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Sea Urchins: Spiny Shelled Sea Spheres

Posted by Greg on December 7, 2011

Purple Sea Urchin. Photo by Janek Pfeifer (Wikimedia Commons)

Just try to say that three times fast!

While I was doing the article on sea stars, my sources referenced sea urchins and sand dollars as related animals (echinoderm). Since I have sand dollar and sea star articles, it follows I should complete the set.  As with the first two, urchins are varied and beautiful in shape and color. According to the Tree of Life web project,  “sea urchins are sea creatures that live in oceans all over the world.  Similar to sea stars, sea urchins have a water vascular system. Their spherical shape is typically small, ranging from about 3 cm to 10 cm in diameter, and their bodies are covered with a spiny shell. The skeleton of a sea urchin is also known as the test. The shells within the test of these creatures are made up of packed, fitted plates which protect them from being damaged.  As for the spines outlining their shell, these are movable and help the sea urchin to camouflage or protect itself from predators. Sea urchins can vary greatly in colour.  Some of the most frequently seen colours are black, red, brown, purple and light pink.  On the bottom side of a sea urchin there are five teeth that these organisms use to ingest algae and break down other foods they consume to survive. These five teeth continually grow throughout the sea urchin’s life.  On the outside of their body, they also have hundreds of transparent tubes that emerge which allow them to stick to the bottom of the ocean or to move at a very slow pace.  These unusual tubes are called ‘tube feet.’  Their tube feet are much longer than the spines outlining their shells and they are also used by the sea urchin to trap food and in respiration.”

Two Urchins in a Coral Reef off the Coast of Kona, Hawaii. Photo by Mila Zinkova (Wikimedia Commons)

Sea urchins are found worldwide in both warm and cold ocean environments.  E.G.D. tells me she often saw them clinging to coral reefs and to crevices in rocks when she went snorkeling off the coast of Oahu. The Tree of Life web project article referenced above mentions that they also live in rock pools, kelp forests and sea grass beds and that they like to “lodge themselves half way into the surface of sand, mud or holes.”  They are able to access food in these areas since they feed on algae, sea grass and seaweed.

While admiring these creatures in tide pools and rock pools, please always follow tide pool etiquette.  Also, here’s a note from E.G.D.: if you’re snorkeling in Hawaii, please be careful not to step on the sea urchins!  They’re all over the place, and they aren’t always obvious to the eye.

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