The sand dollar is a marine invertebrate recognized for its distinct, flattened, disc-like shape. While commonly mistaken for a shell, the sand dollar itself is a living animal. Its unique appearance makes it a subject of curiosity.
The Sand Dollar as a Living Organism
A sand dollar is a type of echinoderm, making it a relative of sea urchins, starfish, and sea cucumbers. Unlike the rigid, spiky appearance of a dead sand dollar, living specimens are covered in a dense layer of tiny, velvety spines. These spines are crucial for the sand dollar’s movement and feeding, working with cilia to transport food particles.
The familiar white object found on beaches is not a shell, but rather the endoskeleton, or “test,” of a sand dollar that has died. This test, composed of calcium carbonate, is what remains after the soft tissues decompose and the spines fall off. The five-pointed, petal-like pattern visible on the test mirrors the internal structure of the living animal.
Life in the Sandy Seafloor
Living sand dollars inhabit shallow, sandy seafloors, often burying themselves just beneath the surface. They are found in both temperate and tropical coastal regions worldwide. This burrowing helps them find shelter, avoid predators, and access their primary food source.
Sand dollars are filter feeders, consuming microscopic algae, organic debris, and detritus. They use their spines and cilia to move food particles towards a central mouth. This feeding process can be slow, with digestion taking up to two days. Their movement involves using their spines to creep along the sand or to drive themselves edgewise into the sediment.
Sand Dollar’s Place in the Ecosystem
Sand dollars contribute to their marine environment through their feeding habits and interactions with other organisms. By consuming detritus and microscopic particles, they help in nutrient cycling and keep the seafloor clean. Their burrowing also aerates the sediment, benefiting smaller creatures within the sand.
Despite their protective burrowing, sand dollars serve as a food source for various marine animals. Their natural predators include certain fish like flounders, as well as crabs and sea stars. The larvae of sand dollars can even clone themselves for defense when sensing nearby predators, increasing their chances of survival.
Unpacking Common Misconceptions
A common misconception is that small, bird-like figures, called “doves,” live inside sand dollars. These “doves” are not living organisms. Instead, they are the five internal skeletal plates, or ossicles, part of the sand dollar’s jaw structure, called Aristotle’s lantern.
When a sand dollar dies and its soft tissues decompose, these durable calcified elements can become loose inside the hollow test. If the test is broken open, these V-shaped pieces may fall out, resembling tiny birds. This phenomenon has led to various folklore and symbolic interpretations, but nothing living resides within the sand dollar’s test.