Sand dollars are distinctive marine invertebrates, a type of flattened sea urchin belonging to the echinoderm family. They typically inhabit sandy seafloors in shallow coastal waters, often found partially buried in the sediment. Their unique appearance comes from a rigid, disc-shaped internal skeleton, called a test, which in living individuals is covered by a soft, velvety skin of tiny spines and hair-like cilia. These features allow sand dollars to move across the seabed and burrow, making them a prey item for various ocean inhabitants.
Marine Animals That Prey on Sand Dollars
Several marine animals prey on live sand dollars. Sea stars are notable predators, including species like the sunflower sea star and the pink spiny sea star. Fish also target sand dollars, with triggerfish being particularly effective due to their strong jaws. Other fish species, such as cod, flounder, sheepshead, and haddock, also consume sand dollars.
Crabs represent another group of predators, including sand crabs, hermit crabs, king crabs, and stone crabs. While larger crabs directly prey on sand dollars, some smaller species, like the sand-dollar pea crab, exhibit a parasitic relationship, feeding on the sand dollar’s tissues directly. Otters and octopuses are also known to occasionally consume sand dollars.
Methods of Consumption
Predators employ diverse methods to consume sand dollars, often overcoming their tough exterior. Triggerfish, for instance, blow a stream of water downward to expose a buried sand dollar. Once exposed, they grab the sand dollar with their teeth, lift it, and drop it repeatedly until it lands upside down, exposing its softer underside. The triggerfish then crushes the center of the sand dollar with its strong jaws to access the soft internal tissues.
Sea stars, such as the sunflower sea star, use their powerful tube feet to dislodge sand dollars from the seafloor. They then evert their stomach over the sand dollar, digesting it externally. This process liquefies the sand dollar’s soft parts, which the sea star absorbs, leaving behind the empty test.
Predatory snails, including moon snails and cassid snails, consume sand dollars by drilling a hole through their test. They use a specialized ribbon-like tongue called a radula, often aided by acidic secretions, to bore through the hard skeleton and then suck out the soft body of the sand dollar.
Crabs typically use their powerful claws to pinch and tear apart the sand dollar’s body, allowing them to access the edible portions. The sand dollar’s hard test and spines can present a challenge.
Factors Affecting Sand Dollar Predation
Several factors influence the vulnerability of sand dollars to predation. The size of the sand dollar influences vulnerability; juvenile sand dollars are more susceptible due to their smaller size and less developed tests compared to larger, more robust adults. Different sand dollar species possess varying defense mechanisms, such as tougher tests or more pronounced burrowing abilities.
Their habitat influences their survival. Sand dollars can rapidly burrow into sandy substrates, often burying themselves completely to hide from predators. This burrowing behavior, combined with their natural camouflage that allows them to blend with the sandy seafloor, reduces their chances of being spotted. The presence and density of specific predators in a given area also directly impact predation rates on sand dollar populations.
Another defense mechanism, particularly among sand dollar larvae, is the ability to clone themselves when sensing dissolved mucus from predatory fish. This cloning process results in smaller, more numerous larvae, which can make them harder for predators to detect and consume, although it may increase their vulnerability to smaller predators like crustaceans.