Few animals seem less animal-like than sea cucumbers. The long, slender beings often appear motionless like the vegetable they resemble as they bask on the seafloor. They lack a brain, eyes or obvious features apart from a mouth and anus.
Neither are sea cucumbers especially appealing to human senses. Their bodies, often drab gray or brown, can be wart-studded and rough or slimy to the touch. “They’re not charismatic animals,” admits Annie Mercier, a researcher in marine biology at Memorial University in Canada. That doesn’t stop her from loving them.
Scientists like Mercier are learning there’s a lot more to these simple creatures than meets the eye. Some care for their young, others reproduce by dividing in two—they have an impressive ability to regrow lost tissues. To boot, many of the 1,700-plus species of sea cucumber are important for ocean health because they hoover up ecosystem-smothering algae and bacteria. “If you look at the diversity of shapes and forms and behaviors across all the sea cucumbers that exist, it’s pretty incredible,” says Mercier, who coauthored a 2025 article on the creatures in the Annual Review of Marine Science.
But many are rapidly vanishing. About eighty species are harvested commercially. Dried sea cucumbers are especially prized in East Asian cuisines; some species are being fished to the brink of extinction and are targets of illegal trade.
Scientists hope that better understanding sea cucumbers and the threats they face will help to forge paths to protect them.
To Be a Sea Cucumber
Sea cucumbers are echinoderms, invertebrates including starfish and sea urchins that have a five-rayed body arrangement. One could think of a sea cucumber as an elongated urchin that has largely lost its crunchy external skeleton. It has two openings: the food-ingesting mouth and the waste-excreting cloaca, or anus, which also hosts a breathing organ for extracting oxygen from seawater.
Beyond this basic form, sea cucumbers are astonishingly diverse. Many are brightly colored. Some are transparent, others twinkle with bioluminescent light. Some are shorter than a fingernail, others as long as a person is tall.
Though their larvae can swim, adults are slow-moving, using sticky tube feet to peruse the surfaces they meander over. Some arch their bodies upwards, extending feathery tentacles into the water to capture plankton. Others rhythmically burrow through and ingest seafloor sediment, extracting bacteria and algae and leaving moundlike trails of cleaner excreted material as they go.
With these laborious methods of locomotion, sea cucumbers were long thought to never exceed speeds of about a yard a day. But scientists recently spotted them detaching from the seafloor and moving much faster. For instance, a deep-sea species called the headless chicken sea monster was observed using winglike veils to swim, “like ethereal aquatic angels,” as noted in The World of Sea Cucumbers, a 2023 book cowritten and coedited by Mercier.
And in 2019, Mercier’s lab discovered that the North Atlantic orange-footed sea cucumber and an Indo-Pacific species known as the sandfish can float by taking up water through the mouth and anus, making themselves more buoyant. Detaching from the seafloor, they can bob along in the current for hundreds of meters and escape crowded conditions or dirty or not-salty-enough waters. “They can essentially move away from a situation that is unfavorable to them,” Mercier says.
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The Little Plankton That Could
Sea cucumbers have similarly diverse strategies for reproduction. In some species, the males release pheromones to attract aggregations of other sea cucumbers. The females release thousands to millions of eggs from a hole toward the front of their bodies; these are then fertilized by sperm. In other species, eggs stay attached to the female; she nourishes and protects the developing embryos.
And, about 20 species “literally just divide into two,” says marine biologist Sven Uthicke at the Australian Institute of Marine Science. In a remarkable feat of regeneration, the separated sea cucumber halves regrow the missing portion of themselves within weeks.
Sea cucumbers use their regenerative powers for survival, too. Some species spit out their guts though their mouths, “which might be a way to get rid of pollution,” Uthicke says. Others excrete parts of their innards through their anus when attacked, in order to distract predators. In both instances, they neatly regenerate the discarded body parts.
And when the silky sea cucumber senses a bite or tear, it pinches off the injured body part within 30 seconds. Other species simply dissolve their body walls when grabbed and removed from the water.
Beyond their alluring biology, sea cucumbers are critical ocean denizens—and not just as prey. Their burrowing and sediment-gobbling habits aerate the seafloor, making it a healthier habitat for crabs, mollusks and worms. They do this at massive scale: One 2021 study on an Australian reef estimated that lollyfish, or black sea cucumbers, turn around 71,000 tons of sediment per year, heavier than five Eiffel Towers.
By vacuuming microbes out of the sand, sea cucumbers reduce the growth of ecosystem-smothering algae as well as nasty bacteria that cause coral disease: A 2024 experiment found that removing sea cucumbers on two Pacific reefs led to a surge in sickness and death among many staghorn corals. Sediment-gobbling also releases nutrients like nitrogen, phosphorus and calcium carbonate from the sand, to be used by other animals.
On top of this, many sea cucumbers host dozens of life-forms in and on their bodies—including the cloaca-inhabiting pearlfish. Sea cucumbers “are not just an animal,” says marine biologist Chantal Conand, an honorary associate at the National Museum of Natural History in Paris. “They have a whole ecosystem living with them.”
Conserving these animals is no easy task. Demand for dried sea cucumbers has skyrocketed, especially in China, where they’re considered to have health-boosting properties and are traditionally served as a symbol of status and generosity. As the Chinese population has become wealthier, “the market is growing for these products,” says marine biologist Alessandro Lovatelli, an aquaculture officer at the Food and Agriculture Organization of the United Nations.
While demand initially focused on temperate species along the Chinese coast, burgeoning demand has been driving traders to look further afield. Sea cucumber harvesting for East Asian buyers is now an important livelihood in South America, Africa, the Pacific and the Caribbean. The animals are easy to catch, dry and store, and can fetch hundreds of dollars per pound, driving locals to harvest many tropical species to the brink. As they become rarer, fishermen are using more dangerous diving tactics to get at deeper-living sea cucumbers. In Mexico, fishermen have engaged in violent disputes over sea-cucumber fishing territories.
Reversing these trends is difficult, especially since there are sparse data on species declines, which are key to getting them legal protections. And even where species have been assessed as endangered and placed on lists that prohibit their trade, illegal smuggling persists.
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Lovatelli and his colleagues have helped many countries to develop management plans for sustainable sea cucumber harvesting—but many populations have collapsed regardless because governments lack the resources to enforce fishing rules. Lovatelli’s team has also crafted guides to help custom officers catch endangered species crossing borders, but many exports from small islands are collected by boats and bypass customs.
Some countries have started farming sea cucumbers in contained areas, but that hasn’t entirely stopped the harvesting of wild animals. “It’s a difficult industry to control,” Lovatelli says.
It will take concerted effort by international organizations, governments and fisheries to ensure that threatened sea cucumbers continue to exist. But at least there’s now an uptick of interest in studying and protecting these ugly yet prized creatures of the sea, and that gives Mercier hope. “I think that’s definitely a step in the right direction,” she says.
This article is republished from Knowable Magazine under a Creative Commons license. Read the original article.
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