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Mellita quinquiesperforata

By Lauren Sweeten

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. (Fotheringham, 1989; Meinkoth, 1981)

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. (Fotheringham, 1989; Meinkoth, 1981)

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. (Bullough, 1950; Fox, 2001; Gosner, 1978; Hyman, 1995; Meinkoth, 1981; The Assateague Naturalist, 2001)

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. (Fotheringham, 1989; Hyman, 1995; McHenry, 1998)

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. (Fotheringham, 1989; McHenry, 1998)

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. (Fotheringham, 1989; McHenry, 1998)

  • Breeding season
    Late spring into the summer months

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. (McHenry, 1998)

  • Typical lifespan
    Status: wild
    10 (high) years
  • Average lifespan
    Status: wild
    4 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. (Bullough, 1950; Fotheringham, 1989; Hyman, 1995; McHenry, 1998)

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. (Hyman, 1995; McHenry, 1998)

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. (Fotheringham, 1989; Hyman, 1994; The Assateague Naturalist, 2001)

  • Animal Foods
  • aquatic crustaceans
  • other marine invertebrates
  • 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. (Fotheringham, 1989; Gosner, 1978; McHenry, 1998; The Assateague Naturalist, 2001)

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. (Hyman, 1994; McHenry, 1998)

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. (Fotheringham, 1989; The Assateague Naturalist, 2001)

Contributors

Renee Sherman Mulcrone (editor).

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

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

macroalgae

seaweed. Algae that are large and photosynthetic.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

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.

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