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What is a Sea Cucumber?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 01 November 2016
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A sea cucumber is an echinoderm in the class Holothuroidea that can be found in all of the oceans of the world at depths ranging from the inter-tidal zone to the bottom of oceanic trenches. There are over 1,000 species of Holothurians, although all of them have the long cucumber-shaped bodies that give the animals their common name. Like other echinoderms, sea cucumbers display five sided radial symmetry along the length of their bodies. The rather bizarre ocean creatures are also prized in some Asian culinary traditions.

From a distance, one could be forgiven for mistaking a sea cucumber for an actual cucumber. Most of the time, the animals rest on the ocean floor, filtering food from the water that flows around them. Their dark, knobbly bodies do strongly resemble cucumbers, although some exotic species give away their animal nature with warts in colors like orange and blue. They eat plankton and other organic material, either through filter feeding or by sifting through the sediment on the ocean floor with the flexible tentacles that surround their mouths.

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Like other ocean-going animals, the sea cucumber filters out oxygen from seawater to breathe. In this case, an apparatus to extract oxygen — called a breathing tree — starts at the base of the anus and runs along the animal's body. The cucumber intakes small amounts of water through the anus and cloaca, and then expels the waste back through the cloaca. The animal's simple breathing and digestive system suggests that it has been around for millions of years, and there are fossilized examples to support this hypothesis from the Silurian period, 400 million years ago.

Some species in this class do have a trick up their sleeve when self-defense is needed. Under moderate amounts of stress, one will squirt water from both ends. Under extreme stress, however, the animal will regurgitate its stomach and escape in the subsequent chaos. The stomach will regrow within a relatively brief period of time. Other echinoderms are also able to force their stomachs inside out, usually to feed, but the stomach usually retracts back into the body.

In Asian cuisine, sea cucumber appears both fresh and dried, and it is commonly used in soups and stews. The flavor is bland, and the texture is rubbery and slightly gelatinous. The animal is also used in some traditional medicines, and Western pharmaceutical companies have begun to study it to see if it contains useful properties. Some species appear to have anticoagulant, anti-inflammatory, and antimicrobial compounds that may be usable in medical treatment.

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GlassAxe
Post 2

@ Anon73328- Sea cucumbers have both bilateral and radial symmetry. Sea cucumbers are purely pentaradial in symmetry during the larvae stage, but as the sea cucumber matures, it evolves a bilateral symmetry that is secondary to their pentaradial symmetry.

To understand this, think of the sea cucumber like a five-armed starfish. The anus of a sea cucumber is on one end of the animal, while the mouth is on the other. In between the mouth and the anus are five separate strips of flesh that make up the dermal layer of the sea cucumber. If you imagined those strips as unconnected, you could stand the creature up on its mouth, push the mouth and anus together, and you would

have an animal with an anatomy similar to a starfish.

The adult sea cucumber spends its life lying horizontally, so it has developed a bilateral symmetry. If you cut the creature in half from mouth to anus the creature would be bilaterally symmetrical. That was tough to articulate, but I hope it helps.

anon73328
Post 1

I am curious how you would respond to those sites that indicate that sea cucumbers have bilateral symmetry. My students and I are in "discussion" regarding the symmetry. I was taught that the sea cucumber has 5 parts, therefore radial symmetry. Can anyone help solve this?

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