Starfish
Asteroidea
Quick answer
Starfish, also called sea stars, are marine invertebrates in the class Asteroidea, named for their typically five-armed, star-shaped bodies. They have no brain or blood and move using hundreds of tiny tube feet powered by seawater. Many can regrow lost arms, and some can regenerate an entire body from a single limb. Most live around 5 to 35 years depending on the species.
Starfish facts at a glance
| Scientific name | Asteroidea (class) |
|---|---|
| Diet | Carnivore (mollusks, clams, mussels) |
| Habitat | Ocean floors, from tide pools to the deep sea |
| Lifespan | 5–35 years depending on species |
| Size | Commonly 12–25 cm (5–10 in) across |
| Top speed | Very slow; centimeters per minute |
| Conservation status | Least Concern (IUCN, most species) |
| Kingdom | Animalia |
|---|---|
| Phylum | Echinodermata |
| Class | Asteroidea |
Body without a brain
A starfish has no brain and no blood. Instead of blood it pumps filtered seawater through a water vascular system that powers movement and circulation. A simple nerve ring and nerves running down each arm let it sense light, touch, and chemicals, and a tiny eyespot at the tip of each arm detects light and dark.
Tube feet and movement
The underside of each arm is lined with hundreds of small tube feet that extend and grip using water pressure and suction. These let a starfish creep slowly across the seabed and pry open the shells of clams and mussels. Movement is gradual, measured in centimeters per minute rather than any quick dash.
Regeneration
Starfish are famous for regrowing arms lost to predators or injury, a process that can take months to over a year. In some species a severed arm that keeps part of the central disc can grow into a whole new animal. This regenerative ability is one of the most remarkable in the animal kingdom.
Feeding and habitat
Many starfish feed by pushing their stomach out through their mouth to digest prey such as mussels and clams outside the body, then drawing the liquefied meal back in. They live on ocean floors worldwide, from shallow tide pools and reefs to the cold deep sea. A few species, such as the crown-of-thorns starfish, can damage coral reefs during population outbreaks.
Frequently asked questions about the Starfish
Is a starfish a fish?
No. Despite the name, a starfish is not a fish. It is a marine invertebrate called an echinoderm, related to sea urchins and sand dollars, which is why many people now call them sea stars.
Can a starfish regrow its arms?
Yes. Most starfish can regrow arms lost to predators or injury, a process that can take months to more than a year. In some species, a single arm with part of the central disc can even regenerate an entire new starfish.
How do starfish eat?
Many starfish push their stomach out through their mouth to digest prey like clams and mussels outside their body, then pull the partly digested food back in. They use their tube feet to pry shells open first.
Do starfish have brains?
No. Starfish have no brain and no blood. They use a simple ring of nerves and a water-filled vascular system to sense their surroundings and move their tube feet.
How long do starfish live?
Lifespan varies widely by species, ranging from about 5 years to 35 years or more. Larger, deep-sea species tend to live the longest.
How many arms does a starfish have?
Most starfish have five arms, but some species have many more. The sunflower star, for example, can have up to 24 arms.
