Sand dollar

Lore & Background

Sand dollars diverged from other irregular echinoids during the early Jurassic, with the first true sand dollar genus, Togocyamus, arising during the Paleocene. Soon after, more modern-looking groups emerged during the Eocene. Their rigid skeleton, called a test, consists of calcium carbonate plates arranged in a fivefold symmetric pattern. In living individuals, the test is covered by velvet-textured spines with small hairs (cilia), which appear in colors such as green, blue, violet, or purple depending on the species. Dead individuals are commonly found with empty tests bleached white by sunlight.

Reader's Guide

Sand dollars are significant as a distinct group of echinoids adapted for burrowing beneath the seabed (endobenthos), evolving from ancestors that lived on top of the seabed (epibenthos). Their flattened test allows them to burrow into sand and avoid predators such as cod, flounder, sheepshead, and haddock. They reproduce through broadcast spawning, with males releasing sperm and females releasing eggs into the water column; the sex of an adult cannot be determined visually but only by the color of its gamete exudate (yellow for eggs, white for sperm). Sand dollars also clone themselves during normal asexual reproduction when food is plentiful or temperature conditions are optimal. Their presence in both temperate and tropical zones worldwide makes them a common subject of beachcombing and scientific study.

Did You Know?

• The term 'sand dollar' derives from the appearance of the bleached white test, which suggested a large silver coin such as the old Spanish dollar (38–40 mm in diameter).
• In South Africa, sand dollars are known as 'pansy shells' because their pattern suggests a five-petaled garden flower.
• The anus of sand dollars is located at the back rather than at the top, unlike most other urchins.