Red Sea Urchin

A sea urchin's spherical body is completely covered by sharp spines. These spines grow on a hard shell called the "test", which encloses the animal. It can vary in color from red to dark burgundy. Rarely, albino specimens are found. It has a mouth located on its underside, which is surrounded by five teeth. During larval development, the body of a sea urchin transitions from bilateral to radial symmetry.

This bilaterally symmetrical larva, called an echinopluteus, subsequently develops a type of pentaradiate symmetry that characterizes echinoderms. It crawls very slowly over the sea bottom using its spines as stilts, with the help of its tube feet. Scattered among its spines are rows of tiny tube feet with suckers that help it to move and stick to the sea floor.

This animal has a mouth with special jaws (Aristotle's lantern) located on the bottom (oral) surface. Its preferred diet is seaweeds and algae, including giant kelp (Macrocystis pyrifera) and bull kelp (Nereocystis luetkeana), which it scrapes off and tears up from the sea floor. Adults may consume plankton and algae (particularly Lithothamnion sp. and Bossiella sp.) if other food sources are not available. During larval development, urchins use bands of cilia to capture food (namely zooplankton) from the water column. Red sea urchins found in the channel adjacent to San Juan Island have been found to live a uniquely sedentary lifestyle with the heavy currents bringing an abundance of food.

Sea urchins are often found living in groups of five to ten individuals. They can regenerate lost spines. Their lifespan frequently exceeds 30 years, and marine biologists have discovered some species with dramatically longer lifespans; certain individuals have been estimated to live for more than 200 years. Red sea urchins are notoriously ravenous kelp-eaters and are implicated in devastating kelp beds by forming grazing fronts. The intense grazing pressure exerted by urchins is an important link in a trophic cascade often observed along the west coast of North America in which sea otter predation influences urchin abundance, which in turn influences kelp devastation.