Biology/Natural History: Feed primarily on kelp (especially Nereocystis or Macrocystis) but can eat sessile invertebrates. Often forms large subtidal aggregations in or near kelp beds. A prime food for sea otters. Other predators include the sunflower star Pycnopodia helianthoides, leather star Dermasterias imbricata, red rock crab Cancer productus, spiny lobster Panulirus interruptus and sheephead fish (in S California), and humans. Commensals include the rhabdocoele flatworm Syndesmis (or Syndisyrinx) franciscanus, less than 1 cm long and lives inside the test, the isopod Colidotea restrata, which clings to the spines, and the amphipod Dulichia rhabdoplastis which builds rods of its fecal pellets that extend out from the urchin's spines (photo). The amphipod feeds on diatoms, which it seems to "farm" on the spines. Plots from which urchins were excluded became overgrown by large algae. Small urchins (less than 5 cm test diameter) often hide under the adults. In Puget Sound they spawn spring and summer. Pelagic echinopluteus larvae metamorphose into benthic juveniles after about 6-10 weeks. Young urchins often are found under larger individuals. Nishizaki and Ackerman (see ref below) found that adult urchins release a chemical cue that causes the young to aggregate underneath them when the adults detect the presence of Pycnopodia helianthoides. Ebert (1998) and Ebert and Southon (2003) determined that these urchins live over 100 years, and found some near Vancouver Island that may be 200 years old. They seem to reproduce best when in dense aggregations (Ebert 1998). Aggregations from which smaller individuals have been harvested recover much faster than those from which larger individuals have been removed (Rogers-Bennet et al., 1998).