Ten Questions: Sea Stars

Starfish are not fish. They are echinoderms, related to sea cucumbers, sea urchins, and sand dollars. They are called sea stars. There are more than 2000 species of sea stars. I have ten questions to answer and answers to go with them. Are you ready to learn more about sea stars?


01. Just how many arms does a sea star have?
Many have five arms. We are most familiar with those. However a sea star can have from 4 to 40 arms.

02. How much blood does a sea star have?
None. A sea star uses a hydraulic vascular system that transports nutrients and waste. It is also the way it moves. Tubular feet are contracted and relaxed to allow a sea star to seem to glide through water.

03. How many eyes does a sea star have?
It has no eyes but there are eyespots on the tip of each arm. An eyespot is sensitive to light and gives the sea star the ability to see shapes and movement.

04. Where is a sea star's mouth?
The mouth is located on the underside of the central disc. The anus is on top of the disc.

05. How does a sea star eat?
When the sea star locates the shell fish it feeds on, it opens the shell with it's arms. Then it pushes its stomach out through its mouth and into the shell to capture the meat. There are actually two stomachs in a sea star. The first one that grabs the food is known as the cardiac stomach. Once the cardiac stomach is back inside the sea star, it transfers the food to the pyloric stomach for the final digesting of its prey.

06. Can a sea star really regenerate an arm?
Most certainly. As a matter of fact, if a sea star is cut into pieces it has the capability to begin a new sea star from each segment. So besides being able to replace an arm that is lost, each arm can replace the rest of the sea star.

07. How else do sea stars reproduce?
Sea stars can change their sex pretty much at will. They can even reproduce hermaphroditically. The female sea star can release as many as 2 million eggs at one time. The eggs are then fertilized by the male. Fertilized eggs become larvae which swim around for about three weeks. That is when they settle and begin the metamorphosis that makes them look like stars.

08. Where are sea stars found?
Sea stars are found in every ocean of the world. They are salt water animals so you will not see them in fresh water.

09. How large is the brain of a sea star?
A sea star has no brain. Each arm contains a complex nervous system. Each nervous system meets to form a central nerve ring to control the esophagus and other central functions.

10. How does a sea star protect itself from predators?
A sea star has a spiny, leathery outer skin made of calcium carbonate. This makes it difficult for birds and other animals who might eat them.

Perhaps you would like to share anything you know about sea stars. Please do. We are all looking to know more about our world.