Sea cucumbers

Common Name:

Sea cucumbers

Scientific Name:

Holothuroidea

Type:

Invertebrates

Diet:

Omnivore

Average Life Span In The Wild:

5 to 10 years

Size:

0.75 inches to 10 feet

Size relative to a 6-ft human:

What is a sea cucumber?

Sea cucumbers might look like the lumpy cylindrical vegetable but these bizarre animals are echinoderms—an invertebrate like starfish, sea urchins, and feather stars.

There are around 1,200 known species of sea cucumber. They live in salt water on or near the seafloor—sometimes partially buried beneath it. They can be found in every ocean in the world, usually in the shallows although there are some deep-water species. For an animal that has no brain, they have evolved very clever ways of moving through the ocean currents, defending themselves against enemies, and finding and eating food.

Appearance and behavior

Sea cucumbers have bumps over their bodies but, unlike a cucumber, they come in a range of colors: brown, red, orange, yellow, white, blue, or even patterned. Ten species are known to be bioluminescent and scientists believe 200 more might be able to emit light.

These strange animals—which don’t have eyes or a brain—come in a variety of sizes, usually ranging from 0.75 inches to 6.5 feet long. However some species, such as the Synapta maculata, or snake sea cucumber, can reach a staggering 10 feet.

Sea cucumbers typically hold onto the seabed, or crawl along slowly, using tiny tube feet—projections from their body with suckers at the end that allow them to move, anchor themselves, or hold onto prey. However, when they need to pick up the pace, they can flood themselves with water to become more buoyant and float away on the ocean’s currents at speeds of up to 56 miles (90 kilometers) per day.

(Watch a sea cucumber poop—and clean the seafloor in the process.)

Diet and feeding

These animals feed on tiny particles of organic matter like algae, plankton, or waste materials. They gather their food with the modified tube feet tentacles surrounding their mouths—they have between eight and 30—then break the particles down into even smaller pieces. 

Some sea cucumbers eat the plankton floating past them, waving their feathery, tentacle-like tube feet in the water until plankton get stuck on them. Then they bring it to their mouth to eat like someone licking their fingers one by one after a delicious meal.

Other sea cucumbers walk across the seafloor looking for food in the sediment and pooping as they go. By digesting these materials, they clean the sand, filter the water, and recycle nutrients—a bit like earthworms on land. They play an important part in keeping the ecosystem healthy, preventing algal blooms, and reducing ocean acidification.

Defensive adaptations

Although sea cucumbers can’t run away from predators or mount an attack, they’re not entirely defenseless. When threatened, some species discharge sticky white threads—called Cuvierian tubules—that act like glue and entangle their enemies.

(Meet a fish that lives inside a sea cucumber's bum.)

Others can mutilate their own bodies as a defense mechanism. They violently contract their muscles and jettison some of their internal organs through their anus as a way of scaring off an attacker. The missing body parts are quickly regenerated—an ability which makes them an interesting model animal in regenerative medicine.

Some deep-sea species of cucumber are even able to use a forceful expulsion of waste to propel themselves through the water when in need of an escape.

Reproduction

Sea cucumbers can breed sexually or asexually. Sexual reproduction is more typical, but the process is not very intimate. Females release eggs into the water while males nearby release their sperm. Fertilization occurs when the eggs and sperm meet. There must be many individuals in a sea cucumber population for this reproductive method to be successful.

Threats to survival

Sea cucumbers, particularly eggs and young larvae, are prey for fish, crabs, turtles, and other marine animals. They are also eaten by humans—around 80 species are edible—and often made into a dried product called bêche-de-mer. Sea cucumber is especially popular in China where, like shark fins, they are seen as a status symbol and believed to have medicinal properties.

These slow-moving, shallow water animals are easy to catch. This puts them at risk of overfishing and more than 70 species are exploited for profit. Between 1996 and 2011, the number of countries exporting sea cucumbers increased from 35 to 83.