The Echinoid Directory

Lifestyle - sand dollars

Sand dollars and their close relatives mostly live infaunally, usually just underneath the surface of the sea floor. True sand dollars are restricted to very shallow settings and inhabit fine to medium sands. None tolerate muds or silts. They feed on the fine organic particles that are to be found among the sediment grains. These fine particles are sieved from the superficial layer of sediment that is passed over the top of the animal as it burrows. The spine canopy is dense enough to prevent sand grains from falling between the spines, but allows very fine particles through which are caught up in ciliary currents and transported to the mouth. One species, Dendraster excentricus, can become a suspension feeder, by lifting its test obliquely out of the sediment to filter-feed from sea-water currents. Aboral tube-feet are specialised for gaseous exchange while oral tube-feet are minute and used for deposit feeding.

Some sand dollars have a series of slits or perforations, the lunules, that act as pressure release valves and help prevent the animal from being washed out from the sediment by wave surge. They are very effective burrowers and can occur in high densities.

Sand dollars are preyed upon by crabs, fishes and birds. They generally live for only a few years, to judge from growth ring analyses. Although a few species are found today living in relatively deep-water settings (200 metres plus), the vast majority are confined to the immediately sublittoral environment. The sexes are separate, but cannot usually be told apart from external appearance.