Why do starfish have five arms?
For grown-ups: the science
Many sea stars have five arms because that shape helps them move, hold on, and reach around them.
Sea stars have radial body symmetry, usually arranged in five sections, called pentaradial symmetry. Five arms are common in many species, but arm number varies widely among sea stars. Their water vascular system powers tube feet, which help with movement, attachment, and handling prey.
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Transcript
A starfish rests on a rock under the sea. Why does it have five arms?
Most sea stars grow with five arms around a middle spot. This body shape helps them reach in many directions.
Tiny tube feet line the undersides of the arms. The feet grip rocks and help the sea star move and find food.
Some kinds of sea stars have more than five arms. But five is the usual number for many kinds.
So, five arms are a helpful body shape for many sea stars. They can hold on, move, and explore all around!