall to the ocean floor. Many sea cucumbers sift through sandy sea floors, and excrete a lovely homogenized product that helps shape the ecology of the ocean bottom.

A lovely picture of a sea cucumber’s mouth. Image by Drow male, CC BY-SA 4.0, via Wikimedia Commons

Though most species of sea cucumber tend to stay on the ocean floor, not all do. There are some that are capable of making ‘jumps’ in the water, because their bodies’ densities are about the same as the water around them. Of course, propelling yourself into the water with no control isn’t such a good idea, so these species have special appendages  that help them ‘swim’ — such as umbrellas or lobes.

Jumping sea cucumbers may have reasonable escape methods from predators, but more sedentary species have to have different ways of protecting themselves. Many species are toxic to all but the most specialized predators, and some actively dispel a toxin into the water that can kill nearby predators. Other species can expel a bunch of sticky threads from their body that entrap attacking predators.

One species of ‘jumping’ sea cucumber, which looks nothing like a cucumber. Image courtesy of Expedition to the Deep Slope 2007., Public domain, via Wikimedia Commons

Sea cucumbers are used both in cuisine and in traditional medicines, particularly in the Asian markets. Some species have been threatened by over-fishing, and others are grown in aquaculture farms.  I don’t know why someone would want to eat one of these guys — they don’t look very appetizing. Still, I’m not much of a seafood fan in general, so I’m not the best judge. I think I’ll stick to nice green land cucumbers.

Cover image by Peter Southwood, CC BY-SA 3.0, via Wikimedia Commons, cropped to fit