10 Curiosities About the Sea Cucumber

Written and verified by the biologist Samuel Sanchez

Last update: 18 November, 2023

There are many curiosities about the sea cucumber, but very few people will come across one in their lives. These echinoderms are found in marine substrates around the world, and in many cases, it’s necessary to put on your diving goggles and dive into the coastal waters to see a specimen. Still, its strange physiology makes it worth the effort.

As not all humans have a beach at their disposal, sometimes we have to settle for getting to know majestic creatures through words and photos. Keep reading, because below we’ll show you 10 curiosities about the sea cucumber that will fascinate you.

1. The sea cucumber is an echinoderm

The term “sea cucumber” doesn’t refer to a single species, but to a group of echinoderms of the class Holothuroidea. Due to their taxonomic classification, they’re considered close relatives of starfish and sea urchins. As we’ll see in later lines, their anatomy justifies this kinship.

There are about 1700 species of cucumbers and holothurians in the world, and the greatest variety is found in the marine region of the Asian Pacific. In turn, the Holothuroidea class is divided into 6 orders and 160 different genera, so it’s considered a fairly diverse taxon at a genetic level.

2. It lives up to its name with its shape!

Sea cucumbers get their common name for a very specific reason. In general, all holothuroids have an elongated shape and a vermiform (wormy) silhouette, but retain the typical pentaradial symmetry of echinoderms. This means that its body can be divided into 5 equal sections starting from the mouth.

As indicated by the Animal Diversity Web portal, depending on the species, the sea cucumber can have a cylindrical to spherical shape. The anterior part of the animal contains the mouth and corresponds to the oral pole of sea urchins, while its anus is the aboral section of other echinoderms. Therefore, it could be said that sea cucumbers live “sideways”.

3. Curiosities about the sea cucumber: fascinating animals of all sizes

The average size of sea cucumbers ranges between 10 and 30 centimeters (4 to 12 inches), but there’s some variability. Holothuria thomasi is the largest species in its group, with an impressive 2-meter (6.6 foot) span. However, it camouflages itself so well on the seabed that it wasn’t discovered by humans until 1980.

Other species of sea cucumber (such as Thyonina bijui) are much smaller and reach a maximum size of 2 centimeters (less than an inch)

4. Very primitive eating habits

One of the curiosities about sea cucumbers is found in its eating habits. All the species included in its group are suspensivores or detritivores, which means that they feed on the free organic matter present in the water column or on the seabed.

In their sedentary forms, some holothuroids “pull” special tentacles out of their mouths, with which they trap microparticles and plankton that are suspended in water. Some of them are strategically placed in areas with currents, thus maximizing the flow of water through their tentacles with minimal effort.

In some areas, so many sea cucumbers are concentrated that they can filter 190 kilos (420 pounds) of organic matter per square meter in a year.

5. A simple digestive system

Now you know that sea cucumbers have a series of oral tentacles that in normal situations aren’t visible, but we now need to talk about the rest of their digestive system. Behind the mouth there’s a pharynx with calcareous plates and, in many cases, this is the only hard part of the body that allows the insertion of the muscles.

Otherwise, its digestive system is very primitive. Some sea cucumbers have an esophagus and a stomach, but others directly discharge the contents of the pharynx into an intestinal network. This is rolled up on itself and ends in a very simple anal opening.

6. The sea cucumber doesn’t have a brain

One of the most fascinating curiosities about the sea cucumber is that it doesn’t have a brain nor a complex nervous system. As studies indicate, its sensory network consists of a nerve ring around the mouth and 5 radial nerves under each of the ambulacral areas, which refers to the pentarradial symmetry described previously.

If the central nervous ring of the sea cucumber is damaged, it’s still able to maintain coordination and autonomy. This means that there’s no area in which its nervous system is centralized, as is the case in the vast majority of living beings (the brain).

7. How do sea cucumbers move?

Despite their archaic shape and absence of limbs, many sea cucumbers move relatively efficiently across the seabed. To “crawl” in the sediment, they use a series of podiums or pedicels, expansions of the body wall in the shape of small tubes that contain a branch of the hydrovascular system.

Beyond these structures, some species use muscular contractures to advance and others (in the Elasipodida order) “jump” more than 1000 meters above the seabed floor thanks to their density being comparable to that of the water that surrounds them. In addition, their dermal structure can be modified according to need, allowing them to fit in very tight spaces.

8. A curious reproduction

Another of the curiosities of the sea cucumber lies in its reproductive system. All holothurians have a single gonad and their system is usually dioecious, that is, the specimens are either male or female (but not both at the same time). In many cases, reproductive individuals release eggs and sperm into the aquatic environment, waiting for them to disperse with ocean currents.

When a specimen of the opposite sex finds a gamete (sperm or egg), it picks it up with its tentacles and it becomes fertilized without further complications.

9. Spilling their guts?

If you knew something about sea cucumbers, then it may have to do with their defense capacity. Well, in case you don’t know, these animals stand out from the rest for their very curious method of avoiding predation. These invertebrates are famous for secreting their intestines to scare their attackers when they are in danger.

This archaic defense method is effective and represents a moderate cost for the sea cucumber, as it’s capable of regenerating its intestines afterwards. As you can imagine, its ability is being investigated in the medical field to provide answers to questions in the field of tissue implants (and much more).

10. An endangered species

As the last of the curiosities about the sea cucumber, it should be noted that many of the species are endangered by overfishing. These echinoderms are considered a delicacy in many regions where they’re endemic, so they are hunted indiscriminately in order to prepare dishes only available to the chosen few.

Sea cucumbers, or holothurians, recycle organic matter from the seabed and are an important part of the diet of many fish, so we can’t allow them to disappear. They’re a vital part of the marine ecosystem that deserves preservation.