The question of whether a sand dollar is a type of starfish is common, as both creatures are often found on the beach and share a distinctly marine appearance. The straightforward answer is no, a sand dollar is not a starfish, but the confusion is understandable because they are close relatives. Both belong to the same large group of invertebrates known as echinoderms, a phylum that represents a unique branch of the animal kingdom. These organisms share an ancient common ancestor, which accounts for the deep biological similarities hidden beneath their different external forms.
The Echinoderm Phylum: Establishing the Family Tree
The phylum Echinodermata, which translates from Greek to “spiny skin,” is the overarching biological classification that connects sand dollars and starfish. This phylum is exclusively marine, meaning no species can survive in freshwater or on land. It is a large and diverse group of animals with about 7,000 living species found in every ocean worldwide, from shallow intertidal zones to the deepest abyssal trenches.
Within this phylum, there are five major classes of living echinoderms, each representing a different evolutionary body plan. This hierarchical structure clarifies that while starfish and sand dollars share the phylum, they occupy separate branches of the family tree at the class level. The five major classes are:
- Asteroidea (sea stars or starfish)
- Echinoidea (sea urchins and sand dollars)
- Ophiuroidea (brittle stars)
- Holothuroidea (sea cucumbers)
- Crinoidea (sea lilies and feather stars)
Defining the Classes: Starfish Versus Sand Dollar Anatomy
The differences in outward appearance between a starfish and a sand dollar stem from their assignment to separate classes: Asteroidea and Echinoidea. Starfish typically exhibit a flattened, star-shaped body with five or more distinct arms radiating from a central disc. Their skeletal plates, called ossicles, are not rigidly fused, giving them a flexible body structure that allows them to move their arms independently and conform to irregular surfaces.
Sand dollars, classified as irregular echinoids, have a structure adapted for burrowing. Their ossicles are tightly fused together to form a rigid, flattened, disc-shaped shell known as a test. This fused structure lacks the flexible arms of the starfish, instead presenting a compact body that is circular or oval in outline. The surface of a living sand dollar is covered in a dense coating of tiny, hair-like spines, which are much smaller than the prominent spines found on their sea urchin relatives.
The five-part radial symmetry is externally obvious in the starfish’s arms, but in the sand dollar, it is seen as a distinct, five-petaled pattern on the upper surface of the test. This petaloid pattern marks the areas where specialized tube feet are located. Starfish tube feet are spread across the underside of each arm in grooves, serving as their primary means of locomotion and predation.
Shared Biological Features
Despite their different forms, the underlying biological systems of sand dollars and starfish are governed by three features shared by all echinoderms. The first is pentaradial symmetry, meaning the body is organized in five equal parts around a central axis. While clear in the starfish’s five arms, this five-part organization is less visible in the sand dollar, yet it remains fundamental to their internal structure.
The second defining feature is the water vascular system, a unique hydraulic network of canals and reservoirs found in no other animal group. This system uses seawater, which enters through a small sieve plate called the madreporite, to control the extension and retraction of the tube feet. In both organisms, this system facilitates movement, gas exchange, and the handling of food particles.
The final shared trait is the endoskeleton, which is an internal skeleton composed of calcium carbonate plates, or ossicles, embedded in the body wall. These ossicles form the hard, spiny structure in the starfish and the solid, fused test in the sand dollar, confirming their shared ancestry.
Ecological Roles and Behaviors
The distinct anatomy of starfish and sand dollars reflects their specialized ecological roles and behaviors in the marine environment. Starfish are active predators or scavengers, using their muscular tube feet and flexible arms for movement across the seafloor and to pry open the shells of prey like clams and oysters. Their body plan allows them to move relatively quickly and pursue their food sources.
In contrast, the flattened, disc-like shape of the sand dollar is an adaptation for a sedentary, burrowing lifestyle in soft, sandy bottoms. They use their small, dense spines to slowly move and partially or completely bury themselves beneath the sediment. Sand dollars are primarily deposit feeders or filter feeders, collecting microscopic organic particles and plankton from the surrounding sand or water column using specialized tube feet and cilia.