How Do Starfish Breathe? (Explained)

starfish brain

Starfish are fascinating marine creatures that adapted so well to their environment that they don’t need a lot of organs we would expect them to have. If you wonder how do starfish breathe and if they have lungs or gills to do that, in this post we’ll talk all about that. However, let’s start with a quick answer:

Starfish use their papulae (skin gills) and tube feet to breathe. They absorb the oxygen directly from the seawater and exchange gases through the process called diffusion.

However, this doesn’t tell the whole story. Below I’ll explain what organs starfish use for breathing and how they do it. Furthermore, I’ll explain if starfish can breathe out of the water. Read on!

What do starfish use for breathing?

Starfish don’t have lungs or gills like fish do. What’s important to note is that starfish are not fish, and scientists prefer to call them sea stars instead. They also don’t have blood, heart, or brains. So, what do they use for breathing?

Starfish use two organs for breathing: papulae (also called skin gills or dermal gills) and their tube feet.  Let’s take a look at these organs in more detail.

Starfish gills
Starfish Papulae And Tube Feet

Papulae are numerous thin-walled extensions on the starfish’s aboral surface (on top of their bodies). They contain extensions of the coelom (main body cavity) and enlarge the surface area for respiratory exchange. These skin gills are very soft tissue patches supported by collagen fibers.

They’re set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the crevices. This arrangement allows both the starfish’s arms to bend easily and the rapid onset of the stiffness and stiffness required for stressful breathing activities.

Tube feet play a very important role not only in respiration but in locomotion and circulation system. They are small flexible appendages located on the underside of their body. They connect the water vascular system which is a part of the starfish’s circulatory system. You can read more about it in my other blog post “Do Starfish Have Blood And A Heart?”.

The water travels through small canals of the water vascular system and exchanges gases. During that exchange it also allows the starfish to move around the seafloor by extending the feet.

Starfish respiratory system

Although starfish don’t have actual gills, they did develop a respiratory system.  Let’s now take a closer look at each part of it and see how it works.

Starfish have an open respiratory system which means that the fluid moves freely in the body cavity.  The papulae are lined with cilia that move to produce a current. Next, this current brings water abundant in oxygen, and it enters the interior of the starfish’s body by diffusion. Skin gills, due to their very thin-walled outer layer, help in the proper diffusion process.

The carbon dioxide that is more abundant in the interior of the starfish’s body travels by diffusion into seawater together with other wastes, such as ammonia. Therefore, gases and other tiny substances move from places where they’re most abundant to where they’re least. This is how gases are exchanging.

Coelom

Coleom is a part of the respiratory and digestive system of the starfish. It is an open, fluid-filled body cavity lined with tissue. The hollows of the papulae are continuous with the coelom.

Therefore, a respiratory water current is created during the continuous exchange of gases, which passes through the papulae by cilia lining their outer surface towards the coelomic cavity. The cilia that line the celomic cavities cause the flow of the celomic fluid into them.

As a result, oxygen is delivered to the celomic fluid, and carbon dioxide is carried outside the body. Therefore, it is said that in a starfish, gas exchange occurs through branched protrusions of the body wall connected at the base to the celomic cavity.

The coelom is not only part of the respiratory system but also functions as an excretory system. When the liquid collects, the waste inside the starfish body, such as CO2, provides them with oxygen and small nutrients. This way, starfish breathe but also clean themselves.

Water vascular system’s function

Now let’s take a closer look at the water vascular system that the tube feet are part of. Tube feet are known as starfish “secondary gills” as they take part in gases exchange. They’re connected to a water vascular system that is a system of canals inside sea stars’ bodies.

starfish water vascular system
Water Vascular System

During the circulation, the water enters through madreporite (also called a sieve plate) – a small, smooth plate located on the aboral side of the sea star, slightly off the center, and flows to the tube called a stone canal.

The stone canal connects with the ring canal that circulates the sea stars’ center of the body and connects with a radial canal. Next, the radial canal runs throughout the length of each arm of the sea star, and it terminates as the lumen of a terminal tentacle. The canal goes immediately to the oral side of the ambulacral muscles.

The radial canal spreads into two series of short and narrow branches called lateral canals. Each of these canals has a valve inside to prevent a backward flow of the water into the radial canal. Next, they connect with the base of tube feet and their suction cup, called ampulla.

The ampulla is located at the end of sea stars’ tube feet (also called podia) that create contractions. When the sea star wants to make suction, the ampulla pulls water out of the podia. When it wants to extend the tube feet, the suction cup pushes the water into the end of each foot. 

This is how the water vascular system works to exchange gases and nutrients. It also plays a significant role in sea star’s movements, catching food, and attaching to the rocks. Thanks to the tube feet starfish can walk to find food and shelters, mostly in the coral reefs.

What’s interesting, they can locate coral reefs but seeing large dark structures with their eyes. If you’re interested in reading more about this, check out my other article: “Do Starfish Have Eyes?”.

Can starfish breathe out of water?

Because starfish don’t have lungs, they cannot breathe out of the water. Most species can hold their breath only for about 30 seconds, so it’s important not to take them out of the water when you see them.

If you do, starfish won’t be able to absorb the oxygen that they need, and also such condition will prevent the sea stars to emit carbon dioxide and carbon monoxide from their body. It’ll lead to carbon dioxide or carbon monoxide poisoning, which means it’ll will lead them to death.

It’s important to not take starfish out of the water even for a moment because some of them can die immediately because of the stress or chemicals we have on our skins. We can transport diseases and poison them by our natural oil, bacteria, or chemicals, such as sunscreen.

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