Mellitidae Family of Keyhole Sand Dollars

One Keyhole Sand Dollar from the Mellitidae Family can be found in this website:

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 are in the subphylum Echinozoa, the class Echinoidea, the subclass Euechinoidea, the infraclass Irregularia, the subterclass Neognathostomata, the superorder Luminacea, the order Echinolampadacea, the suborder Scutelloidea, the infraorder Scutelliformes, and the superfamily Scutelloidea. The Mellitidae Family is one of four families in this superfamily. The Mellitidae Family contains four genera and twenty species. The name Mellitidae comes from the Greek and Latin words for “honey”, “sweet” or “honey bee”. It is also a female name. How any of those meanings apply to species in this family is a mystery.

Morphology:  Keyhole Sand Dollars have five, or six holes or slits (lunulae) that pass through the body. The test is rigid with a round shield/star outline. They have a flattened profile. The dorsal surface is slightly humped and the ventral surface is flat. 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 is a modified Aristotle’s Lantern (complex jaw structure), and is located centrally on the ventral surface. Grooves on the ventral side, lined with cilia, direct food to the mouth. The more central lunula is utilized for waste removal. The petalloid design on top is made of pairs of pores which allow for respiration through specialized tube feet. Keyhole Sand Dollars are burrowing urchins that have an epidermis that 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. Living animals 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.

Habitat and Distribution:  Keyhole Sand Dollars are found on sand or muddy sand bottoms from the intertidal zone to depths up to 200 m (656 feet). They live worldwide, in temperate to tropical seas. At least seven species from this family are found in Mexican waters.

Reproduction: Mellitidae Sand Dollars are gonochoric (male or females for life). The reproduce sexually through broadcast spawning with external fertilization. The 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. Some species of Keyhole Sand Dollars have life spans in excess of ten years.

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