The characteristic spines of the purple sea urchin have a number of roles in feeding, locomotion, defence against predators and protection from damage during bad storms (5) (8). The spines are also used by the purple sea urchin to dig cavities in the substrate, although not all individuals demonstrate this behaviour. Using its Aristotle’s lantern, the purple sea urchin is capable of biting pieces out of the rock, and the spines slowly erode away at the surface, creating a small pit in which the urchin may remain for long periods of time (2) (7).
Feeding mainly on algae, such as the giant kelp Macrocystis pyrifera (3) (4) (5) (6), the purple sea urchin spends much of its time stationary, feeding on kelp strands that drift past. However, kelp forests are very sensitive, and are often decimated by storms, or by warm water temperatures and nutrient reductions associated with El Niño events (9) (10). During these periods, shortages of drifting kelp may cause the purple sea urchin to switch its foraging behaviour, and it will actively feed on young kelp plants. This can prevent the kelp forest from re-establishing, and the area may become a sea urchin ‘barren’, with very high densities of urchins and very little algae (4) (6) (10). Predators of the purple sea urchin, including the sea otter, spiny lobster, sea star and the California sheephead, all play an important role in limiting the abundance of the purple sea urchin in areas prone to becoming sea urchin barrens (4) (10).
The purple sea urchin spawns seasonally, usually during winter and early spring. The larvae are planktonic, drifting along with the current during the first 6 to 12 weeks of development. After reaching a certain stage in maturity, the larvae of the purple sea urchin will settle on the sea bed and progress to the juvenile life-stage (4) (5) (11). Once settled, usually in late spring or early summer, the juvenile will begin to grow rapidly (2), although the rate of growth is highly variable and dependent on food availability. After one year, the young purple sea urchin tends to reach between one to three centimetres in size (4).