Purple Sea Urchin
Overview
The Purple Sea Urchin, Strongylocentrotus purpuratus, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico to British Columbia, Canada. This sea urchin species is deep purple in color and lives in lower intertidal and nearshore subtidal communities. It normally grows to a diameter of about 4 inches and may live as long as 70 years. Its genome was completely sequenced and annotated in 2006. The sea urchin genome is estimated to encode about 23,300 genes. Many of these genes were previously thought to be vertebrate innovations or were only known from groups outside the deuterostomes.
Skeletal System
Like all Echinoderms, the Purple Sea Urchin has a mesodermal skeleton composed of calcareous plates or ossicles. Each one of these, even the articulating spine of a sea urchin, is composed mineralogically of a crystal of calcite. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereo. Ossicles may be fused together, as in the test of sea urchins, or may articulate with each other as in the arms of sea stars, brittle stars and crinoids. The Purple Sea Urchin builds its spicules, the sharp, crystalline "bones" that constitute the animal’s endoskeleton, in the larval stage. The fully formed spicule is composed of a single crystal with an unusual morphology. It has no facets, and within 48 hours of fertilization, assumes a shape that looks much like the Mercedes-Benz logo. In other echinoderms, the endoskeleton is associated with a layer of muscle that allows the animal to move its arms or other body parts. This is entirely absent in sea urchins, which are unable to move in this way.