Aquarium of the Pacific - Online Learning Center - Species Print Sheet
Conservation Status: Safe for Now
Red Sea Urchin
(Strongylocentrotus franciscanus)Echinoderms • Invertebrates
Sea urchins belong to a group of marine invertebrates called echinoderms, which means spiny-skinned animals. They are related to sea stars, sea cucumbers, and sand dollars. Like their relatives, sea urchins do not have a brain or heart.
Alaska to Baja California, Mexico
These urchins are found in rocky subtidal habitats from areas just below the low tide line where the wave action is not extreme to about 90 m (300 ft) deep. They avoid locations with sand or muddy sediments.
The red sea urchin’s body is domed above and flat below. Its external skeleton, called a test, is made up of 10 fused, chalky plates covered with spines. Every other section has holes through which the urchin’s tube feet are extended among the spines. These feet are long, thin, flexible tentacles ending in tiny suction cups. Spines are connected to the test by ball joints that enable them to point in all directions. Also located among the spines and near the mouth are pincer-like structures called pedicellaria.
The mouth of urchins, located on the test’s flat bottom (oral side), is called Aristotle’s Lantern, named after the Greek philosopher who first described it. It has five tooth-like plates that point inward and is moved by 60 muscles. On the top (aboral side) waste is excreted through an anus, and eggs and sperm secreted through 5 openings called gonopores.
The color of these sea urchins varies between a uniform red and a dark burgundy.
These urchins are the largest of all the sea urchins. In California, the maximum test diameter is about 127 mm (5 in) and spine length 50 to 75 mm (2-3 in). However, in British Columbia they grow to a diameter of over 180 cm (7 in) with 8 cm (3 in) long spines. Although annual growth rates in older urchins are very slow, less than 0.01 cm/yr (0.004 in), researchers believe they continue growing all of their lives.
Diet and Feeding
Red sea urchins graze on attached or drift seaweed and kelp. Southern California urchins prefer giant kelp. The northern California and north Pacific urchins eat bull and brown kelp. Their “teeth” are used to scrape algae off rocks. They hold onto kelp with their tube feet as they eat. If algae lands on top of an urchin, it uses a combination of its spines and tube feet to pass the food to its bottom located mouth.
Sexes are separate. A female sea urchin releases orange colored eggs and and the male releases white sperm through the gonopores to unite in the water column. At hatching, the planktonic larvae are bilaterally symmetrical (a single line divides the larvae into two mirror images). When they settle to the bottom, they develop the features of radially symmetrical adults (can be divided in equal parts from a central axis).
Sea urchins use a water vascular system to manipulate their tube feet by controlling water movement to and from the feet through muscular tubes. As the tube feet press against an object, removal of water from the tube creates a vacuum. When the tube is refilled with water, the vacuum is broken and the grasp of the foot is released.
All of a sea urchin’s appendages are sensitive to touch and may also respond to chemicals released into the water by predators. If the urchin’s body is touched with a sharp object, all its spines point toward the region being touched. However, if touched with a blunt object, the spines turn away, allowing the pedicellariae to be the primary mode of defense. The pedicellaria remove parasites and other unwanted material from the urchin’s surface and may possibly assist in obtaining food.
Sea urchins have a general sensitivity to light all over the body. They show a shadow response, making rapid movement with their spines if a shadow suddenly appears.
Original estimates of the life span of red sea urchins were 7 to 10 years. More recent studies have shown that they live much longer and that age is dependent on geographic location. Southern California red sea urchins can live to be about 50 years old whereas those in British Columbia, Canada can reach more than 100 years. These same studies determined that Canadian urchins over 19 cm (7.5 in) in diameter were probably about 200 years old!
Red sea urchins are harvested commercially for their reproductive organs primarily for export to Japan. In California, the industry is regulated by the California Department of Fish and Game for number of licenses, harvesting time, and size. South of Monterey County, the diameter of the test must be at least 8.3 cm (3.3 in) and in northern California, 8.9 cm (3.5 in). In British Columbia the legal size is 7 cm (2.7 in), that of a 5-10 year old urchin.
In addition to humans, they are preyed on by sea otters, octopus, sunflower stars, wolf eels, and some crabs and birds.
Southern California red sea urchins can live to be about 50 years old whereas those in British Columbia, Canada can reach more than 100 years.