A real sand dollar is a species of flattened, burrowing sea urchin belonging to the order Clypeasteroida. Sand dollars live on sandy or muddy flats offshore, at depths of up to 90 meters. There are approximately 200 extant species of sand dollars, which vary in size, coloration and surface detail. Despite their name, living sand dollars are rarely completely round and flat; their shapes range from oval or heart-shaped to star-shaped. The unmistakable and intricate surface pattern of a sand dollar aids in camouflage and makes each species unique.
What do sand dollars eat?
Sand dollars are omnivores and feed on tiny organic particles from sediments as well as algae. Their primary food source is microalgae, which they scrape from sandy surfaces using their specialized mouthparts. They particularly seek out diatoms, a type of single-celled algae. Diatoms and other microalgae help impart the vibrant colors often seen in live sand dollars. In addition to microalgae, sand dollars will feed on tiny animals like foraminifera, crustaceans and detritus.
| Food Source | Details |
|---|---|
| Microalgae | Primary food source, particularly diatoms. Scraped from sediments. |
| Foraminifera | Single-celled organisms, additional food source. |
| Crustaceans | Microscopic crustaceans, supplemental food. |
| Detritus | Decaying organic matter, opportunistic food source. |
What is the anatomy of a sand dollar?
Sand dollars have a rigid, flattened skeleton called a test, covered with movable, spine-studded plates. The test provides protection but is also optimized for efficient burrowing: they can quickly dig themselves nearly completely beneath the sandy seafloor when threatened. On the top surface of the test is a flower-like pattern with five petals. This flower pattern surrounds the anus, located in the center. The underside of a sand dollar contains its mouth. Around the mouth are hair-like pods called podia, which have several functions including locomotion, gas exchange and particle manipulation for feeding.
| Anatomical Feature | Description |
|---|---|
| Test | The rigid skeleton made of calcium carbonate plates. |
| Flower pattern | The star-shaped pattern on the top surface surrounding the anus. |
| Anus | Waste opening located in the center of the flower pattern. |
| Mouth | Centrally located on the underside. |
| Podia | Hair-like extensions used for locomotion, gas exchange and feeding. |
Where do sand dollars live?
Sand dollars are found along sandy coastlines around the world, from temperate to tropical seas. They live partially buried in the upper few inches of the sand during the day to avoid predators and discourage settlement of algae. At night they emerge to feed. Sand dollars thrive in areas with consistently shifting sands, where tidal currents and wave action keep the seabed sediments in motion. This ensures fresh organic particles are always available for the sand dollar to consume. Different species inhabit different ranges of water depth, from the intertidal zone down to 90 meters deep.
| Habitat Factors | Preferences |
|---|---|
| Location | Sandy coasts worldwide. |
| Water depth | Intertidal to 90 meters deep depending on species. |
| Substrate | Sandy or muddy bottoms in areas with shifting sediments. |
| Temperature | Temperate to tropical zones. |
How do sand dollars move?
Sand dollars move using the numerous hair-like podia that extend through pores in their test. Each podium has a bulbous tip with a suction-like ampulla that sticks to the sand, providing grip. The spines on the test can also be used to gain purchase in loose sediments. To move, rows of podia on one side of the sand dollar contract, pulling that side of the body forward. Rows on the opposite side then extend and grip, followed by contraction to continue the inching movement. Sand dollars can also coordinate podia movement in a gentle waving motion to swim short distances just above the seabed.
| Locomotion Method | Description |
|---|---|
| Podia | Undulating contraction sequence moves body slowly along sand. |
| Spines | Provide grip and traction in loose sand. |
| Swimming | Podia used for gentle waving motion to swim short distances. |
How do sand dollars reproduce?
Sand dollars are dioecious, meaning each individual is distinctly male or female. Reproduction relies on external fertilization when eggs and sperm are released into the water column. This typically occurs in spring and summer months when food is abundant. Eggs and sperm unite either by chance or facilitated by pheromones. Fertilized eggs rapidly develop into free-swimming larvae. After 4-6 weeks the larvae settle to the seabed and metamorphose into juvenile sand dollars. Juveniles initially have a round shape and three-part skeleton. As they mature, their skeleton divides into the definite petal-like segments of the adult test. Lifespan is thought to range from 5-10 years depending on species.
| Reproduction and Life Stages | Overview |
|---|---|
| Sexes | Male and female (dioecious). |
| Fertilization | External, eggs and sperm released into water. |
| Larvae | Planktonic for 4-6 weeks before settling. |
| Juveniles | Initially round shape, mature into adult form. |
| Lifespan | Typically 5-10 years. |
What eats sand dollars?
Despite their elaborate defenses, sand dollars fall prey to a variety of marine animals. Crustaceans including crabs, lobsters and shrimp will readily feed on live sand dollars if they can be caught before burial. Large predatory snails use chemicals or brute force to penetrate the tests. Bottom-feeding fish like flounders, halibut and rays crush or dislodge sand dollars to expose the soft internal parts. Sea stars and other echinoderms are also major sand dollar predators. Even gulls have been known to snatch exposed individuals at low tides. Juvenile sand dollars suffer especially high mortality rates from predation before reaching maturity.
| Predators | Notes |
|---|---|
| Crabs | Quickly dismember unburied sand dollars. |
| Snails | Drill through or crush the test. |
| Fish | Crush tests and consume soft tissues. |
| Sea stars | Forcefully pry open tests. |
| Gulls | Opportunistically feed on exposed individuals. |
What adaptations help sand dollars survive?
Sand dollars employ an array of defensive adaptations to improve their odds of survival in sandy seafloor habitats filled with potential predators:
- Their flattened, circular shape makes them masters of rapid burial in sand.
- Spines covering the upper test provide protection and traction for burrowing.
- Cryptic coloration and patterns let them disappear against sandy bottoms.
- Toxic chemicals in their tissues deter some predators.
- Quick regeneration of lost spines and damaged tissue aids recovery after attacks.
- Burial at night and during the day minimizes exposure.
- Thick, rigid tests resist crushing by fish and crabs.
- Juveniles exhibit faster burial reflexes than adults.
| Adaptation | Benefit |
|---|---|
| Flattened, circular shape | Allows rapid, efficient burial |
| Covering of spines | Protection and traction for digging |
| Camouflage coloration | Concealment against seabed sediments |
| Toxins in tissues | Deterrence of some predators |
| Tissue regeneration | Recovery after damage and losses |
Conclusion
Sand dollars have evolved a remarkable array of specializations to survive in their hazardous, predator-filled environment. Their unique flattened shape, intricate patterns and prompt burial behavior all help them avoid becoming a meal. While they appear delicate, their hardy skeletons and tenacious survival abilities are a testament to the success of these animals. Getting to see a living sand dollar in its natural habitat is a special treat that reveals just how masterfully adapted they are.
