Starfish, or Sea Star, a bottom-dwelling marine animal. Despite its name, the starfish is not a fish; it is an echinoderm, a type of marine animal that is spiny-skinned. There are about 2,000 species, found in all seas, and most often near rocky shores. Starfish typically have five or more tapering arms radiating from a central point. The mouth is in the center on the underside. Extending from the mouth to the tip of each arm are grooves lined with tubelike feet. In certain species, the feet have suction disks. The starfish uses its feet to crawl along the ocean bottom. When an arm is lost, a new one soon grows in its place. Most starfish are brightly colored; red, orange, yellow, and pink are common colors. They vary in size from less than 1/2 inch (13 mm) across to more than four feet (1.2 m).
Starfish typically have five or more arms.
Starfish feed mainly on such invertebrates as coral and shellfish, especially clams and oysters. Starfish with suction disks use them to hold and force open the shells of shellfish. Once it has opened a shell, the starfish extends its stomach membranes from its mouth, inserts them into the shell opening, and secretes digestive juices that break down the shellfish's body into a form that can be absorbed into the stomach membranes. Starfish without suction disks swallow their prey whole, and eject the parts that are not digestible. Starfish are not eaten by humans.
What Makes a Sea Star an Echinoderm?
Unlike fish, sea stars don’t have backbones. Instead, sea stars and their relatives have hard plates under their skin. Some of these hard plates have spines. That’s one reason a sea star is an echinoderm.
The sea star’s center is round. Arms grow out from the center—like spokes in a wheel. The body of an adult sea star has several nearly identical sections. It can be divided into similar pieces—like the slices of a pie. Some other echinoderms have other shapes. Some look like balls. Others look like barrels. But they even have bodies with many similar sections.
Like all echinoderms, sea stars have a system of tiny tubes inside their bodies. The tubes extend outside the animal’s body. The closed tips of these tubes are called tube feet. Sea stars have rows upon rows of tube feet—as do most other echinoderms.
How Does a Sea Star Use Its Spines?
A sea star’s spines are sharp. If eaten, they can make for a very painful meal. That’s why many predators avoid sea stars. Still, a few animals—such as king crabs, sea otters, and gulls—eat sea stars. Somehow, they seem to be able to handle the sea star’s spines and bony plates.
The sea star called the crown-of-thorns has spines all over the top of its body. Like other sea stars, it also has shorter spines on its underside. Those spines lie along the rows of tube feet. If the sea star is in danger, it can close the spines together to protect its soft feet.
Sea stars use their spines for protection. Some sea stars also have tiny pinchers in between the spines on top of their bodies. These sea stars use their pinchers to snap at intruders. They can also use their pinchers to clean the sand off their bodies.
Where Is a Sea Star’s Head?
A sea star has no head. It has no brain either. But a sea star doesn’t need a brain to sense what is going on around it. Special cells on the sea star’s skin gather information about its surroundings. These cells then send signals through a network of nerves inside the sea star’s body. These signals trigger the animal to take some kind of action, such as to turn or to crawl.
A sea star also has another kind of network inside its body—a network of tubes. Tiny tubes extend from a sea star’s center to the tip of each of its arms. These tubes carry seawater throughout the animal’s body. A sea star uses the network of tubes to move its tube feet.
A sea star’s mouth is on the underside of the animal’s round center. The mouth leads directly to a large, baglike stomach. The sharp spines found all over the sea star’s body also surround its mouth. These spines help protect a sea star’s soft insides.
The Daily Starfish
What Do Sea Stars Eat?
Many sea stars are carnivores (kahr nuh vawrz). That means they eat meat. Some sea stars prefer to eat animals such as mussels, clams, and oysters. Others like to eat snails, sponges, sea anemones (uh NEHM uh neez), coral, or other sea stars. And some will eat any kind of small animal they can get.
Some sea stars eat in a most amazing way. First, the sea stars use their arms to force open clams or oysters. Then the sea stars push their stomachs out through their mouths and into the open shells. They digest the soft bodies of their prey this way.
Other sea stars swallow their prey whole—shells and all. The sea stars don’t use the shells as food. So, once the sea stars eat the rest of the prey, they pass the empty shells back out through their mouths.
How Do Sea Stars Find Food?
Sea stars can “smell” food in the water. But, of course, they don’t use noses to do this smelling. Sea stars don’t even have noses. Instead, they use their skin. Sea stars have sensitive cells on their skin. Some of these cells can detect chemicals in the water that come from food. Some sea stars can also “smell” food with their tube feet.
Once a sea star detects food, it can move toward its prey. But a sea star isn’t going to win many races. Normally, a sea star crawls very slowly—less than 1 foot (30 centimeters) in a minute. When it is “chasing” prey, it can speed up. Then it may be able to crawl 2 1/2 feet (76 centimeters) in a minute.
Can Sea Stars See?
Sea stars do not have eyes as people do. But, they do have eyespots. These spots are groups of cells that can detect light. Sea stars have these eyespots at the tips of their arms.
Eyespots help a sea star survive. When a sea star senses light from above, it can tell which way is up. And if something suddenly blocks the light, the sea star senses that an enemy may be nearby.
Eyespots also help a sea star when it’s out of water. A sea star may end up on the beach at low tide. If it stays in the sun too long, its body may dry out. That would cause the sea star to die. With its eyespots, the sea star can find its way to shade. In the shade, the sea star can stay cool and damp until the water level rises again at high tide.
How Many Arms Does a Sea Star Have?
Most sea stars have 5 arms each. But some have as many as 40 arms. The number of arms sea stars have often is a multiple of 5—5, 10, 15, 20, and so on. Why do sea stars have so many arms? With arms on all sides, sea stars can react in almost any direction to their surroundings. This helps keep sea stars safe.
A sea star also has special sense cells all over its body. For a sea star, this is better than having a head with a brain, nose, and eyes. With sense cells getting signals from everywhere, a sea star can readily detect predators nearby. The sense cells also help a sea star find food.
Can Sea Stars Really Grow New Arms?
Yes, they can!  Sea stars are always in danger of losing arms, so being able to grow new ones is important to them. Sometimes predators bite the arms of sea stars.
Sometimes rocks fall and crush them.
A few species of sea stars are so good at growing new arms that a cut-off arm can grow into a whole new sea star. The sea star you see here is actually growing four new arms. It is going through a process called regeneration (rih jehn uh RAY shuhn). Regeneration means replacing a body part with a new one. The process of regeneration doesn’t happen overnight, though. It can take up to a year for a sea star to replace missing parts.
How Many Feet Does a Sea Star Have?
A sea star has hundreds of tube feet. But these feet don’t look like your feet. You can see them on the underside of the animal’s body. Find the groove that runs down the middle of each arm. The sea star’s tube feet are in these grooves.
A sea star uses its tube feet for walking, clinging, and grabbing food. Some sea stars have tube feet that can grip like suction cups. Here’s how that works. First, seawater enters the sea star through a hole on top of its body. The water then flows through the tube system and into the tube feet. The water causes the tube foot to lengthen and spread out. When the foot is pressed against a hard surface, it shrinks. This helps the tube foot stick. Then the sea star releases a substance like glue from its foot.
When the sea star is ready to let go, it stretches its foot out again. It releases another substance. This new substance stops the glue from sticking.
How Are Sea Stars Born?
Female sea stars make eggs. Male sea stars produce sperm. The females release their eggs about the same time that the males send clouds of sperm into the water. The sperm fertilizes the eggs. The fertilized eggs hatch into larvae (LAHR vee).
Sea star larvae are not much bigger than pinheads. The larvae float in the water, going wherever water currents carry them. Over time, the larvae change into many odd-looking shapes. As they grow, the larvae sink to the ocean floor. There, they eventually become adult sea stars.
Do Sea Stars Care for Their Young?
Most sea stars do not care for their young. The females just release their eggs into the water. The larvae grow up on their own. Fish and other animals eat many of them. However, many females produce millions of eggs at a time. This makes it very likely that some of their young will survive.
In some species of sea stars, females do care for their young. Those mothers protect their eggs beneath their bodies. When the eggs hatch, the mothers guard the larvae until they are big enough to go off on their own.
Female sea stars that hold onto their young produce fewer eggs than those that don’t. These kinds of sea stars release only about a few hundred eggs at a time. But many of the larvae will live to be adults.
Which Sea Star Looks Good Enough to Eat?
The chocolate chip sea star may look good enough to eat, but it isn’t. That’s because it is very spiny. So, despite its name, this sea star is not a snack.
The chocolate chip sea star is covered with brown spines. But having spiny skin isn’t the only reason sea stars aren’t good food. Some of these echinoderms are poisonous. The crown-of-thorns sea star, for example, can harm even humans who touch its long spines. A sting from one of these sea stars can be very painful. A victim may lose feeling at the site of the sting.
Who Stands Out in the Sea Star Family?
You can’t always divide the number of arms a sunflower sea star has by 5. And a sunflower sea star may grow up to 24 arms. The older a sunflower sea star gets, the more arms it grows. This echinoderm grows to 3 feet (90 centimeters) across. That makes it one of the biggest sea stars in the world. This big sea star has a very large appetite. And it is not a picky eater. The sunflower sea star likes clams, mussels, and snails. It also eats other echinoderms, such as sea urchins, sea cucumbers, and other sea stars. The sunflower sea star often swallows its food whole.
Is the Leather Sea Star Made of Leather?
No, it isn’t, but the leather sea star feels as if it is. This colorful sea star lives among rocks in intertidal zones. Unlike most other sea stars, a leather sea star’s spines do not stick out from below its skin. Because of this, its skin feels smooth and leathery to the touch.
A leather sea star isn’t prickly to the touch, but it sure is slippery. This sea star releases mucus over much of its body. And this sea star isn’t just slimy—it’s also smelly. Some people think it gives off a smell just like garlic.Leather sea stars come in a variety of colors. Besides the colors you see here, these sea stars may also have gold, blue, and red on them.
Starfish make up the class Asteroidea of the phylum Echinodermata.