Sand dollar

Sand dollars live in the sea.

They stay under the sand to hide. They use tiny spines to move and dig. These spines can be purple or blue.

When a sand dollar dies, it loses its spines. The sun turns its shell white. It looks like a small silver coin.

These sea animals are very special.

Sand dollars are flat sea urchins. They live on sandy ocean floors.

Most are 80 to 100 mm wide. They have a hard skeleton called a test. This test is made of many small plates.

Living sand dollars are covered in velvet-like spines. These spines help them move and dig. Some species have slits called lunules. These slits help them stay in the sand. This stops waves from washing them away.

When they die, they lose their spines. The sun turns the white test bright. It looks like a silver coin.

Sand dollars reproduce by broadcast spawning. This means they release eggs and sperm into the water. You can tell the sex by the color of the fluid. Yellow fluid has eggs. White fluid has sperm. This way, many sand dollars can meet in the water. It helps more babies grow. Some tiny larvae even clone themselves to hide from fish.

Sand dollars are special, flat sea urchins that live on the ocean floor. They are part of a group called Clypeasteroida. You might find them in warm tropical waters or cooler temperate zones. They like to live on sandy or muddy bottoms.

These animals are usually small. Most are between 80 and 100 mm wide. They have a hard skeleton called a test. This test is made of many calcium carbonate plates.

Some species have slits called lunules. These slits help the animal stay tucked in the sand. This keeps them from being swept away by waves.

Living sand dollars have a soft, velvety look. This is because their test is covered in tiny spines. These spines have even smaller hairs called cilia. The sand dollar uses the spines to move across the seabed.

They use them to burrow into the sand for safety. This helps them hide from predators like cod or flounder. They eat algae and organic matter found on the ocean floor. Some species even tip on their side to catch food in currents.

This way of living is called being an endobenthos.

Sand dollars have a very old history. They split from other sea creatures during the early Jurassic period. The very first true sand dollar genus was named Togocyamus. It appeared during the Paleocene period.

Later, during the Eocene, more modern groups began to emerge. This long history shows how they changed over time. They changed from living on top of the sand to living beneath it. This change helped them survive in the ocean.

These creatures have many interesting facts about their bodies. A living sand dollar can be green, blue, violet, or purple. Their mouth is located on the bottom of their body.

They reproduce through a way called broadcast spawning. This is when they release eggs or sperm into the water. You can tell the sex by the color of the fluid. Yellow fluid contains eggs, while white fluid contains sperm. In November and December, many sand dollars spawn together.

When you walk on a beach, you might find a sand dollar. It might not look like a living animal. Dead sand dollars lose their spines and become very smooth. The sun bleaches their white skeletons.

Long ago, people thought they looked like silver coins. They called them "sand dollars" because they reminded them of the old Spanish dollar. In other places, they have different names. In South Africa, people call them pansy shells. This is because they look like garden flowers.

Sand dollars are specialized, flat species of sea urchins belonging to the order Clypeasteroida. They are found in temperate and tropical zones across all continents. Most live in waters below the mean low tide line. They prefer to inhabit sandy or muddy areas on or just beneath the surface.

These animals are important members of the ocean floor ecosystem. They have evolved from creatures that lived on top of the seabed, known as epibenthos, to those that burrow beneath it, known as endobenthos.

The physical structure of a sand dollar is centered around a rigid skeleton called a test. This test is composed of calcium carbonate plates arranged in a fivefold symmetric pattern. In living individuals, the test is covered by a velvet-textured layer of spines. These spines are covered in even smaller hairs called cilia.

The sand dollar uses coordinated movements of these spines to move across the seabed. Some species possess slits in their tests called lunules. These lunules help the animal remain embedded in the sand so it is not swept away by ocean waves.

Sand dollars exhibit unique biological symmetries. While their bodies display radial symmetry like other echinoids, they also show secondary front-to-back bilateral symmetry. This means they have a distinct left and right side. The mouth is located on the bottom of the body at the center of a petal-like pattern. This pattern consists of five paired rows of pores. These pores are perforations in the endoskeleton that allow podia to project through for gas exchange.

Unlike many other urchins, the anus is located at the back of the body.

The evolutionary history of these animals spans millions of years. Sand dollars diverged from other irregular echinoids, such as the cassiduloids, during the early Jurassic period. The first true sand dollar genus, Togocyamus, arose during the Paleocene epoch.

More modern-looking groups of sand dollars emerged later during the Eocene. This long history of adaptation allowed them to master life within the sediment. Today, they average between 80 and 100 mm in diameter.

Reproduction in sand dollars occurs through a process called broadcast spawning. In this method, adults release gametes directly into the water column for external fertilization. This typically happens in groups to increase the chances of successful fertilization.

You can identify the sex of a sand dollar by the color of its gamete exudate. A yellow exudate contains eggs, while a white exudate contains sperm. In many species, the highest gonad index occurs in August, with spawning occurring in November and December. Males release sperm through the gonopores, and the process may last about 10 minutes.

After fertilization, the life cycle follows several complex stages. Cell division, or cleavage, begins roughly 80 minutes after fertilization. The cells then become blastulae and then gastrula. During the gastrula stage, the embryo's height decreases while its width increases slightly. The larvae then enter the pluteus stage.

During this stage, the larvae have an esophagus, stomach, and intestine, but they cannot yet feed. Towards the end of this stage, the larvae undergo metamorphosis. This process takes about 90 minutes as larval tissues are resorbed and the juvenile develops its own skeleton and teeth.

Sand dollars have developed fascinating survival strategies, including asexual reproduction. Biologists discovered in 2008 that certain sand dollar larvae can clone themselves. When larvae sense mucus from a predatory fish, they may split in half to create clones.

This cloning process can take up to 24 hours. It results in larvae that are only two-thirds the original length, which helps them hide from predators. While this helps them escape fish like cod, flounder, sheepshead, and haddock, it may make them more vulnerable to smaller crustaceans. This ability to clone also occurs when food is plentiful or temperatures are optimal.

Cultural names for these animals reflect their various appearances. The name "sand dollar" comes from the bleached white tests found on beaches. These skeletons lack the colorful spines of the living animal and resemble silver coins like the old Spanish dollar. In South Africa, they are called pansy shells because they look like five-petaled flowers. In other regions, they are known as sea cookies, snapper biscuits, or cake urchins.

These diverse names highlight how humans have viewed these unique sea creatures throughout history.