Common Name: Sea Cucumbers, Holothurians
A.K.A.: Class Holothuroidea
- Approximately 1250 species
- Size: 2-200cm (¾” to 6.5’)
- Lifespan: 5-10 years in the wild
Found: Throughout the oceans, in both shallow and very deep regions
It Does What?!
Where to begin? This is an odd one… To start, despite the name sea cucumber, this isn’t a plant but an animal; a relative of starfish and sea urchins. One could be forgiven for mistaking the holothurians for plants, however. Most spend their lives lying on the ocean floor, looking like a sunken vegetable, and covering a distance of a couple metres or less per day in their search for food. The creatures feed on small particles, like algae and plankton. There is a tiny mouth at one end of their body, surrounded by between eight and thirty tentacle-like feet with which they grab their food and which can actually be retracted into their mouth. But that’s not really the interesting end of a sea cucumber, as we’ll see.
Lacking both eyes and any rapid means of locomotion, holothurians are tempting prey for crabs, fish, and other large sea creatures. When threatened, they have the single most bizarre and seemingly impractical defence mechanism ever evolved: self-evisceration. As a predator approaches, the sea cucumber violently contracts the muscles around its body wall and actually expels its own internal organs via its anus (demurely labelled as the ‘aboral pole’ in the diagram). Yes, really. In some species, these organs include most of the creature’s respiratory system, which takes the form of sticky threads that blanket and ensnare the predator. And just to add genuine injury to the insult, this discharge is accompanied by a toxic chemical known as holothurin, which kills whatever’s nearby. Disgusting, but effective. Once expelled, the missing organs can be regenerated in 1-5 weeks, depending on the species. Some researchers speculate that this ability may even be used as a means of ridding the organism of accumulated waste or parasites. The ultimate detox regime, if you will.
One such parasite is the pearl fish. You see, holothurians actually breathe through their rear end as well, so when one of them, umm… opens up… to take in some fresh, oxygenated water, in goes the fish, which then feeds on the sea cucumber’s internal organs. You can see why they might want to rid themselves of this visitor.
Strange as it all seems, the sea cucumber’s strategy is quite a successful one. At depths below five and a half miles (8.8km), they make up fully 90% of the mass of all macrofauna (i.e. any animal that’s not microscopic). Among the species that live at shallower depths, populations can reach a density of 1000 cucumbers per square metre. And it’s a good thing, because they’ve got one predator with whom spewing out their guts won’t work: humans. Sea cucumbers are a popular ingredient in Chinese and other Southeast Asian cuisines, although only about ten species are used for this purpose. These species are farmed commercially in artificial ponds, and are also used in traditional Chinese medicine. Perhaps not surprisingly, they are considered to improve male sexual health.
[Fun fact: Sea cucumbers have a body wall made up of collagen fibres which they can ‘unhook’ at will, essentially liquefying their interiors and allowing them to squeeze into very small cavities as a means of hiding from predators. Once inside the cavity, they re-solidify themselves, making the creature very difficult to extract from its hideout.]
- Byrne (2001) Journal of Experimental Biology 204: 849-863.
- National Geographic
- New Zealand National Institute of Water and Atmospheric Research
- Piper (2007) Extraordinary Animals. Greenwood Press: Westport, Connecticut.
- Vandenspiegel et al. (2000) Biological Bulletin 198(1): 34-49.