Mellita quinquiesperforata

Geographic Range

Mellita quinquiesperforata , or keyhole urchins, are dispersed along the east coast of the United States from Virginia heading south, surrounding the Florida peninsula, west into the Gulf of Mexico to Texas, south to Mexico, and along the coast of Brazil. Keyhole urchins are also found along the coasts of Bermuda, Jamaica, and Puerto Rico.

• Biogeographic Regions
• atlantic ocean atlantic ocean
  • native native

Habitat

Mellita quinquiesperforata is found in shallow waters below tide lines in sandy bottoms. The keyhole urchin favors habitats where the salinity of the water is greater than twenty three parts per thousand. The urchin is most commonly found in bay areas near inlets.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• coastal coastal

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Physical Description

Mellita quinquiesperforata can grow to 140 mm long and 15 cm wide. Smaller keyhole urchins average at 75 mm. Keyhole urchins differ greatly from the closely related sea urchin. Keyhole urchins do not have arms, but have much shorter spines. Their bodies are flattened with an upward slope towards the center of the aboral (top) side, but not perfectly circular in shape. The oral (bottom) surface is usually flat or concave with the mouth directly in the center and the anus to one side. The mouth is made up of five teeth arranged in a circle that form what is called, "Aristotle's lantern". Mellita quinquiesperforata skeletons are called a test and are covered with epidermis, spines used for burrowing, tube feet for locomotion, and cilia. On the aboral side, petalloids, specialized tube feet, are arranged in the shape of five flower petals. The petalloids are used as gills. There are five oval shaped holes, called lunules, that completely pierce the tests of keyhole urchins. There are two pairs of lunules, one pair toward the top and one large longer hole in between the second pair that are towards the bottom. Younger keyhole urchins have notches that will later develop into the lunules. Coloration of M. quinquiesperforata includes tan, brown, and occasionally gray or greenish. Male and female keyhole urchins cannot be distinguished by external characteristics.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• radial symmetry radial symmetry

Development

The life cycles of keyhole urchins are complex and pass through several stages of metamorphosis. After the eggs have been fertilized they will develop into swimming larvae that are bilaterally symmetrical. These swimming larvae are covered with cilia, drift about the sea water as plankton for four to six weeks, and feed on tiny organisms until they metamorphosize into young keyhole urchins that will grow into adults on the ocean floor. The young keyhole urchins eat sand and store it in their gut to weigh them down while developing into adults. Adult keyhole urchins are radially symmetrical. Like all Echinoderms, keyhole urchins are coelomates and deuterostomates.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Mellita quinquiesperforata breed annually from the late spring into the summer months. The success of fertilization is affected by external factors such as temperature, light, salinty, and location of adults.

Keyhole urchins reproduce sexually by external fertilization. Millions of eggs and sperm are released into the water where fertilization occurs. They are also capable of regeneration of spines and areas of their test.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external

There is no parental investment post-spawning.

• Parental Investment
• pre-fertilization
  • provisioning

Lifespan/Longevity

The average lifespan of M. quinquiesperforata is four years, but they have been known to survive for up to ten years.

Behavior

Keyhole urchins are free-living social animals. They live in large groups which can serve as means of protection and successful reproduction, since fertilization is external. Living in large groups is thought to be influenced by food availablity. In contrast to other echinoderms with arms, the keyhole urchin can only right itself when turned over by burying itself vertically and then falling over right side up.

• Key Behaviors
• motile motile
• social social

Communication and Perception

Keyhole urchins sense the surrounding environment, light, and temperature by touch through the many sensory cells that are located on the epdermis. These cells are also used to communicate socially.

• Communication Channels
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

Keyhole urchins are omnivorous and pull food particles from the sand with their tube feet. The food particles are then guided toward the mouth by cilia and mucous. Foods eaten include microorganisms, algae, marine plants and shellfish.

• Primary Diet
• omnivore omnivore

• Animal Foods
• aquatic crustaceans
• other marine invertebrates

• Plant Foods
• algae
• macroalgae macroalgae

• Other Foods
• microbes

Predation

Keyhole urchins passively defend themselves. The tough skeleton and spines serve as defense mechanisms. They also burrow under the sand to hide.

Ecosystem Roles

Mellita quinquiesperforata have an important role in their ecosystem. Keyhole urchins control the populations of many small organisms. By their mass production of gametes that turn into larvae, they serve as food for the many other organisms within the same ecosystem. They often disturb several layers of sediment by sifting for food and burrowing.

Economic Importance for Humans: Positive

There is no known positive importance for humans.

Economic Importance for Humans: Negative

There is no known negative importance for humans.

Conservation Status

Other Comments

Keyhole urchins are most commonly called sand dollars. They are mostly found washed ashore on beaches and are widely collected by beachcombers and bleach white once they die. Sand dollars have also been used by Christians as a symbol of their beliefs.

Additional Links

Encyclopedia of Life

Contributors

Renee Sherman Mulcrone (editor).

Lauren Sweeten (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

References

Bullough, W. 1950. Practical Invertebrate Anatomy . London: Macmillan & Co..

Fotheringham, N. 1989. Beachcomber's Guide to Gulf Coast Marine Life . Houston, TX: Gulf Publishing Company.

Fox, R. 2001. "Mellita quinquiesperforata" (On-line). Accessed 12/14/04 at http://www.lander.edu/rsfox/310mellitaLab.html .

Gosner, K. 1978. A Field Guide to the Atlantic Seashore . Boston: Houghton Mifflin Company.

Hyman, L. 1995. Echinoderm. Pp. "505-506" in Collier's Encyclopedia Vol.8 . New York: P.F. Collier.

Hyman, L. 1994. Sand Dollar. Pp. "392-393" in Collier's Encyclopedia Vol. 20 . New York: P.F. Collier.

McHenry, R. 1998. Echinoderms. Pp. "857-865" in The New Encyclopedia Britannica Vol.17 15th Edition . Chicago, IL: Encyclopedia Britannica Inc..

Meinkoth, N. 1981. The Audubon Society Field Guide to North American Seashore Creatures . New York: Chanticleer Press.

The Assateague Naturalist, 2001. "Keyhole Sand Dollar" (On-line). Accessed 12/14/04 at http://www.assateague.com/sand-dol.html .