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.

Purple sea urchins are found on the pacific coastline from Alaska to Cedros Island, Mexico. (Olhausen and Russo, 1981)

Biogeographic Regions: pacific ocean (Native )

Strongylocentrotus purpuratus has a round body that consists of a radially symmetrical test, or shell, covered with large spines. The test itself ranges from 50mm in diameter to an occasional 100mm in diameter. This test is covered with spines that are generally bright purple for adults. Younger urchins have purple tinged spines that are mostly pale green in color. Also covering the test or shell, are tube feet and pedicellariae. The oral side of the urchin, on which the mouth is located, is usually the side facing the substrate (down). The aboral side of the urchin is usually the side of the urchin facing the observer (up). The body of S. purpuratus is radially semetrical. Male and female urchins are monomorphic; they are not physically distinguishable from one another. (Abbott et al., 1980; Olhausen and Russo, 1981; Ebert and Russell, 1988)

Strongylocentrotus purpuratus is primarily found in the low intertidal zone. The purple sea urchin thrives amid strong wave action and areas with churning aerated water. The giant kelp forests provide a feast for S. purpuratus. Many sea urchins can be found on the ocean floor near the holdfast of the kelp. (Calvin et al., 1985; Olhausen and Russo, 1981)

Aquatic Biomes: coastal

Habitat: _Lower intertidal and subtidal, on pilings. Usually in areas of at least moderate wave action (rarely found in Puget Sound or the inner Straits of Juan de Fuca).

As a sedentary invertebrate, S. purpuratus primarily feeds on algae. Bits of algae are a common food that urchins snag out of the water. The tube feet, spines, and pedicellariae that cover S. purpuratus are used to grab the food and aid it into the mouth. In addition to grabbing food out of the water, S. purpuratus scrapes algae off the rocks or substrate. It's mouth consists of a strong jaw piece called Aristotle's lantern. The mouthpiece itself has five bony teeth that are instrumental in scraping the algae off the substrate. While any algae will satisfy the appetite of the purple sea urchin, this species prefers the giant kelp Macrocystis pyrifera. (Calvin et al., 1985)

Average lifespan

Status: captivity: 20 years.

January, February, and March are the primary reproductive months of S. purpuratus. It has been noted, however, that ripe individuals can be found even into the month of July. Purple sea urchins reach sexual maturity at the age of two years. At this time they are about 25mm in diameter or greater. Once sexually mature, females and males release their gametes into the ocean where fertilization occurs. The fertilized egg then settles and begins to grow into an adult. After the egg is fertilized and settles onto a substrate, the urchin begins to develop. The test develops quickly to protect the young urchin. The plates of the test begin to form individually and grow tighter together to form the test. As with most echinoderms, the sexes are usually separate. There is however an occasional hermaphrodite. (Abbott et al., 1980; Calvin et al., 1985; Mead and Denny, 1995)

Breeding interval: Sea urchins breed yearly.

Breeding season: January, February, and March are the primary reproductive months of S. purpuratus.

Average age at sexual or reproductive maturity (female): 2 years.

Average age at sexual or reproductive maturity (male): 2 years.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); oviparous

Average age at sexual or reproductive maturity (male)

Sex: male: 730 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 730 days.

The calcite crystals in purple sea urchin teeth are co-oriented thanks to propagation of existing crystallinity through an amorphous precursor.

The body of purple sea urchins may allow spatial vision due to diffuse photoreceptors on the body surface and spines that shield wide-angle light.

There is no special status listed for Strongylocentrotus purpuratus. However, the harvest of sea urchins poses some concern for the wellfare of the sea urchin population. As mentioned previously, sea urchins are being exported to Japan and other countries in astounding numbers. This leads some to believe that the populations of sea urchins are drastically declining. The California Department of Fish and Game is trying to control the harvesting of the sea urchin now, to insure that urchin populations do not become endangered. For now, they are simply limiting the number of permits available to fisheries. They are discussing other conservation techniques as well, that have not yet been implimented. (Sea Urchin Harvesters Association, 2000)

US Federal List: no special status

CITES: no special status

A negative effect directly on humans is not evident with S. purpuratus. It does, however, have an adverse effect in an indirect way. Purple sea urchins feed on the giant kelp, as mentioned previously. In their feeding, they can destroy entire forests of kelp. These kelp forests are commercially important for fisheries. They are even more important in that the blades of the kelp can be harvested for algin. Algin is a product that is used in the manufacturing of plastics and paints. It is also used as a thickening agent in foods such as gravy and pudding. Another use for algin is in making fibers that are instrumental in the manufacturing of fire resistant clothes. Without the kelp, algin could not be harvested. Strongylocentrotus purpuratus aides in the demise of the kelp forests that provide us with so many different products. (Readdie, 1998)

Strongylocentrotus purpuratus is actually used in many seafood recipes. Sea urchin is common in sushi. It is also considered a delicacy in some countries, especially Japan. The primary urchin harvesting company in California sends 75% of the harvest to Japan.The market value for urchins in Japan ranges from $2.20 per tray to $43.00 per tray. In 1994, Japan imported 6, 130 metric tons of sea urchins at a total value of 251 million dollars. Sea urchin harvesting has become one of the highest valued fisheries in California, bringing 80 million dollars in export value per year. (Calvin et al., 1985; Sea Urchin Harvesters Association, 2000)