Biology/Natural History: Like all our local urchins, this species eats algae. The teeth grow fast enough in the laboratory to be completely replaced in about 75 days. The species frequently holds rocks, shells, algae, or other debris over itself by its tube feet (picture), or is found in holes eroded into the bedrock, which they apparently dig with their spines and teeth (pictures). In areas where the giant brown kelp Macrocystis grows, that species is the preferred food. They sometimes eat chitons such as Katharina tunicata. In great numbers the urchin can have serious effects on the survival and regrowth of kelp beds after a storm destroys the full-grown kelp. Predators include otters, the seastar Pycnopodia helianthoides (which may swallow them whole), and occasionally the seastars Pisaster ochraceous and Dermasterias imbricata; some crabs, the anemone Anthopleura xanthogrammica, and sheephead fish. The urchin defends itself against seastars by lowering its spines, retracting its tube feet, raising its pedicellariae, and moving to another spot. Sea otter bones are stained purple from the pigment in this urchin's spines. The species tolerates a wide range of temperatures and salinities, from 5 to 23.5C and 80-110% seawater in the lab. Mass mortality occurs if temperatures exceed 26 C. The species requires well-oxygenated water, and obtains oxygen mainly through the tube feet, which are usually extended at least partway when under water. Symbionts include ciliated protozoans and the flatworm Syndisyrinx franciscanus in the gut, and externally the purple polychaete Flabelligera commensalis and the isopod Colidotea rostrata which live among the spines. Most spawning occurs in Jan-March (October-November off northern Baja California), though some reproductive individuals can be found in other seasons. Sexes are usually separate, though some hermaphrodites are found. They are easily induced to shed gametes, which are extensively used to study fertilization and early embryonic development. Urchin eggs fertilized with sand dollar sperm begin development but die as embryos. Off San Diego the gonadal index indicates reproductive activity in spring to summer, with much decreased gonadal index in late fall and early winter. The species grows slowly, with large size reached in about 10 years.
Recent scientific developments (See Scientific Articles section): This species' DNA sequence was reported in 2006 (Sea urchin gene sequencing consortium, 2006). It appears to have about 23,500 genes, similar to the number humans have. Although the larva (pluteus) of this species, which forms by about 2 days after fertilization, has only about 1500 cells of about 12 types, it takes a great deal of gene transcription to get it to that stage. About 11,500 protein-encoding genes and another 51,000 RNA's with some other function had been transcribed by that time. During this time period about 80% of the species' 283 transcription factor genes had already been used at least once. Others were used during development of the egg inside the mother. Together, the use of such a large proportion of the transcription factors so early in development implies that such regulatory genes may be used repeatedly and for different purposes at different stages of life. Other genes heavily involved during development include genes coding for general cytoskeletal and metabolic proteins, while fewer of the genes coding for immunity or sensory functions were transcribed during this period (Samanta et al. 2006).
Even though the urchin has no known organs specialized for light or chemoreception and does not even have a centralized brain, the species has 979 genes associated with sensing light or odors. This includes 6 genes for opsins, which are receptors for sensing light. In the adult at least some of these opsins seem to be concentrated in the pedicellariae and on the tips of the tube feet, while the pluteus has light-sensitive spots on the arms.
The urchin has a remarkably well-developed immune system (Rast et al. 2006). In its innate immune system, which is shared by both vertebrates and invertebrates, it has 218, or more than 10 times as many SRCR genes as do vertebrates and has 222 toll-like receptors compared with 10 for humans. It also has genes which have been associated exclusively with the vertebrate adaptive immune system, such as Rag genes which are involved in producing antibodies in vertebrates (the urchin does not produce antibodies). It also has genes associated with interleukins and tumor necrosis factors, which normally act as signals for vertebrate immune cells which the urchin lacks.