Dendrasteridae Family of Sand Dollars

Eccentric Sand Dollar, Dendraster excentricus. A representative of the Dendrasteridae Family of Sand Dollars.

Phylogeny:  The Dendrasterid Sand Dollars are members of the Dendrasteridae Family and along with Sea Stars, Basket Stars, Sea Cucumbers and Sea Urchins are in the Phylum Echinodermata. They differ from Keyhole Sand Dollars by lacking the five or six lunulae through their test. They are Sand Dollars are, alone with heart urchins and sea biscuits,  Irregular Urchins in the Class Echinoidea. They differ from regular urchins in that they are not radially symmetrical, but instead have bilaterally symmetrical. They are in the Order Order Clypeasteriodea.

Distribution: The Dendrasteridae Family has eleven known members  that range from the northeast Pacific Ocean from Alaska to Baja California Sur. Three species are found along the Pacific Coast of Mexico. They are found on sand, or muddy sand bottoms from the intertidal zone to unknown depths.

Morphology: The Dendrasteridae Sand Dollars that have an irregular circular profile, with a flat margin posteriorly, that have dorsal sides that are slightly humped and ventral sides that are flat. They have a rigid test (external skeleton) that is perforated with multiple pores and contains all of the organs, except for the epidermis. They have a “round shield star” or the outline shape and the 5-petalled pattern on the dorsal surface with a very low profile and resemble the United States silver dollar coin. These Sand Dollars are depositional feeders that consume algae, larval crustaceans, detritus, diatoms, and mollusks. In turn they are preyed upon by fish, mollusks and starfish. 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. Young sand dollars ingest sand and store it in their gut, to serve as ballast. They can reach 10 cm (3.9 inches) in diameter and 10 mm (0.4 inches) in height. Living Sand Dollars are brown, gray or purple in color. The tests of dead specimens quickly bleach to white.

Reproduction:  Dendrasteridae 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. These 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.