Over 1,500 known species of starfish, also known as Asteroidea or sea stars, are found in ocean waters around the world. Starfish typically have five or more arms that surround a central body. Unlike actual fish, starfish are not good swimmers. Instead they climb around sand, rocks and coral using what are known as tube feet.
The Big Picture
If you flip a starfish upside down, you will see what looks something like a flower. A deep groove runs along each starfish leg, which slightly resembles a vein going through a flower petal, surrounded by hundreds of raised bumps. You will also see a hole in the middle of the starfish. What you're looking at is called the water vascular system of a starfish. The hole in the center of a starfish is known as a madreporite. This is where water is taken in from a starfish's surroundings. The water travels through a circular ring canal near the starfish's mouth, which then distributes the water to the radial canals, those deep grooves in the middle of each leg. The rows of little raised bumps surrounding the radial canals are known as tube feet. Each tube foot is comprised of a sucker, an ampulla and a podium.
How It Works
The water vascular system of a starfish is dependent on the flow of water. This hydraulic system enables the starfish to carry out important functions like walking, clinging to surrounding substrates and catching prey for eating. In order for the starfish to do these things, water must be taken in through the madreporite. After water travels into the radial canals from the ring canal, it is stored there until the starfish needs to move. The starfish creates or removes pressure by contracting or relaxing its muscles in the chambers of the radial canal.
Locomotion in the Ocean
In order for a starfish to walk on surfaces, the water stored in the radial canals is controlled by valves that lead to the hundreds of tube feet. These valves, known as ampullae, create pressure by contracting and resting. To extend a tube foot, an amupulla contracts, increasing the pressure around that tube foot. When the ampulla releases pressure, a tube foot is withdrawn back toward the starfish's body. By controlling tube feet in a wavelike motion, releasing and contracting the tube feet in a series, a starfish can grip and walk along surfaces. Some species of starfish, like the northern sea star, can travel a mile in one week. Starfish also depend on their tube feet's suckers to hold them in place when currents are strong.
Starfish also depend on their tube feet to feed. The suckers on their tube feet are especially important. When a starfish finds prey, such as a snail, clam or oyster, it uses tube feet to capture it, then hold it close to its mouth. Once the prey is close and secure, the starfish pushes its stomach out through its mouth to digest the prey. If the prey is a creature with a shell, the starfish pushes its stomach through a gap in the shell before beginning to secrete digestive enzymes that break down flesh that is then absorbed by the starfish's stomach. Some starfish do not invert their stomachs to consume prey. Instead they swallow it whole and break it down internally.
- University of Alaska Southeast: Water Vascular System
- University of Michigan: Asteroidea
- Marine Education Society of Australasia: Sea Stars
- The Madreporite Nexus: How Starfish Move -- and the Water Vascular System
- Memorial University of Newfoundland: Northern Sea Star
- National Oceanic and Atmospheric Administration: Sea Stars, Commonly Called "Starfish," Are Not Fish
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