A sand dollar is a flattened sea urchin, belonging to the class Echinoidea, which also includes sea stars and sea cucumbers. While many recognize the familiar bleached white “shell” found on beaches, a living sand dollar is a marine invertebrate with a distinct life and purpose.
Life Beneath the Surface
Sand dollars navigate their sandy environment using specialized body structures. Their bodies are covered in thousands of tiny, flexible spines and miniature tube feet, which allow them to slowly move across the ocean floor or burrow into the sediment. This burrowing behavior is a defense mechanism, helping them hide from predators and strong currents.
The flattened, disk-like shape of a sand dollar is an adaptation for sandy or muddy bottoms in shallow coastal waters. In calm conditions, a sand dollar might stand on end, partially buried in the sand. When waters become rough, it lies flat or burrows completely, sometimes even ingesting sand to weigh itself down against strong currents.
Essential Life Functions
Sand dollars are filter feeders, gathering food particles from the water and sediment. They use tiny hair-like cilia and mucus on their underside to transport organic particles, detritus, and microscopic organisms towards their central mouth. Their mouth contains a jaw structure known as Aristotle’s lantern, which has five tooth-like sections. This apparatus grinds food, and digestion can take up to two days.
Respiration in sand dollars occurs through a process of diffusion rather than gills. Oxygen is absorbed through the body walls, particularly through specialized petal-like patterns called petaloids on their upper surface, which are modified tube feet. Carbon dioxide is simultaneously released, making this gas exchange efficient. Sand dollars reproduce through broadcast spawning, where males and females release eggs and sperm into the water column, leading to external fertilization. This synchronized release increases the chances of successful fertilization and population maintenance.
The Sand Dollar’s Enduring Contribution
Sand dollars play a role in their marine ecosystems. Their feeding habits, consuming organic matter and detritus, contribute to nutrient cycling and keep the ocean floor clean. Their burrowing activities also aerate the sediment, creating a more oxygenated environment that can support other organisms. Sand dollars also serve as a food source for certain predators, including some fish, crabs, and sea stars.
After a sand dollar dies, its soft tissues decay, leaving behind its rigid, calcium carbonate exoskeleton, known as a “test.” These tests often lose their spines and are bleached white by the sun as they wash ashore. The familiar star-shaped pattern becomes more visible on these smooth, dried tests. These remnants become a part of beach sand, contributing to coastal ecosystems.