Sand dollars are marine invertebrates, with disc-shaped bodies. These creatures are a type of sea urchin, closely related to starfish and sea cucumbers. The familiar white, star-patterned discs often found on beaches are not living animals but rather the calcium carbonate skeletons, known as tests, left behind after the animal dies. Their unique appearance has led to various common names, including sea cookies, pansy shells, and sand cakes.
Understanding Their Natural Environment
Sand dollars primarily inhabit sandy or muddy bottoms. They prefer areas free from dense seagrass or other underwater vegetation, which could impede their movement and burrowing activities. These animals are most commonly found in shallow coastal waters, typically below the mean low tide line. While most species remain in shallower depths, some can reside as deep as 1500 meters.
Sand dollars thrive in gentle water currents. In calm conditions, they may stand partially buried in the sand, while in rougher waters, they lie flat or burrow deeper to maintain their position. Younger sand dollars can even ingest sand grains to increase their weight and anchor themselves against strong currents. They often form large aggregations, sometimes with hundreds of individuals clustered together in a single square yard. This communal living supports their feeding and reproductive success.
These creatures are proficient burrowers, using their spines and tiny cilia to move and dig just beneath the sand surface. This burrowing behavior provides protection from predators and strong currents, and it also positions them optimally for filter feeding. They feed on microscopic particles, such as algae and detritus, found in the sand and water column.
Optimal Conditions for Locating Sand Dollars
Finding sand dollars on the beach requires timing and observation. The best opportunities arise during low tide, particularly an hour before and after the lowest point. This period exposes a greater expanse of the sandy seafloor, making them easier to spot. Early mornings are advantageous, as they often coincide with low tides and fewer beachgoers, increasing the chances of discovery.
Searching after a storm can also be productive. Strong waves and currents dislodge sand dollars from their buried positions and wash their tests ashore. When scanning the beach, look for subtle circular depressions or slight mounds in the sand, which might indicate a buried sand dollar. They can also accumulate along the high tide line mixed with seaweed and other debris.
For a more active search, gently shuffle your feet through the shallow water just offshore. This technique can help you feel for buried specimens beneath the sand. Moving slowly allows you to distinguish sand dollars from other shells or natural elements. Only the bleached white tests of deceased sand dollars should be collected.
Identifying Live Versus Collected Sand Dollars
Distinguishing between a live sand dollar and a discarded test is important for ethical beachcombing. A live sand dollar appears significantly different from the familiar white souvenirs. Living specimens are covered in a dense, velvety layer of tiny spines (cilia), giving them a fuzzy appearance. These spines are used for movement and feeding.
The coloration of a live sand dollar varies, ranging from gray, brown, or purplish to reddish hues. If you gently hold a live sand dollar, you might feel the subtle movement of its spines. Additionally, some species may secrete a harmless yellowish substance called echinochrome, which can temporarily stain your skin.
In contrast, a collected sand dollar test is bleached white or light tan, lacking any spines or movement. The distinctive five-pointed star pattern, which is obscured by spines on a live animal, is clearly visible on a cleaned test. Live sand dollars cannot survive out of water, so if one is found alive on the beach, it should be gently returned to its natural habitat for survival.