Five-hole Sand Dollar
Mellita quinquiesperforata

Large-holed Sand Dollar
Encope grandis

Small-holed Sand Dollar
Encope micropora

The Keyhole Sand Dollars of the Mellitidae Family are in the Phylum Echinodermata  indicative that they have spiny skin, along with Sea Stars, Basket Stars, Sea Cucumbers and Sea Urchins. Keyhole Sand Dollars are considered Irregular Urchins, in the Class Echinoidea, that includes the  heart urchins and sea biscuits. They differ from regular urchins in that they are not radially symmetrical, but instead have bilaterally symmetrical. The Echinoids have a rigid test (external skeleton) and all of the organs, except for the epidermis, are located within the test. The test is perforated with multiple pores. Keyhole Sand Dollars are in the Order Clypeasteriodea referring to their “round shield star” or the outline shape and the 5-petalled pattern on the dorsal surface. Clypeasteroids are greatly flattened burrowing urchins that have dorsal sides that are slightly humped and ventral sides that are flat. 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 epidermis is covered with small spines, which facilitate movement over, and through, soft substrates. The petalloid design on top is made of pairs of pores which allow for respiration through specialized tube feet.

The Mellitidae Family includes the Keyhole Sand Dollars that have five, or six, holes or slits (lunulae) that pass through the body. 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. The more central hole is utilized for waste removal. Living Keyhole Sand Dollars may be gray, greenish or purple in color. The tests of dead specimens quickly bleach to white.

Keyhole Sand Dollars can reach 14 cm (5.5 inches) in diameter. They are found on sand, or muddy sand bottoms from the intertidal zone to depths up to 200 m (656 feet). They are depositional feeders that consume algae, larval crustaceans, diatoms, detritus and mollusks. In turn they are preyed upon by fish, mollusks and starfish. They 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 10 years.

Keyhole Sand Dollars are found worldwide, in temperate and tropical seas. There are currently 18 species in the Mellitidae Family of which 6 are found along the Pacific Coast of Mexico and 1 species is found along the coast of the Gulf of Mexico.