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.
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
Perception Channels: visual ; tactile ; chemical
US Migratory Bird Act: no special status
US Federal List: no special status
CITES: no special status
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
There is no known negative importance for humans.
There is no known positive importance for humans.
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.
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.
Animal Foods: aquatic crustaceans; other marine invertebrates
Plant Foods: algae; macroalgae
Other Foods: microbes
Primary Diet: omnivore
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 (Native )
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 ; tropical ; saltwater or marine
Aquatic Biomes: coastal
Other Habitat Features: intertidal or littoral
The average lifespan of M. quinquiesperforata is four years, but they have been known to survive for up to ten years.
Typical lifespan
Status: wild: 10 (high) years.
Average lifespan
Status: wild: 4 years.
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 ; heterothermic ; radial symmetry
Keyhole urchins passively defend themselves. The tough skeleton and spines serve as defense mechanisms. They also burrow under the sand to hide.
Known Predators:
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.
Breeding season: Late spring into the summer months
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External )
There is no parental investment post-spawning.
Parental Investment: pre-fertilization (Provisioning)
