Red Urchin (Strongylocentrotus franciscanus)
Contributed by TPERP Carol Holt
Where to find them: They reside along the Pacific Coast from Alaska to Baja California. They frequent the exposed outer coast from the lower-most rocky intertidal to depths of up to 300 feet. They tend to avoid sandy or muddy areas.
What they eat: They like to graze on attached and drifting seaweed and kelp. Some of their favorite foods are the kelp Macrocystis and Nereocystis. Thus, they tend to reside in kelp beds. They have specialized jaws which they use to scrape algae off rocks. They hold on to kelp with tube feet while they eat. If the urchin has algae land on top of it, it can use its spines and tube feet to get the food to its mouth, which is located on its underside. Urchins are also scavengers and eat about anything they can manage including parts of dead fish.
Who eats them: Smaller and younger urchins can be eaten by some crabs, some fish (including wolf eels) and sea stars. The larger-sized mature urchins are less threatened due to their size. Note: mature red urchins can have a diameter up to 7 inches without including the spines and can have spines up to 3 inches long. The main predators of mature urchins are sea otters. In areas where sea otters are in abundance and flourish, red sea urchins do not. Otters crack urchins open on rocks and then place them on their stomachs where they dismantle and consume the urchins. There is also a commercial fishery in southern California for these urchins. The gonads are used in sushi.
Adaptations: The urchin’s skeleton is made up of fused plates that encircle the animal, similar to the slices of an orange. Every other section has holes through which three kinds of appendages can extend – spines, tubed feet, or pedicellariae. The tubed feet allow the animal to move and are controlled by a water vascular system. The animal extends or contracts its feet by changing the amount of water inside the feet. The pedicellariae are thin flexible stalks which have three jaws apiece. Each jaw has a poison gland and a stiff sensory hair. The urchin uses the pedicellariae to defend itself against a predator’s attack.
Reproduction: The main spawning period from Central California south is springtime usually between April and May. In the northern areas of the range (B.C., Canada to Alaska) it can run from June – September. Females release orange colored eggs and males release white sperm through gonophores into the water for chance fertilization. Any fertilized eggs develop into planktonic larvae and settle onto the bottom where they can develop into juvenile sea urchins. The planktonic larvae are “bilaterally symmetric” – having a single line which divides them into two mirror images. As they develop, they gain the features of the adults and are “radially symmetric” – can be divided into equal parts from a center axis.
What their life is like: Red urchins can take longer than a year to reach reproductive size and typical lifespan can vary from a few years to decades. Some in the northern portions of the range can reach ages of well over 100 years. They tend to inhabit only deeper pools and rocky shores extending downward from the low tide line. They are constantly searching out kelp and seaweed to consume for food. Often a sea urchin’s long intestine can be inhabited by a small flatworm which also feasts on whatever the urchin consumes.
Interesting facts: The sea urchin gets its name from an old English word for spiny hedgehog.
The mouth of urchins, located on the flat bottom of the animal, is called “Aristotle’s Lantern” and is named after the Greek philosopher who first described it. The mouth has five tooth-like plates that point inward, and it is moved by 60 muscles.
The gonads of the Red Sea Urchin are considered a delicacy in Japan and other parts of Asia.
Scientific Name: Strongylocentrotus franciscanus
Common Name(s): Red urchin
Sources of Information:
Brandon, Jeffrey L. Rokop, Frank J. 1985. Life Between the Tides.
San Diego: American Southwest Publishing Company of San Diego
Ricketts, Edward F., Calvin, Jack, and Hedgpeth, Joel W. 1985.
Between Pacific Tides Fifth Edition. Stanford: Stanford University Press
Denny, Mark W. & Gaines, Steven D., editors 2007.
Encyclopedia of Tidepools & Rocky Shores. Berkeley: University of California Press
Last revised 26-Nov-15