Sea Urchins and Sand Dollars: Echinoidea
SEA URCHINS AND SAND DOLLARS: EchinoideaLONG-SPINED SEA URCHIN (Diadema savignyi): SPECIES ACCOUNTS
PEA URCHIN (Echinocyamus pusillus): SPECIES ACCOUNTS
WESTERN SAND DOLLAR (Dendraster excentricus): SPECIES ACCOUNTS
SIX KEYHOLE SAND DOLLAR (Leodia sexiesperforata): SPECIES ACCOUNTS
TUXEDO PINCUSHION URCHIN (Mespilia globulus): SPECIES ACCOUNTS
Sea urchins and sand dollars are spine-covered sea animals with five arms and prickly skin. Most sea urchins are almost spherical and have five-way symmetry. Sand dollars usually are flat and have two-way symmetry. Sea urchins and sand dollars have rows of tube feet that run from the anus (AY-nuhs), which is on the top of the animal, to the mouth, which is on the bottom of the animal. At the top of the shell is a small plate with holes that allow seawater to pass into a system of tubes running through the body to the tube feet. Muscles contract to draw seawater into the tubes and send it to the tube feet, which then extend under the force of the water pressure.
The shell of sea urchins and sand dollars is made up of tightly packed plates. This design keeps cracks from spreading if the shell is damaged. The shell of the smallest urchin is 0.2 inches (5 millimeters) across, and that of the largest is about 15 inches (38 centimeters) across.
All sea urchins are covered with moveable spines, but the structure of the spines varies among species. Some spines are thick and blunt; others are long, pointed, and venomous. Spines are used for movement, for fighting predators, for camouflage from predators, and to make shade for protection from direct sunlight.
Sea urchins and sand dollars live all over the world.
Sea urchins live on wave-exposed rocks, in crevices within rocks, in rock pools, on coral reefs, in sandy lagoons, in sea grass beds, and in kelp forests. Sand dollars live in sand and coarse gravel.
Some sea urchins eat only algae, sea grass, and seaweed. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves. Others also eat small animals and waste particles. Sand dollars eat diatoms and plant particles that accumulate in sand. Diatoms (DYE-uh-tahms) are a type of algae that have a shell.
BEHAVIOR AND REPRODUCTION
Sea urchins move slowly and usually search for food at night to avoid predators. They use their teeth to bite and scrape their food. Sand dollars sift the sand for food while burrowing. Their spines are dense enough to prevent sand grains from falling through yet are fine enough to allow food particles to drop out onto strings of mucus before being placed in the mouth. Sea urchins and sand dollars use their tube feet for trapping food particles, for movement, for prey capture, for attaching to the material they live on, and for breathing.
Sea urchins and sand dollars have separate sexes. Females release millions of eggs into the water, where they unite with sperm from the males and develop into larvae. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults. Sea urchin and sand dollar larvae drift before settling to the bottom and transforming into young animals with a body form that looks like that of adults. In a few species of sea urchins and sand dollars the young develop inside the females. These species do not have free-floating larvae.
STRENGTH IN NUMBERS
On wave-exposed shores, sea urchins group together and interlock spines with one another, thus reducing the risk of being swept away by strong waves.
SEA URCHINS, SAND DOLLARS, AND PEOPLE
In the United States, red, purple, and green sea urchins are harvested for their eggs, which people eat. In Japan, urchin eggs and reproductive organs are eaten as delicacies. In areas where predators of sea urchins have been over-fished, huge numbers of sea urchins damage the environment. In the Caribbean Sea, for example, sea urchins have caused ninety percent of the erosion of coral reefs. Because they are so efficient at consuming unwanted algae and waste, sea urchins are used in aquariums. Sand dollar shells are prized by beachcombers and collectors.
The spines of long-spined sea urchins puncture human skin, leaving deep and painful wounds. The tips of the spines easily break off under the skin and are almost impossible to remove.
The World Conservation Union (IUCN) lists one species of sea urchins and sand dollars as Low Risk/Near Threatened, or at risk of becoming threatened with extinction in the future.
Physical characteristics: The body of long-spined sea urchins is somewhat flattened and is about 3 inches (8 centimeters) in diameter. The long, thin, black or white spines are 2 to 4 inches (5 to 10 centimeters) long. The shell and spines are fragile. There is a shiny blue ring around the anus.
Habitat: Long-spined sea urchins live in shallow water on rocks in sheltered areas of coral reefs, in sandy lagoons, and sometimes in sea grass beds. Darker urchins live in the open on sand; paler urchins live in crevices or in cloudy water.
Diet: Long-spined sea urchins eat algae.
Behavior and reproduction: Long-spined sea urchins hide during the day in rocky crevices but look for food at night. These urchins are greedy grazers on algae. When large numbers of the urchins die, algae grow rapidly and harm coral reefs. The shell color of long-spined sea urchins often changes according to changes in light. To protect themselves from predators, a variety of sea animals, such as shrimps and young fish, live among the spines of long-spined sea urchins. Long-spined sea urchins form groups when it is time to release eggs and sperm.
Long-spined sea urchins and people: The spines of long-spined sea urchins are poisonous and easily puncture human skin, often causing infection. Long-spined sea urchins protect coral reefs from overgrowth of algae.
Conservation status: Long-spined sea urchins are not considered threatened or endangered. ∎
Physical characteristics: Pea urchins are one of the smallest urchins. The tiny egg-shaped shell is only about 0.6 inches (1.5 centimeters) long. These urchins are usually grayish green to bright green and have very short spines, which give the animal a velvety texture.
Geographic range: Pea urchins live in the coastal waters of northern Europe.
Habitat: Pea urchins live buried in gravel and sand.
Diet: Pea urchins eat food particles they find in the sand or gravel.
Behavior and reproduction: Scientists know little about how pea urchins behave. These urchins release eggs and sperm into the water, where they unite and develop into free-living larvae, which transform into young urchins that grow into adults.
Pea urchins and people: Pea urchins have no known importance to people.
Conservation status: Pea urchins are not considered threatened or endangered. ∎
Physical characteristics: Western sand dollars have a rigid shell about 3.5 inches (9 centimeters) across and covered with moveable spines. These sand dollars are pale grayish lavender to dark purplish black. They have a five-way petal-shaped pattern of tube feet on the upper surface of the shell.
Geographic range: Western sand dollars live along the west coast of North America from southern Alaska to Mexico.
Habitat: Western sand dollars live on sandy bottoms in sheltered bays, lagoons, and open coastal areas.
Diet: Western sand dollars eat plankton, or microscopic plants and animals drifting in water.
Behavior and reproduction: Western sand dollars live in groups that form a thick carpet of animals. Scientists believe these sand dollars form the groups to influence the water current so that more food flows toward them. Mucus strands help the sand dollars trap food particles. Young Western sand dollars take in sand when feeding to help weigh them down.
Western sand dollars release sperm and eggs into the water, where they unite and develop into larvae, which transform into young animals that grow into adults. Some scientists believe adult Western sand dollars eat their larvae but not the eggs, which have a protective coating. Western sand dollars live about fifteen years.
Western sand dollars and people: Western sand dollars are prized by beachcombers and collectors.
Conservation status: Western sand dollars are not considered threatened or endangered. ∎
Physical characteristics: Six keyhole sand dollars are thin, flat disks with six slot-like holes and a five-way petal-like pattern of tube feet on the top side. They are about 4 inches (10 centimeters) across and are yellow to light brown.
Habitat: Six keyhole sand dollars live in open sandy areas clear of algae.
Diet: Six keyhole sand dollars eat algae and waste particles.
Behavior and reproduction: Six keyhole sand dollars burrow several inches (centimeters) straight down into the sand. They use mucus strands to collect food. Six keyhole sand dollars release eggs and sperm during the rainy season between late summer and autumn.
Six keyhole sand dollars and people: Six keyhole sand dollars are prized by beachcombers and collectors.
Conservation status: Six keyhole sand dollars are not considered threatened or endangered. ∎
Physical characteristics: The shell of tuxedo pincushion urchins is about 2.5 inches (6 centimeters) across and has five to ten broad bands of bright blue with bands of reddish brown spines.
Geographic range: Tuxedo pincushion urchins live in the coastal waters of Asia from India to southern Japan.
Habitat: During the day tuxedo pincushion urchins live among rocks and rubble as well as in crevices on coral reefs.
Diet: Tuxedo pincushion urchins eat algae and coral.
Behavior and reproduction: Tuxedo pincushion urchins hide during the day and look for food at night. They live alone rather than in groups. They camouflage their shell with found shell fragments and algae held on with hairlike fibers between their spines. These urchins reproduce by releasing sperm and eggs into the water, where they unite and develop into larvae which transform into young urchins that grow into adults.
Tuxedo pincushion urchins and people: Tuxedo pincushion urchins are used in home aquariums.
Conservation status: Tuxedo pincushion urchins are not considered threatened or endangered. ∎
FOR MORE INFORMATION
Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.
Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.
Samarri, Fariss. "Helping Urchins May Benefit Coral." Sea Frontiers (winter 1995): 16–17.
Fautin, Daphne G., and Judy Follo. "Echinoidea." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/information/Echinoidea.html (accessed on March 1, 2005).
"Sand Dollars." Seashells.org.http://www.seashells.org/identcatagories/sanddollarstypes.htm (accessed on March 1, 2005).