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Why the starfish is star-shaped

Why the starfish is star-shaped

by Wilma Blom
Tue, 12 May 2015

Room 5 at Halsey Drive Primary School wanted to know why starfish are star-shaped and what a mussel's foot does for the mussel. Wilma Blom, Curator Marine Invertebrates, suggests some answers.

Why are starfish a star shape?

Thank you for your question 'Do you know why starfish are a star shape?'

Do you know that is a really tricky question! Scientists think that the sea star got its star-shape more than 400 million years ago. But they are really not sure why this is so. One theory is that it helps the sea star get food because it can move in any direction it wants.

Radiaster gracilis from the Aldermen Islands Marine Region.

© CC BYAuckland War Memorial Museum - Tāmaki Paenga Hira.

Have a look at humans. We have a front and back, and we have a head with eyes that let us see in front. We find food (or thousands of years ago we might have hunted down prey) because our eyes let us see where the food is. And then we can collect food (or catch prey) because we can move forward (fast) on our long legs.

Now imagine you are a sea star living on the seafloor. Sea stars have a top and bottom, but not a front and back. They don’t have eyes, or even have a head and they can only move slowly. And they are quite flat and live close to the seafloor. How do you get your food?

Some scientists think that by being wide and flat the sea star can touch more of the seafloor so it has a better chance of finding food. And because sea stars are more or less shaped the same in all directions it means they can move in any direction without having to turn around.

Another theory is that by being wide and flat with long arms the sea star can cling tightly to the seafloor. That might be an advantage if big waves were washing over it, or if a predator was trying to get it.

What does a mussel's foot do for the mussel?

Phew! Now for the second question 'What does a mussel's foot do for the mussel?'

The mussel's foot is really a strong muscle, which the mussel (hope you are not getting confused here!) can use to move about. When it has found a suitable spot to live, a small sac on the side of the foot makes strong anchor ropes.

Blue mussels, or Mytilus galloprovincialis planulatus.

Boom, Wilma (n.d.)Auckland War Memorial Museum - Tāmaki Paenga Hira.

These ropes are called byssus threads, and it uses these to glue itself to anything hard, such as rocks, wood or other mussels. And if it decides it doesn't like the spot, it can unglue the byssus threads and then it uses the foot to move to a new location.

  • Post by: Wilma Blom

    Wilma is the Curator Marine Invertebrates at Auckland Museum. She also looks after palaeontology and geology.

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