Sea Urchin Definition
Sea urchins are small marine invertebrates of the Echinodermata category of animals. They are distinguished by having a globular body covered in multiple spines. There are approximately 700 different species of sea urchin which have adapted to many different environments, meaning that sea urchins are present in all of the world's oceans.
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Echinoderm Features
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Sea urchins belong to the family of animals known as "echinoderms," which includes Starfish and Brittle Stars, and they share certain characteristics with all members of this family. The first is that they have a water-vascular system which pumps water through an organ called the madroporite, which is an echinoderm's equivalent of a heart. Their bodies are partly made off hard, chalky plates called ossicles and they all have some form of tube feet. All echinoderms have radial symmetry, small jaws that function via the water-vascular system, and most have the ability to regenerate any limbs that they may lose through disease or predation. No echinoderm has a brain.
Unique Sea Urchin Features
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Sea urchins differ from other echinoderms in that they have multiple spines which they are capable of controlling and on which they move around. These spines can be different lengths depending on the species, although the largest, the Red Sea urchin, can have spines up to 4 inches long. Between these spines, sea urchins have five paired rows of tube feet which help with their locomotion, capturing food, and when conditions require it, anchoring the animal to the sea floor. They also have small pincer-like organs, called pedicellariae, that are tools for catching food and defending themselves. These pincers also contain a toxin.
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Habitat
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Different species of sea urchin have adapted to widely varied marine environments. They are found from the tropical seas of the Caribbean to the waters under the Antarctic ice and anywhere at least one form of food is available. They live on the sea floor and habituate coral reefs, kelp forests, sea grass beds and rock pools.
Diet
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Sea urchins will eat almost any plant and animal matter that they find on the sea floor. These include kelp, algae and sponges as well as algae, mussels and barnacles. They will feed on the decaying flesh of dead fish and mammals that have sunk to the sea bed. A sea urchin's mouth parts are located on its underside and consist of 5 plates that point inwards. This arrangement has been named "Aristotle's Lantern" by scientists.
Sea Urchins As Food
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A number of different species feed upon sea urchins. Besides fish and eels, among sea-dwelling animals, they are consumed by other echinoderms such as starfish, crabs and sea snails. Sea urchins are also preyed upon by non-marine animals such as otters and many types of sea bird. Humans consume sea urchins too, with many believing that they contain aphrodisiac properties.
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References
- Photo Credit sea-urchin image by Piter Pkruger from Fotolia.com
