How Do Starfish Breathe? 5 Shocking Facts Explained by a Marine Biologist

If you ever spot a starfish moving gently in the water during high tides, you might be amazed by how its body works. What’s really interesting is that starfish don’t have lungs or gills like many other animals. So, how do they breathe?

Starfish breathe through tiny soft parts on their upper surface called papulae (also known as skin gills). These papulae stick out slightly through their skin and help them take in oxygen from seawater and release carbon dioxide. This gas exchange happens through a process called diffusion, where gases move naturally across thin walls.

Papulae are connected to the starfish’s internal body cavity, so oxygen spreads easily inside. Starfish also have a special water vascular system, which helps move water around their body, supporting both movement and internal functions. Unlike fish, starfish don’t have lungs, gills, or blood—instead, their body uses sea water to transport oxygen and nutrients.

But that’s not the whole story, there’s more to how starfish breathe! Below, I’ll explain the parts of their body they use for breathing and how it all works.

What Are Starfish Breathing Organs? (Gills or Lungs)

Even though we call them “starfish,” they’re not fish. They don’t have lungs or gills like most animals do to breathe. So, that brings up a big question—how do starfish breathe underwater, and what parts of their body help them do that?

Starfish rely on two primary organs for gaseous exchange:

1. Papulae or Skin Gills

Papulae are lots of thin walls on the top of a starfish. They work like tiny gills. They have extra parts connected to their main body, which helps them breathe better by taking in oxygen and letting out carbon dioxide. Collagen fibers support these delicate dermal gills.

2. Tube Feet (pedicellariae)

On the underside of a starfish, you’ll find hundreds of tiny, flexible structures called tube feet. These aren’t just for walking—tube feet do a lot more! They help the starfish move slowly along the ocean floor, breathe by moving water in and out of their body, grab food like clams and mussels, and even sense their surroundings.

Each tube foot works like a little suction cup, sticking to surfaces and helping the starfish hold on tightly—even in strong ocean currents. They’re powered by water pressure through a system called the water vascular system, which is unique to starfish and their relatives.

So, even without a brain or blood like ours, starfish have an amazing way of getting around and surviving in the sea.

Even though their walls are thin and don’t let much through, starfish breathe through tiny, soft structures called papulae, found on the top side of their body. These papulae work like mini gills, helping them take in oxygen from the water and release carbon dioxide.

Papulae are part of the starfish’s skin gills and are connected directly to the internal body cavity, so oxygen can easily spread through their body. They’re very delicate, which is why they’re supported by tiny collagen fibers that keep them from collapsing.

Fun fact: Because starfish live in seawater, they don’t need a big breathing system like lungs. Instead, their slow lifestyle and unique water-based system are perfect for their needs. They rely on diffusion, meaning oxygen naturally passes into their body from the water around them—no effort needed.

Even though starfish don’t have a real heart system, the water system helps starfish move by stretching and squeezing their tube feet. It also helps them find food and sense what’s around them.

Starfish have many simple organs and move slowly, which helps them take in oxygen from the water around them very efficiently. They don’t need a lot of oxygen at once, so their slow lifestyle works well with their unique breathing system.

Starfish also play an important role in the ocean—they help clean it up. Starfish are natural decomposers of the ocean, feeding on dead plants, animals, and organic matter on the sea floor, which helps keep the ocean healthy.

Do Sea Stars Have Gills? Starfish Respiratory System Explained

Starfish don’t have true gills like fish or lungs like land animals. Instead, they have a unique respiratory system that works well in their underwater environment. Let’s break it down and see how starfish breathe:

Starfish have an open circulatory system, meaning they don’t have blood vessels like humans. Instead, water flows freely inside their body through a fluid-filled space called the coelom.

For breathing, they use tiny structures called papulae (or skin gills), which are found on the top side of their body. These papulae are thin-walled and covered with a semi-permeable membrane, which allows oxygen to pass in and carbon dioxide to pass out through diffusion.

Each papula is equipped with cilia—tiny hair-like parts that move and create a gentle water current. This helps bring fresh, oxygen-rich water to the papulae and carry away waste gases like carbon dioxide.

This entire system allows starfish to get the oxygen they need from seawater—even without lungs, gills, or blood.

As seawater flows into the starfish’s body, the oxygen in the water moves into the starfish’s internal fluid, called coelomic fluid, and then into its cells. The papulae are covered with a thin, soft membrane that allows oxygen to pass through easily—this is how the starfish breathes.

When carbon dioxide (CO₂) and other waste products like ammonia (NH₃) build up inside a starfish’s body, they are removed through a process called diffusion. These wastes move from areas of high concentration (inside the body) to areas of low concentration (the surrounding seawater). This happens through the papulae and the thin body surface, allowing the starfish to get rid of waste without needing special organs like kidneys or lungs.

Coelom of Starfish

The coelom is a fluid-filled body cavity inside the starfish that surrounds its digestive system and other organs. While it’s not open to the outside environment, it plays an important role in helping the starfish stay healthy. The walls of the coelom are lined with cilia—tiny, hair-like structures that move back and forth to keep the coelomic fluid flowing.

This fluid helps carry oxygen, nutrients, and waste to and from the starfish’s cells. It also helps keep the internal body pressure stable and acts like a cushion, protecting the organs. In starfish, since they don’t have blood, the coelomic fluid is especially important for internal transport and support.

Papulae are small, finger-like bumps on the starfish’s body surface that help it breathe by increasing the area where gas exchange happens. These papulae are covered with tiny hairs called cilia that gently move water into the coelom, the internal body cavity. Oxygen (O₂) from the seawater passes through the thin walls of the papulae and enters the coelomic fluid inside, while carbon dioxide (CO₂), a waste gas, moves from the coelomic fluid back into the water.

The coelomic fluid then carries oxygen to all parts of the starfish’s body and takes carbon dioxide away from its tissues. Besides helping with gas exchange, the coelom also supports and cushions internal organs, acts like a skeleton by keeping the body firm, and helps transport fluids and gases inside the starfish.

Unlike special breathing organs such as gills or lungs, the coelom and papulae are not only for breathing—they also have other jobs like supporting the body. The coelomic fluid helps move gases inside the starfish but does not exchange gases directly with the environment. Nutrients, on the other hand, are absorbed through the digestive system and distributed differently, bypassing the coelom.

The Water Vascular System’s Function

Now, let’s examine the structure of the starfish water vascular system.

Starfish have tiny, tube-like structures on their underside called tube feet or podia. These tube feet not only help the starfish move and hold onto surfaces but also work like secondary gills to help the starfish breathe. They allow oxygen to enter and carbon dioxide to leave the body through their thin walls.

Inside the starfish, the tube feet are connected to a special system called the water vascular system, which is a network of canals filled with seawater. This system helps the starfish move its tube feet by using water pressure. Seawater enters this system through a small opening on the starfish’s upper surface called the madreporite. The madreporite acts like a filter, allowing clean seawater in while keeping debris out.

The water vascular system not only helps with movement and breathing but also helps circulate water around the starfish’s body, making sure oxygen reaches all the parts that need it.

The madreporite is located on the upper side of the starfish’s body, a little off-center. When seawater enters through the madreporite, it flows into a short tube called the stone canal. The stone canal connects the madreporite to the ring canal, which is a circular tube that goes around the center of the starfish’s body.

From the ring canal, water moves into several radial canals that extend outward along each arm of the starfish. These radial canals carry water to the tube feet, helping them to move and assist in gas exchange by bringing fresh oxygen-rich water close to the starfish’s body.

This entire water vascular system works together to help the starfish breathe, move, and sense its environment.

Each radial canal runs the full length of a starfish’s arm, ending at the tip. Along the way, the radial canal branches into smaller tubes called lateral canals. Each lateral canal has a tiny valve that stops water from flowing backward into the radial canal.

These lateral canals connect to the base of the tube feet, which end with a sucker or sticky pad. Near the base of each tube foot is a small bulb called an ampulla. When the ampulla muscles contract, they squeeze water into the tube foot, making it stretch out.

When the ampulla relaxes, the elastic walls of the tube foot pull it back. This creates suction, helping the starfish stick to surfaces. This system lets starfish move slowly, hold onto rocks, grab food, and do other important tasks.

Starfish are found mostly on rocky or hard surfaces in different marine environments, like coral reefs and tide pools, where their tube feet help them stay attached even in strong currents.

How Do Sea Stars Breathe Underwater?

Despite their name, starfish aren’t actually fish, and they don’t have lungs or gills like fish do. So, how do these amazing sea stars breathe underwater? What makes their respiratory system work?

Let’s break it down below and take a closer look at this fascinating and beautiful marine creature. The way a starfish breathes may surprise you—it’s a smart mix of simple structures and natural seawater flow.

Thinking about what types of sea stars live in California waters? Check out this blog: 10 Amazing Kinds of Starfish Found in California Waters. It covers some of the most beautiful and unique starfish found along the California coast.

Starfish breathes using two special parts:

Papulae

These are tiny, thin skin-like projections on the starfish’s top side. They work like small gills by increasing the surface area so more oxygen from the water can enter the starfish’s body. Tiny hairs called cilia help move water around the papulae, making it easier for oxygen to get in and carbon dioxide to leave.

Tube Feet

On the starfish’s underside, there are hundreds or even thousands of small tube-like feet. These tube feet help the starfish move, sense its surroundings, catch food, and also help with breathing by moving water around the body for gas exchange.

Together, these features let the starfish breathe underwater without lungs or gills!

How Do starfish Breathe in the ocean?

Seawater enters the starfish’s madreporite, a small pore on its top side, and flows through a special network called the water vascular system. The papulae (tiny skin projections) help by creating small water currents that draw seawater closer to their thin walls.

Oxygen dissolved in the water passes through the thin walls of the papulae into the coelomic fluid, which is the fluid inside the starfish’s body cavity. This fluid carries oxygen to all the starfish’s cells. At the same time, carbon dioxide, a waste gas produced by the cells, moves from the cells into the coelomic fluid, then back out through the papulae into the seawater.

This continuous flow of oxygen in and carbon dioxide out keeps the starfish alive and breathing underwater!

Key Points of Starfish Breathing

• Starfish have an open circulatory system where seawater moves freely inside their body cavity called the coelom.
• Papulae are small, finger-like projections that increase the surface area to help the starfish absorb oxygen and release carbon dioxide.
• Tube feet help circulate water around the starfish’s body and assist in breathing by supporting the papulae’s function.
• This breathing system works best for slow-moving starfish living in places with enough oxygen in the water.

How Do Starfish Breathing Occurs? (Gills or Lungs?)

Starfish lack specialized organs like gills or lungs. They breathe through tiny structures called papulae, which are located on their topside.

Papulae are thin, finger-like projections on the starfish’s topside that allow oxygen from seawater to pass into its body. The starfish’s tube feet also help move water around its body, which makes gas exchange easier. This process, where oxygen moves from the water into the starfish and carbon dioxide moves out, is called diffusion.

Gas exchange happens across the whole surface of the papulae, not just in special membranes. Inside the starfish, a fluid called coelomic fluid carries oxygen to cells and brings carbon dioxide away, but it doesn’t directly exchange gases with the outside water.

Can Starfish Breathe Out of Water?

1. Starefish cannot breathe out of water. It’s very important to never take them out of their natural home—the water. Most starfish can only hold their breath for about 30 seconds, so even being out of water for 20 seconds can be dangerous for them.
2. When starfish are exposed to air, they don’t get enough oxygen and start to suffocate because carbon dioxide builds up inside their bodies. This is not due to poisoning but because they can’t breathe properly.
3. Also, touching starfish can harm them. Chemicals and bacteria on our hands can cause stress or even be deadly for these delicate creatures. So, always leave starfish in the water to keep them safe and healthy.

Respect wildlife

Always leave all marine life, including starfish, in their natural homes. Watch them carefully, but don’t touch Sea Stars, or disturb them. I always make sure to tell people not to bother these gentle creatures for no reason.

Help protect starfish by sharing this message with others: It’s very important to keep them in the water and let them live safely.

Brittle Star Classification – Do they breathe like sea stars?

Brittle stars, or serpent stars or ophiuroids, of phylum Echinodermata, the same group as sea stars, sea urchins, sand dollars, and sea cucumbers. They are classified in the class Ophiuroidea, which contains over 2,000 species. Unlikely, brittle stars don’t have lungs and don’t “breathe” like us. They absorb oxygen and release CO2 through tiny structures on their arms.

Also Read:

1. Are Starfish Poisonous? 8 Exciting Facts You Didn’t Know
2. How to Clean Seashells Like a Pro [12 Bonus Tips ]
3. Top 5 Seashell Identification Apps Recommended By A Marine Biologist
4. Florida Starfish Types: Expert Tips & Guides for Top 8 Species Identification

FAQs( Frequently Asked Questions)

Conclusion

The query how do starfish breathe in the ocean is explained in an easy way, starfish use tiny structures called papulae and tube feet to take in oxygen from the water and get rid of carbon dioxide. This helps their cells get the energy they need.

Starfish can only breathe in water, so it’s important to keep their ocean home clean and safe, starfish themselves are also good decomposers of the oceans.