8 Surprising Facts About Sea Cucumbers

Plankton feeding sea cucumbers
Borut Furlan/WaterFrame/Getty Images

The odd-looking creatures shown here are sea cucumbers. These sea cucumbers are using their tentacles to filter plankton from the water. In this slide show, you can learn some surprising facts about sea cucumbers. 

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Sea Cucumbers Are Animals

Sea cucumber (Bohadschia argus)
Bob Halstead/Lonely Planet Images/Getty Images

One of the most surprising things about sea cucumbers may be that they are animals, not plants. Yes, that blob in the image is an animal.

There are about 1,500 species of sea cucumbers and they display a variety of colors, shapes, and sizes. They can be from less than an inch to several feet in length.

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Relatives of Sea Stars, Sand Dollars, and Urchins

Giant California Sea Cucumber (Parastichopus californicus) 'vacuuming' kelp forest floor of small organisms
Mark Conlin/Oxford Scientific/Getty Images

Although they don't look like it, sea cucumbers are related to sea stars, sea urchins, and sand dollars. This means they are echinoderms. Most echinoderms have visible spines, but the spines of a sea cucumber are tiny ossicles embedded in their skin. For some sea cucumber species, the tiny ossicles provide the only visible clue to the species' identity. The shape and size of these ossicles are examined under a microscope because they are so small.  

Like other echinoderms, sea cucumbers have a water vascular system and tube feet. The water vascular system of sea cucumbers is filled with body fluid rather than seawater.

Sea cucumbers have a mouth at one end and anus at the other. A ring of tentacles (actually modified tube feet) surrounds the mouth. These tentacles that collect food particles. Some sea cucumber filter-feed but many obtain food from the ocean bottom. As the tentacles push into the ocean bottom, food particles attach to mucus.

Although they have five rows of tube feet, sea cucumbers move very slowly, if at all. 

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Sea Cucumbers Breathe Through Their Anus

Swimming crab in sea cucumber anus, Philippines
Close up of swimming crab in sea cucumber anus. Borut Furlan/WaterFrame/Getty Images

Yes, you read that right. Sea cucumbers breathe through a respiratory tree that is connected to their anus.

The respiratory tree lies inside the body on either side of the intestine and connects to the cloaca. The sea cucumber breathes by drawing oxygenated water in through the anus. The water goes into the respiratory tree and oxygen is transferred into the fluids within the body cavity.

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Sea Cucumbers Play an Important Role in Cycling Nutrients

Excretions of Sea Cucumber, Marsa Alam, Red Sea, Egypt
Reinhard Dirscherl/WaterFrame/Getty Images

Some sea cucumbers collect food from the surrounding water, while others find food on or in the ocean bottom.  Some sea cucumbers bury themselves fully in the sediment.

Some species ingest sediment, remove the food particles and then excrete the sediment in long strands. One sea cucumber can filter up to 99 pounds of sediment in a year. The excretions of sea cucumbers help keep nutrients cycling throughout the ocean ecosystem. 

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Sea Cucumbers Are Found From Shallow Tide Pools to the Deep Sea

Orange filter-feeding sea cucumber
Ethan Daniels/WaterFrame/Getty Images

Sea cucumbers live in a wide range of habitats, from shallow coastal areas to the deep sea. They are found in oceans around the world.

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Sea Cucumbers Can Expel Their Inner Organs

Leopard sea cucumber with toxic sticky white tubules (Cuvierian tubules) released from anus for defense
Auscape/UIG/Universal Images Group/Getty Images

Sea cucumbers have a surprising defense mechanism in which they will expel their inner organs if they feel threatened, or even if they are overcrowded or subjected to poor water quality in an aquarium.

Some sea urchins, like the one shown here, expel Cuvierian tubules. These are located at the base of the respiratory tree, the sea cucumber's breathing organ. These tubercles can be expelled if the sea cucumber is disturbed.  

In addition to expelling these tubercles, sea cucumbers can expel internal organs. This process, called evisceration, may occur if the sea cucumber is disturbed or threatened. It may also occur regularly, possibly as a way for the sea cucumber to purge its inner organs of excess wastes or chemicals. Once the organs are discharged, they regenerate within days or weeks.

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There Are Male and Female Sea Cucumbers

Sea cucumber spawning eggs
Franco Banfi/WaterFrame/Getty Images

In most species of sea cucumbers, there are both males and females, although differences aren't externally visible. Many species reproduce by spawning — broadcasting their sperm and eggs into the water column. There, the eggs are fertilized and become swimming larvae that later settle to the ocean bottom.

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Sea Cucumbers Are Edible

Sea cucumber in abalone sauce
Jakob Montrasio/Moment Open/Getty Images

Sea cucumbers are harvested for use in food and medicine. Sea cucumbers have catch connective tissue, which seems to magically go from being stiff to flexible in mere seconds. This aspect of the sea cucumber is being studied for its potential application to the health and repair of human tendons and ligaments. 

These animals are considered a delicacy in some areas and are especially popular in Asian countries. However, the unregulated harvest of sea cucumbers has caused a decline in some areas. In January 2016, rules were put into place to restrict sea cucumber harvesting in Hawaii due to the decimation of nearshore populations in Maui and Oahu.

References and Further Information

  • Coulombe, D.A. 1984. The Seaside Naturalist. Simon & Schuster: New York.
  • Denny, M.W. and S.D. Gaines. 2007. Encyclopedia of Tidepools and Rocky Shores. University of California Press: Berkeley.
  • Lambert, P. 1997. Sea Cucumbers of British Columbia, Southeast Alaska and Puget Sound. UBC Press. 
  • Mah, C. 2013. The Importance of Sea Cucumber Poop. The Echinoblog. Accessed January 31, 2016. 
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Kennedy, Jennifer. "8 Surprising Facts About Sea Cucumbers." ThoughtCo, Aug. 26, 2020, thoughtco.com/surprising-facts-about-sea-cucumbers-2291852. Kennedy, Jennifer. (2020, August 26). 8 Surprising Facts About Sea Cucumbers. Retrieved from https://www.thoughtco.com/surprising-facts-about-sea-cucumbers-2291852 Kennedy, Jennifer. "8 Surprising Facts About Sea Cucumbers." ThoughtCo. https://www.thoughtco.com/surprising-facts-about-sea-cucumbers-2291852 (accessed March 30, 2023).