Sand Dollar
Dendraster excentricus

Physical Description

When dead, their exoskeleton (hard external body covering) is the most recognizable. It is a small white circle, with a five-point star in the center.
When alive, sand dollars are often purple or gray, with densely packed, tiny spines all over their surface, covering the exoskeleton and giving them almost a furry appearance.
Adults are about 3.5 inches in diameter.
Adult females and males look the same externally.
Juvenile larvae look nothing like adult sand dollars; they look like tiny 8-pointed satellites.
Juveniles are bilaterally symmetric (symmetrical on one line of symmetry) rather than pentaradially symmetric (symmetrical on five lines of symmetry) like adults.

Range

Habitat

Reproduction

Eggs and sperm are released externally into the water.
Fertilized eggs are yellow in color and coated in protective gel; they are only 1/500th of an inch in diameter.
Eggs develop into tiny larvae that feed and move in water using cilia (tiny hairs), and settle on bottom several weeks later.

Diet

Crustacean larvae, small copepods, diatoms, algae, and detritus.
They use their tiny spines to move food from their body to a central mouth on their bottom side.

Predators

California sheephead, starry flounders, and large pink sea stars.
When pink sea stars approach sand dollars, the sand dollars bury themselves in the sand.

Interesting Facts

In New Zealand, sand dollars are called sea cookies, and in South Africa they are called pansy shells!
In calm waters, sand dollars stand on one end, partially buried in the sand, to catch particles. In rough waters, they lay flat so they don’t get moved by the waves. In fast currents, young sand dollars eat sand to weigh themselves down.
Scientists can age a sand dollar by counting the growth rings on the exoskeleton plates. They can live to be 10 years old.
Sand dollars often crowd together, sometimes as many as 625 animals in a square yard.
Sand dollars’ mouths have five teeth-like sections to grind up their food. They can “chew” their food for fifteen minutes before eating, and take two days to digest it.

Sources: Monterey Bay Aquarium; Thought Company

Photo: David R Andrew