Do Starfish Have Blood And A Heart?

Starfish are generally simple animals without apparent organs to see or walk, even without actual brains! However, even though it’s not obvious, they do have eyes, and they definitely can walk. So, what about starfish blood, or hearts? What does their circulatory system look like? In this post, we’ll talk all about that but let’s begin with a quick answer:

Starfish don’t have blood or hearts but they use seawater and water vascular system to move nutrients, metabolic by-products, and gases through their bodies. During this exchange, waste products and gases are also being removed.

However, that certainly doesn’t tell the whole story. Below I’ll explain more about the starfish’s circulatory system, how the system works, and what other functions it provides.

Do starfish have blood? (Explained)

Starfish, as part of the phylum Echinodermata, have an open circulatory system. This means that fluid moves freely in the body cavity, but unlike humans or many other animals, this fluid is not blood. What’s more, starfish don’t have hearts as they’re not needed to pump blood.

So, how do they function? Instead of the blood, starfish use seawater and water vascular system (which I will explain more below). The job of their unique circulatory system is to move nutrients, metabolic by-products, and gases through the starfish bodies. Waste products and gases are also being removed from the body during this exchange.

It’s worth mentioning that not having blood is very common in soft-bodied invertebrates such as jellyfish or coral. They don’t need it because they directly exchange their nutrients with the outside environment.

Do starfish have hearts? (Explained)

As I already mentioned above, starfish don’t have hearts. But why is that and how are they able to survive without one? Let’s first understand why a heart is necessary for humans.

Our heart is sort of like a pump. It’s there to send blood around our bodies so the blood provides us with the oxygen and nutrients we need. It also carries away the waste. So how does it sound? It’d say exactly like a water vascular system for sea stars.

Of course, our bodies are completely differently built and we need blood and heart to survive. Starfish, however, don’t. They simply don’t need a heart or blood because their body is adapted to living without them. Instead, they need a water vascular system that provides the same function – provides oxygen, nutrients, and carries away the waste.

What’s more, starfish don’t have brains and they’re able to live without it as well. If you’re interested in that, feel free to read my other post “Do Starfish Have Brains?”.

How does the circulation work?

Circulation takes place in three different organs: 

• The perivisceral coelom, which is the area inside the body but outside the various organs,
• The water vascular system, which is a system of canals connecting sea stars’ thousands of tube feet, 
• The hemal system – a series of channels that move and distribute food materials throughout the body.

Let’s take a more in-depth look at each part of the circulatory system. You can see each step of a water journey in the illustration below.

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, which is a second part of the water vascular system.

Next, the water goes into a ring canal that circulates the sea stars’ center of the body. It’s wide and pentagonal or five-sided. This ring connects the fourth part of the vascular system – a radial canal. 

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 runs immediately to the oral side of the ambulacral muscles. The radial canal spreads into two series of short and narrow branches called lateral canals – the fifth part of the water vascular system.

Each of the lateral canals has a valve inside to prevent a backward flow of the water into the radial canal. Next, these canals are connected to the base of tube feet and their ampulla – the sixth and seventh part of the circulatory system.

The ampulla is the suction cup 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. But this unique system of canals has more functions than “just” that. 

What are the other functions of sea stars’ water vascular system?

Locomotion

Despite the nutrition and gases exchanges function, it also allows sea stars to move around the seafloor. As I already mentioned, sea stars create contractions in their tube feet. There are two double rows of hundreds or thousands of tube feet that line the radial canals. 

When the water goes inside the system and through all the parts discussed above, it eventually reaches tube feet and their ampulla. The ampullae contracts, and the valves at the junction of the lateral canals, and tube feet, prevent the flow of water into radial canals.

Next, the water is forced towards the suckered podium. This causes the feet to extend and reach. It detaches a sea star’s from the surface, and the animal can choose the next point to attach again. 

After reaching the next point, the ampulla relaxes, causing the formation of suction cups. These cups later attach the sea star to the surface thanks to their vacuum-like motion. Then, the ampulla contracts, and the whole process is repeated. 

Attachment

The other function of a sea stars’ water vascular system is attachment. Starfish use their feet to attach themselves to the rock. They do that to protect themselves from predators as their defense system combines physical and behavioral protection strategies.

Starfish have thick, calcified skin that protects them. Some of these animals are even covered with poisonous spines, such as thorn-of-crown starfish. When they combine it with their tube feet firmly attached to the rock, they can avoid attack from some of the predators, like seagulls or otters. 

Being attached to the rocks also help them to prevent from being washed away by the strong currents. 

Food capture

The following function of the water vascular system is food capture. Thanks to their suction-cupped feet, sea stars can attach their arms to open the shells of mollusks. They mainly prey on clams, mussels, and oysters.

Next, they extrude their stomach out through their mouth and digest prey outside their bodies. Once the food is digested, they put the stomach back to their bodies.