The Function or Purpose of a Starfish's Arm

Although most starfish, or sea stars, have five arms, they can have as many as 50.
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Starfish aren't really fish at all, but belong to the echinoderm family of hard, spiny creatures. That's why scientists call them sea stars instead of starfish. Most sea stars have five arms, although some have as many as 50 and a few have only three.


The sea star's arms are hollow but contain many tiny tube feet that are visible within the hollow groove down the center of each arm. These feet help them move from place to place by acting like suction cups. The feet in one arm grab on while the feet in another arm release, creating movement. A sea star moves slowly, though -- about a foot per minute, or maybe double that speed if it's trying to get to its prey.


When the sea star finds a place it wants to stay, water enters the tube feet and spreads them out. As they reach a hard surface, they contract, creating a suction cup. The sea star then sends out a glue-like substance to keep it attached. When it's ready to move on, it secretes another substance to dissolve the glue.


Sea stars don't have noses to smell their food, but the tube feet in the arms have sensors that alert the sea star to nearby prey. The sea star can extend its tube feet down into crevices to extract its meal, which consists mostly of clams, oysters, sponges, plankton and slow-moving fish. It uses its arms to pry open the shell, then sends out its stomach to eat the food.


When a sea star loses an arm, it can grow it back through regeneration. This is important because the arms can easily get caught in cracks, snagged on rocks, crushed by debris or bitten off by predators. It can take as long as a year to regrow an arm, though. An arm that is cut off can also grow into a whole new sea star, which is one of the ways new sea stars are born. To do this, the severed arm must contain part of the sea star's center ring.

Detecting Light

Sea stars don't have eyes to see, but they can detect light through eyespots at the tips of their arms. This is important to the sea star's survival. When the sea star sees light and then the light is blocked, that signals the presence of a predator. Or, if a sea star washes up on shore, it can use the eyespots to find shade, which will keep it from drying out until the tide washes it back into the water.

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