Sea Star Anatomy 101

Although they are commonly called starfish, these animals aren't fish, which is why they are more commonly referred to as sea stars.

Sea stars are echinoderms, which means they are related to sea urchins, sand dollars, basket stars, brittle stars, and sea cucumbers. All echinoderms have a calcareous skeleton covered with skin. They also usually have spines. 

Here you will learn about the basic aspects of sea star anatomy. See if you can find these body parts the next time you see a sea star!

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Sea Star Regenerating four arms, Galapagos / Jonathan Bird /Getty Images
Jonathan Bird/Getty Images

One of the most noticeable features of sea stars is their arms. Many sea stars have five arms, but some species may have up to 40. These arms are often covered with spines for protection. Some sea stars, like the crown of thorns starfish, have large spines. Others (e.g., blood stars) have spines so small that their skin appears smooth.

If they are threatened or injured, a sea star may lose its arm or even multiple arms. Not to worry—it will grow back! Even if a sea star only has a small portion of its central disk left, it can still regenerate its arms. This process can take about a year.

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Water Vascular System

Underside of Spiny Starfish
James St. John/CC BY 2.0/Wikimedia Commons

Sea stars don't have a circulatory system like we do. They have a water vascular system. This is a system of canals in which seawater, instead of blood, circulates throughout the sea star's body. Water is drawn into the sea star's body through the madreporite, which is shown in the next slide.

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Jerry Kirkhart/Flickr

The seawater that sea stars need to survive is brought into their body via a small bony plate called a madreporite, or sieve plate. Water can go both in and out through this part.

The madreporite is made of calcium carbonate and is covered in pores. The water brought into the madreporite flows into a ring canal, which surrounds the sea star's central disk. From there, it moves into radial canals in the sea star's arms and then into its tube feet, which are shown in the next slide. 

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Tube Feet

Tube Feet of Spiny Starfish
Borut Furlan/Getty Images

Sea stars have clear tube feet that extend from ambulacral grooves in the sea star's oral (bottom) surface.

The sea star moves using hydraulic pressure combined with adhesion. It sucks in water to fill up the tube feet, which extends them. To retract the tube feet, it uses muscles. It was long thought that suckers on the end of the tube feet allow the sea star to grasp prey and move along a substrate. Tube feet seem to be more complex than that, though. Recent research (​such as this study) indicates that sea stars use a combination of adhesives to stick to a substrate (or prey) and a separate chemical to detach themselves. An observation that easily confirms this is that sea stars move around as well on porous substances such as a screen (where there wouldn't be suction) as nonporous substances.

In addition to their use in movement, tube feet are also used for gas exchange. Through their tube feet, sea stars can take in oxygen and release carbon dioxide.

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Sea Star with Stomach Everted
Rodger Jackman/Getty Images

One interesting feature of sea stars is that they can evert their stomach. This means that when they feed, they can stick their stomach outside their body. So, although a sea star's mouth is relatively small, they can digest their prey outside their body, making it possible for them to eat prey that is larger than their mouths.

A sea star's sucker-tipped tube feet can be essential in prey capture. One type of prey for sea stars are bivalves, or animals with two shells. Working their tube feet in synch, sea stars can produce the enormous strength and adhesion needed to open up their bivalve prey. They can then push their stomach outside the body and into the bivalve's shells to digest the prey.

Sea stars actually have two stomachs: the pyloric stomach and cardiac stomach. In species that can extrude their stomachs, it is the cardiac stomach that aids in food digestion outside the body. Sometimes if you pick up a sea star in a tide pool or touch tank and it has been feeding recently, you'll still see its cardiac stomach hanging out (as in the image shown here).

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Jerry Kirkhart/(CC BY 2.0)via Wikimedia Commons

Pedicellariae are pincer-like structures on the skin of some sea star species. They are used for grooming and protection. They can "clean" the animal of algae, larvae and other detritus that settles on the sea star's skin. Some sea star pedicellariae with toxins in them that can be used for defense.

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Common Sea Star, Showing Eye Spots / Paul Kay, Getty Images
Paul Kay/Getty Images

Did you know that sea stars have eyes? These are very simple eyes, but they're there. These eye spots are located on the tip of each arm. They can sense light and dark, but not details. If you are able to hold a sea star, look for its eye spot. It is usually a dark spot at the very tip of the arm.

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Your Citation
Kennedy, Jennifer. "Sea Star Anatomy 101." ThoughtCo, Aug. 27, 2020, Kennedy, Jennifer. (2020, August 27). Sea Star Anatomy 101. Retrieved from Kennedy, Jennifer. "Sea Star Anatomy 101." ThoughtCo. (accessed May 26, 2022).