Invertebrates
Sea Cucumber
Holothuroidea · also called sea cucumber
The sea cucumber spends its life quietly eating its way across the ocean floor, one mouthful of sand at a time, cleaning and recycling sediment as it goes. When something tries to eat it instead, a few species fight back by launching their own internal organs at the attacker.
Overview
Sea cucumbers live on ocean floors worldwide, from shallow reefs to the deepest trenches. Ranging from 3 cm to over a metre long, they are close relatives of starfish and sea urchins, sharing the same five-part radial body plan stretched into a tube. They eat sediment and organic particles.
Nature’s vacuum cleaners
By constantly ingesting and excreting seafloor sediment, dense sea cucumber populations recycle nutrients and help keep reef and seabed ecosystems healthy.
Exploding in self-defence
Threatened, some species eject sticky internal organs at a predator through the body wall, then regenerate them over the following weeks.
Status
Sea cucumbers are Data Deficient overall and declining in many reef areas, heavily targeted by the international beche-de-mer trade.
Sea Cucumber: common questions
Why does a sea cucumber eject its own organs?
It is a genuine, deliberate defence mechanism called evisceration, used by several sea cucumber species when they are grabbed, squeezed, or otherwise seriously threatened by a predator. The animal contracts its body forcefully and expels sticky, often toxic internal structures, most commonly specialised tubules from its respiratory system, out through its body wall or anus toward the attacker. These expelled tubules can tangle around the predator's mouthparts or simply provide a startling, unpleasant distraction, buying the sea cucumber time to escape or simply be abandoned as not worth the trouble, after which it slowly regenerates the lost internal organs over the following weeks to months.
Is a sea cucumber an animal or a plant?
It is entirely an animal, despite the plant-like common name that comes purely from its cucumber-like shape. Sea cucumbers belong to the echinoderms, the same major animal group that includes starfish, sea urchins, and sand dollars, and like their relatives they share an underlying five-part radial body symmetry, visible in features such as the five rows of tube feet running the length of the body, even though the overall shape has been stretched out into an elongated tube rather than a star or a ball. They have a simple nervous system, a digestive tract running the length of the body, and no brain, but they are unambiguously animals.
Why are sea cucumbers important to ocean ecosystems?
They function as some of the ocean floor's most important recyclers, continuously ingesting sediment as they crawl and digesting the organic material within it, including algae, dead organisms, and waste particles, before excreting cleaned sand or mud. In dense populations on healthy reefs, this constant processing turns over enormous volumes of sediment, aerating it, recycling locked-up nutrients back into a form other organisms can use, and helping to buffer the acidity of surrounding seawater. Areas that lose their sea cucumber populations to overharvesting often show measurable declines in sediment and water quality as a result.
Why are so many sea cucumber populations declining?
Dried sea cucumber, called beche-de-mer or trepang, is a highly valued food and traditional medicine ingredient across large parts of Asia, and the resulting international demand has driven intense, often unregulated harvesting of reef and shallow-water species across the Indo-Pacific, the Caribbean, and elsewhere. Because sea cucumbers are slow-growing, easy to collect by hand in shallow water, and provide no legal protection in many countries, populations of the most commercially valuable species have collapsed in numerous accessible reef areas within a matter of years, prompting some nations to introduce fishing bans, quotas, or aquaculture programmes to reduce pressure on wild stocks.