Mellitidae Family of Keyhole Sand Dollars

Phylogeny:  The Keyhole Sand Dollars are members of the Mellitidae Family and along with Sea Stars, Basket Stars, Sea Cucumbers and Sea Urchins are in the Phylum Echinodermata. They differ from regular urchins in that they are not radially symmetrical, but instead have bilaterally symmetrical. The Keyhole Sand Dollars, along with heart urchins and sea biscuits,  are irregular Urchins in the Class Echinoidea. They are in the Order Order Clypeasteriodea.

Distribution:  Keyhole Sand Dollars are found worldwide, in temperate and tropical seas. They are found on sand, or muddy sand bottoms from the intertidal zone to depths up to 200 m (656 feet). There are currently eighteen species in the Mellitidae Family of which six are found along the Pacific Coast of Mexico and one species is found in coastal water of the the Gulf of Mexico in the Atlantic Ocean.

Morphology:  The Mellitidae Family includes the Keyhole Sand Dollars that have five, or six holes or slits (lunulae) that pass through the body. The test is rigid with a round shield star profile and a five-petalled pattern on the dorsal surface with perforated with multiple pores this is slightly humped with a flat ventral side. The four, or five, holes around the edge of the test are thought to be utilized to enhance food gathering by increasing the animal’s perimeter. All of the organs, except the epidermis are located within the test.Their mouth and a modified Aristotle’s Lantern (complex jaw structure) is located centrally on the ventral surface. Grooves, lined with cilia, direct food to the mouth. The more central hole is utilized for waste removal. The petalloid design on top is made of pairs of pores which allow for respiration through specialized tube feet.  They are burrowing urchins that have epidermis this is covered with small spines, which facilitate movement over, and through, soft substrates. Young sand dollars ingest sand and store it in their gut, to serve as ballast. Keyhole Sand Dollars can reach 14 cm (5.5 inches) in diameter. Living animals may be gray, greenish or purple in color. The tests of dead specimens quickly bleach to white.

Ecosystem Roles:  The Mellitidae are depositional feeders that consume algae, larval crustaceans, diatoms, detritus and mollusks. In turn they are preyed upon by fish, mollusks and starfish.

Reproduction: The Mellitidae have separate sexes with reproduction by external fertilization. Eggs are brooded by the parents and the planktonic larvae metamorphose through several stages over a long period of time before the test is formed, at which point they become benthic. Sand dollars also have the ability to reproduce asexually via larval cloning, a mechanism of self-defense that doubles their numbers while effectively halving their size making them less attractive for predation. Young sand dollars ingest sand and store it in their gut, to serve as ballast. Some species of Keyhole Sand Dollars have life spans in excess of ten years.