Sea cucumber

Update of post from January 18, 2014


Sea cucumbers are a large, weird, and extremely diverse group of invertebrates living in every ocean in the world. These are in fact animals, not plants as their name implies. They belong to the echinoderm family, as do starfish and sea urchins. The main reason that sea cucumbers are diverse is because there are about 1,250 known species of them! Given that these animals live only in the mysterious ocean, there are probably many more undiscovered species.

Most of these animals are long and tube-like with various textures of skin. Others look completely different, almost more like jellyfish than like the one above. The two animals shown below are actually sea cucumbers!

Sea cucumbers like the one at the top are often confused with sea slugs. It is easy to see why this happens, as they are both ocean-dwelling, soft-bodied, elongated invertebrates.

Sea cucumbers have a wide variety of skin textures. They can range from lumpy, to smooth, to spiny. They also range widely in color. If you think of a color, there is probably a sea cucumber that is that color. They can be red, orange, pink, black, yellow, green, brown, or even multicolored and striped!

Sea cucumbers are odd for animals yet somewhat normal for aquatic invertebrates in that they do not have a brain. Instead, they have a highly complex nervous system that allows them to react to stimuli. They can’t think through stuff, though, and all they do is basically just reflexes or responses to hormones.

Getting around

The two sea cucumbers shown directly above can swim in the open ocean, and many of the 1,250 species can as well. Most of the species can’t swim, and I assume that the one shown at the very top cannot swim. Sea cucumbers like this one get around instead by moving like a slug.


Because they have so many different species, sea cucumbers vary widely in size. Some species grow to just about one inch (2.5 cm) long, others can be as large as 6.5 feet (2 m) long! That is taller than most adult humans! Most sea cucumbers, however, are about 16 inches (40 cm) long. As for how thick these living tubes can be, the largest sea cucumbers grow to over 9 inches (24 cm) across!

Diet and feeding

Since these animals are brainless, they cannot move very fast, and they can’t really hunt their food. Sea cucumbers eat mainly plankton and decaying plant and animal matter.

Most species collect their food with the small tentacles that surround their mouth on one end of their body. They can use these tentacles to grab onto food or sweep it into their mouth. Other species burrow through the substrate on the ocean floor, eating the sediment as they dig through it. They digest the plant and animal matter and let the sand and other indigestible materials pass through their system.

Habitat and range

Sea cucumbers are found in each of the five oceans in the world, though not all species are found everywhere. Since most of them cannot swim, they live on the ocean floor. Even species that can swim usually stay near the ocean floor, but there are a few that live in the open ocean. Most of the time, these animals live with their body buried under the sand and just their head exposed.

Status, threats, and protection

There are many different species of sea cucumbers, and not all of them have the same status. The IUCN Redlist classifies different species anywhere from Least Concern to Endangered. They are fished by humans mostly for use in Asian foods as flavoring. Certain species are hunted more than others, and these species are usually more threatened.

There are several natural predators of sea cucumbers as well. These include fish, starfish, crustaceans, and sea snails. Luckily for the sea cucumbers, they have a few good methods of protection. First of all, if threatened by a predator, this creature can make a disgusting distraction. They can actually squeeze out most of their internal organs so the predator eats them instead. This may not seem like a good strategy, but the sea cucumber can survive temporarily without these organs, and grow them back soon.

Another defense mechanism these creatures have is the ability to shoot sticky threads at the predator that may trap them or discourage them from eating the sea cucumber. They also have toxins in their body that are deadly to some animals.

Use to humans

Sea cucumbers are used by humans for food, as I already mentioned. The toxins in their body are not strong enough to prevent people from eating them safely, and they actually prove to be a problem to the sea cucumbers sometimes. Since the toxins can kill fish, some Pacific islanders capture sea cucumbers, extract their toxins, and place the toxins in the ocean. The toxins will then kill or stun fish making them much easier to be captured for food!

There are other uses for sea cucumbers that are still being researched. Some parts of these animals have been used to kill bacteria, ease pain, and treat cancer! Although the effect of these creatures on cancer and other health issues has been researched since about the year 2000, they are not frequently used due to their obscurity.

Reproduction and young

Sea cucumbers reproduce like most fish do, by spawning. I couldn’t find when their breeding season is, but I would guess it has to do with water temperatures, as they can’t communicate to each other. However it happens, the eggs are released into the water and fertilized. At least, this is how most species do it. In other species, the females may keep the eggs while they develop, or lay them on the ocean floor while males come fertilize them.

The eggs float throughout the ocean, and even after hatching, the juveniles stay afloat. After a few weeks, they make their way to the ocean floor where they start to grow to their adult size over the next few years. The lifespan of sea cucumber varies based on the species, but most live between 5 and 10 years.

Don’t forget to scroll down and comment your guess about what the next animal is!


Photo credits:

  • Sea cucumber – Bernard DUPONT
  • Pink sea cucumber – Public domain
  • Red sea cucumber – NOAA
  • Mystery animal – Public domain
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