No, urchins are not indigenous to the desert. The usual home for Strongylocentrotus purpuratus is the ocean's rocky floor or grazing along the intertidal and subtidal areas. Sea urchins, are one species of the over 6000 echinoderms, which are in the scientific class Animalia and a member of the group known as invertebrates (animals without backbones). Echinoderms are known for their round symmetry. The symmetry most often has five equal parts, easily seen when looking at another echinoderm, the starfish and its five radial arms.

Except for their round shape, the symmetry of sea urchins are more difficult to view. It is not easy to see they have five point radial design that makes them pentamerous like their cousin the starfish. One thing you can see with both the urchin and the starfish are their tube feet, complete with individual suction-cup ends. They use these "feet" to move. By attaching a tube foot to a rock an urchin can pull themselves along the rocky bottom and sides of their ocean home. A single tube foot is not enough to move an urchin, but using large numbers of its hundreds of tube feet it can make its way along the ocean floor.

If you take a closer look at our urchin, still moving across the bottom of the aquarium, you would notice that there are two openings. There is the opening on its bottom where the mouth is located. This area is called Aristotle's lantern , a structure unique to echinoderms. The lantern is a complex system of small bones and muscles that surrounds the esophagus and is used to scrape algae (urchins favorite food) off rocks during feeding. The other opening is located at the top of the urchin and is called the aboral (no mouth) end. Waste products exit the aboral end and is also the place where eggs or sperm are secreted.

Our urchin is part of a group that will be helping scientists understand the mystery of animal development by answering questions about egg fertilization and embryonic development. Sea urchins have been used for many years by scientists to study developmental processes such as fertilization. Because they have relatively big eggs and in large numbers they're ideal animals to study. When secreted, the eggs of S. purpuratus, appear orange in color. Mixing the eggs from a female with the sperm from a male in sea water, developmental processes such as egg and embryo development can be studied.

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Here are some other places to read and learn about sea urchins and echinoderms.

On the Web:

There are hundreds of sites on the web, but their web addresses often change. It is best to look for them using a search engine like HOTBOT using the key words echinoderms and sea urchins. 

Activities: 

Sea Urchin Embryology for high school and lower college division students. 

Books:

Sea Stars, Sea Urchins, and Allies by Gordon Hendler, John E. Miller, David L. Pawson, and Porter M. Kier

Research Page