Sea cucumbers: Weird and wacky natural recyclers

Sea cucumbers are marine invertebrates in the class Holothuroidea that live on the sea floor in both shallow and deep waters. They are echinoderms like sea urchins and starfish, which are also known as sea stars.

There are over 1,000 species of sea cucumber. Many have soft, water-filled bodies, leathery skin and do indeed resemble a cucumber. However, there is huge variety in shape, size and colour between species.

Like other echinoderms, sea cucumbers’ bodies have five symmetrical sections with a mouth at one end and an anus at the other. The average length of a sea cucumber is 20 centimetres.

The holothurian skeletal structure is made of very small particles of calcium carbonate called ossicles. This lack of a rigid structure contributes to their blobby, watery appearance.

‘The sea cucumber looks nothing like a traditional echinoderm,’ explains Hugh Carter, our Senior Curator of Echinoderms.

‘There’s a diversity of forms because they have been evolving on their own for at least 450 million years. Reducing their skeleton has allowed sea cucumbers to adopt a range of shapes that other echinoderms couldn’t.’ 

Sea cucumbers feed on tiny waste particles such as plankton and decaying carcasses. They have 8-30 tentacles surrounding their mouth that help catch and pass food particles into it.  

Predators of sea cucumbers include fish, crabs and turtles. They are also enjoyed as food by people in parts of Asia, especially China, where they are considered a delicacy.

Most sea cucumbers reproduce by releasing eggs into the water to be fertilised. Some species in cooler waters fertilise their eggs inside the body instead and brood them until the young are ready to be released into the water.

Sea cucumbers are part of the ocean’s natural recycling system. They break waste substances down to be reused, in a similar way to worms on land.

‘Sea cucumbers will suck up a whole load of sediment, absorb any nutrients out of that and excrete the rest,’ explains Hugh.

This process is called bioturbation and it helps to make sediment and calcium carbonate to be used by other organisms. Some deep-sea species are capable of swimming short distances and may also use the sudden release of waste as a means of propulsion to help them ‘take off’ into the water column.

Here are eight examples of weird and wonderful sea cucumbers:

1. Sea apple, Pseudocolochirus species

Sea apples are striking creatures that can be found nestled in coral reefs. They have feathery tentacles that pick up food particles. They insert one tentacle into their mouths at a time to collect the morsels and recoat the appendage with plankton-catching mucus.

These sea cucumbers are night feeders and can sense whether there’s food about. If there isn’t much available, they’ll tuck their tentacles away to protect them.

Sea apples are one of several sea cucumber species that can eviscerate. This means they expel white and sticky internal organs called Cuvierian tubules out of their anus to put off potential predators. This isn’t fatal and the sea cucumber can grow them back.

A sea apple can also defend itself by releasing the toxin holothurin into the surrounding water. If really perturbed, they can swell to twice their normal size, allowing themselves to be picked up by water currents and whisked away from danger.

2. Red sea cucumber, Cucumaria miniata

The red sea cucumber is a brightly coloured Pacific species that can be found in the intertidal zone - the area of the ocean between low and high tide. This species is sometimes also called the orange sea cucumber or the red sea gherkin.

The red sea cucumber seeks habitats where the current will bring a wealth of food its way. It is a suspension feeder, which means it waves its bushy, tree-like tentacles in the water to catch detritus and plankton to eat.

Red sea cucumbers often embed themselves into surfaces such as rocks, crevices or coral, with only the food-seeking tentacles exposed. This helps to protect them from predators.

3. Sea pig, Scotoplanes globose

Named for their partly see-through, pinkish bodies, sea pigs live in the deep ocean, found at depths of up to almost 5,000 metres. Their jelly-like body makes them well adapted to life far beneath the surface. On some parts of the deep-sea floor, sea pigs account for more than 95% of the total weight of animals.

Like many sea cucumbers, sea pigs get about on special limbs called tube feet. Tube feet use the hydraulic pressure of water moving in and out of the limbs to power their movement. Sea pigs have between five and seven pairs of enlarged tube feet.

Sea pigs are hard to retrieve in good enough condition to study, which makes them rather mysterious. We’re not sure how they mate or how long they live. 

4. Headless chicken monster, Enypniastes eximia

Most sea cucumbers live on the seabed. Only a few, such as Enypniastes eximia, are pelagic as adults, meaning they spend most of their time up in the water above the seafloor.

Enypniastes means ‘dreamer’, owing to the dreamlike way that they swim. The deepest Enypniastes has ever been recorded is at almost 7,000 metres down in the Java Trench.

This sea cucumber has evolved webbed fin-like structures that help it move through the water. It is thought they developed this adaptation to seek mates for reproduction or to escape predators. This swimming species is sometimes called the headless chicken monster due to its unusual appearance.

Another genus of pelagic sea cucumber is Pelagothuria. These beautiful, semi-transparent creatures move gracefully through the water. Their fragile, umbrella-shaped bodies mean they're often mistaken for jellyfish.

5. Snake sea cucumber, Synapta maculata

The snake sea cucumber lives in shallow tropical waters in the Indo-Pacific Ocean.

It belongs to a family of sea cucumbers called Synaptidae. Synaptids have long, delicate bodies which move in undulating, snake-like motions. Synaptids don’t have tube feet but instead stick to things using tiny hooks in their skin called sclerites.

Some synaptids can grow up to three metres in length, making them the largest sea cucumbers. At the other end of the scale, the smallest recorded sea cucumber, Psammothuria ganapatii, is only four millimetres long and can inhabit the space between individual grains of sand.

6. Japanese spiky sea cucumber, Apostichopus japonicus

The Japanese spiky sea cucumber is the most in-demand holothurian. It’s harvested for food and traditional medicine, particularly in China. An especially prized delicacy is the dried outer wall of the sea cucumber, which is called beche-de-mer or trepang.

Japanese spiky sea cucumbers have been over-harvested over the past twenty years. They are one of seven sea cucumber species listed as endangered on the IUCN Red List and are now commercially farmed to try to meet demand.

7. Blue sea cucumber, Actinopyga caerulea

This colourful sea cucumber has a large white body covered in bright blue tube feet. They can be found in deeper tropical waters.

Sea cucumbers’ bodies are hollow and water-filled, and creatures such as pearlfish are known to shelter in the anal opening. However, blue sea cucumbers and others in the genus Actinopyga have five sharp, heavily calcified tube feet to guard the cavity from unwanted houseguests. These are sometimes called anal teeth.

8. Gummy squirrel, Psychropotes longicauda

Psychropotes longicauda resembles a large, squirrel-shaped gummy bear. Gummy squirrels have yellow bodies and a bright red underbelly on which they have 18 short feeding tentacles.

Their tail-like appendage may be used to move through the water, possibly by catching deep ocean currents that zip them along the seafloor.

Even sea cucumbers living in the deepest oceans can be affected by human activities. A recent research project catalogued over 5,000 incredible creatures that live in an area called the Clarion-Clipperton Zone that could become a hotspot for deep-sea mining. This includes the gummy squirrel and other sea cucumber species.