These radially symmetrical invertebrates are armed with long, movable spines which prevent consumption by predators. These adaptations have made the sea urchins to be one of the most successful competitors in the intertidal community. Their sizes range from 1 to 12 cm in diametre. Movement of sea urchins is related to their feeding activity. They rarely move when food is abundant, but can move up to 50cm per day if food supply is low.
Sea urchins are herbivorous grazers, algae such as kelp is their main diet.
Sea urchins can reach sexual maturity very quickly and their lifespan is fairly long. History of population and distribution of sea otter Before the great commercial hunt, sea otters were found in the intertidal zone of Northern Japan, Kuril Islands, Aleutian Islands, and along the entire West coast of North America from Alaska all the way down to Mexico. Their total population was estimated to be around 150,000 to 300,000. Under the Fur Seal Agreement signed in 1911, sea otters have been protected and their total population has increased slowly to around 150,000. Sea otters affect the abundance of kelp indirectly by consuming sea urchins at a high rate because sea urchins are usually more abundant than other herbivorous species.