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📘 How a starfish moves and eats

A sea star is an echinoderm, a marine animal related to sea urchins and sea cucumbers, not a fish. Its body usually has a central disc with five arms, although some species have many more. The mouth sits on the underside, in the middle of t

4
lessons
~20 min
to learn
Adults
level
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What you’ll learn

  1. A body built for the seafloorIdentify the sea star's body plan, underside mouth, tube feet, and madreporite.A sea star's radial body and water vascular system distribute sensing, gripping, and feeding around the seafloor.
  2. Walking with waterExplain how water pressure and tube feet produce crawling, clinging, and righting.Hydraulic tube feet attach and retract in sequence, allowing a sea star to move slowly and resist waves.
  3. Opening the mealDescribe how sea stars use tube feet and an everted stomach to digest shelled prey.A sea star pulls a bivalve open, everts its stomach through the mouth, and digests the prey outside its body.
  4. The whole sequenceConnect the water vascular system, body shape, locomotion, and feeding into one process.The same distributed hydraulic design powers movement, attachment, prey handling, and a remarkable feeding method.

Questions this course answers

What is the water vascular system mainly used for in a sea star?

Water-filled canals and pressure changes extend and retract the tube feet used for movement and gripping.

Where is a sea star's mouth?

The mouth lies on the oral underside, in the middle of the central disc.

How can a sea star eat a mussel that is too large for its mouth?

The cardiac stomach turns inside out and enters the opened shell, where enzymes digest the soft tissues.

What does the madreporite do?

The perforated madreporite acts as an entry point and sieve for water entering the canal system.

Why can many small tube feet pull open a mussel shell?

A sea star applies continuous opposing pull with many feet, eventually tiring the bivalve's adductor muscles.

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