Sea Urchin

Sea Urchin

Red sea urchins, Strongylocentrotus franciscanus, belong to the Phylum Echinodermata and class Echinoidea. This phylum also includes sea cucumbers and sand dollars, one of their common traits is their radial symmetry. They vary in color between a uniform red and dark burgundy and crawl slowly over the sea bottom using their spines as stilts.

The red sea urchin is the largest of the sea urchins, with a maximum "test", or outer skeleton, diameter of more than 18 cm and a maximum spine length of 8 cm. The test is made up of 10 fused plates that encircle the sea urchin like the slices of an orange. Every other section has holes through which the sea urchin can extend its tubed feet. These feet are controlled by a water vascular system. By changing the amount of water inside, the animal can extend or contract the feet. The tip of the tube foot is shaped so it can act like a suction disc. Spines can also be used for locomotion.

Reproduction occurs between March and September in Southeast Alaska. Urchins are broadcast spawners with external fertilization and aggregate during spawning. Female urchins may produce 100,000 to 2,000,000 eggs into the sea where they are fertilized. After fertilization, they develop into a morula and eventually become 8-armed echinopluteus larvae which are herbivorous, feeding on phytoplankton. After the larvae stage, they develop into juveniles and eventually settle onto the substrate. After settling, a rapid metamorphosis occurs including development of spines and tube feet and then internal organs form similar to an adult sea urchin. They seem to reproduce best when in dense aggregations.

Small urchins (less than 5 cm test diameter) often hide under the adults. Adult urchins can release a chemical cue that causes the young to aggregate underneath them when the adults detect the presence of certain kinds of starfish. Some research suggests that urchins can live over 100 years, and found some near Vancouver Island that may be 200 years old.

Field studies of annual growth rates in Southeast Alaska indicate an annual growth increment between 0 and 20 mm. Growth rate is generally greatest among urchins between 20 and 40 millimeters, with large variation among locations and years. Slower growth occurs in areas exposed to open ocean conditions. By age ten urchins have almost stopped growing in diameter and growth slows considerably.