FIA is community supported website. When you purchase through links on our site, we may earn an affiliate commission. Learn More.

Are Fish Cold-Blooded?

Fish are aquatic vertebrates. As they dwell in water throughout their lives, it’s pretty obvious for curious minds to come up with the question, are fish cold-blooded or are they more complicated than cold-blooded reptiles?

What is the truth behind it?

In summary, yes. Fish are cold-blooded poikilothermic vertebrates, which implies they can obtain their body temperature from the environment they are living in.

Are Fish Cold-Blooded or Warm-Blooded? The Detailed Explanation!

Fish are poikilothermic vertebrates.

They get their body temperature from the surrounding aquatic habitat, unlike having a fixed body temperature. In other words, being cold-blooded means the water temperature of their surroundings controls their body temperature.

Fish’s body heat has a direct connection to the water temperature. When the water temperature changes, it brings about a difference in their body temperature as well. And if this change occurs in a negative direction, it affects their bodily functions, making it pretty stressful for them.

Warm water holds a lesser amount of dissolved oxygen, which tends to hinder their respiration and affects their metabolic process.

Being poikilothermic is an excellent solution to many issues related to heat adjustment in their body. They can easily do away with the problem of retaining heat in their body in the cold aquatic environment.

The water, which is the habitat of fish, works more like a heat sink. Due to its higher specific heat, water drains out heat from every object present in it.

In the case of fish, residing in the water for their entire lifetime tend to quickly draw off their energy unless they have adequate amounts of body fat like seals or whales.

Besides, fish have direct contact of their blood with the surrounding water through their gills. So, it tends to lower their blood temperature, resulting in a decrease in their body temperature.

Now, heightening their metabolism to keep themselves warm in such an environment costs them a lot of energy. Thus, being cold-blooded or poikilothermic gives them an added advantage to thrive freely in the water.

So, what would be the exact answer for “are fish cold-blooded or warm-blooded animals?” It is entirely dependent on the fish species and the water temperature it is living in.

Frequently Asked Questions

Are all fish cold-blooded?

No! Not all fish are cold-blooded.

Scientists have revealed that there are many warm-blooded fish that produce higher amounts of heat in their bodies to stay active throughout the day. For example, moonfish (opah) and tuna are fully warm-blooded fish.

However, the warmth in their blood is not as similar as the blood of birds, humans, and other mammals.

Opah is capable of circulating heated blood throughout its body. Their body and blood heat help them cope with cold ocean depths of 150-1300 feet below the surface.

Opah with temperature sensor
Opah with temperature sensor. Image Credit: NOAA Fisheries West Coast

Many fish live in dark and chilly places and rely upon their ambush for catching prey. But warm-blooded fish are faster in such situations.

Fish like opah keep flapping their fins, which serves enough heat to their body and speeds up their metabolism, rate of movement through the water, and reaction times.

Are tropical fish cold-blooded?

Yes, tropical fish are cold-blooded. The blood temperature of tropical fish entirely depends on their surrounding environment. As per the water temperature of the tropical area, their body temperature tends to vary.

The tropical areas of the Earth are neither too cold nor too hot. The water bodies of these regions maintain an optimized balance of temperature.

Regardless of the weather condition and water temperature, tropical fish species do not hold many differences in their traits compared to other non-tropical fish.

Are goldfish cold-blooded?

Goldfish are cyprinids, which are also known as coldwater herbivorous fish. These fish are poikilothermic by nature, i.e., they are cold-blooded.

Are betta fish cold-blooded?

Yes, indeed! Betta fish are also cold-blooded species, but they usually prefer a bit slightly warm aquatic environment. Thus, it is recommended to add a heater to the tank of your bettas.

These tropical fish are best kept in moderately heated water. If they are kept in too cold water, their metabolism can significantly slow down, resulting in inactivity and decreased immunity.

Are pufferfish cold-blooded?

Just like any other fish, pufferfish are also cold-blooded creatures. Of course, they do not have cold blood, but jokes apart, their body temperature is regulated by the water temperature.

Are clownfish warm-blooded?

Clownfish are much unique from any other fish species with their look and various characteristics.

These fish species have scaly skin and live in the moderate temperature of the water. But they are not warm-blooded creatures.

Clownfish are cold-blooded like any other fish species.

Are jellyfish cold-blooded?

Since jellyfish have the term “fish” in their name, you might think them to have similar traits to fish.

The fact is that jellyfish are not fish; instead, they are dome-shaped marine invertebrates.

Jellyfish are ectothermic organisms that fail to generate body heat using high metabolism. Their body temperature matches the temperature of the aquatic environment they reside in. So, yes, like fish, jellyfish are also cold-blooded.

Are starfish cold-blooded?

Like fish and jellyfish, starfish are ectotherms; hence, they are also cold-blooded creatures.

Their body temperature is utterly dependent on the heat of the surrounding environment. But starfish showcase a unique trait here; their entire body does not have the same temperature.

In the case of a starfish, when the water warms up, it does not distribute the heat evenly throughout its body.

When their body temperature reaches 88-95 ℉, while their arms get heated up to 91-102 ℉, they shuck out their limbs.

Just imagine how drastically the rising water temperature can harm cold-blooded sea creatures.

Can koi fish live in cold water?

Koi fish are cold-blooded.

However, even if poikilothermic fish are highly adjustable in the relatively colder aquatic environment, they require a stable temperature for their living.

Usually, koi fish are coldwater species. But they can thrive in a higher range of water temperatures as well. Koi fish typically prefers the water temperature between 59-77 ℉.

With the rise in water temperature, the metabolism rate of koi fish gets poorly impacted. It is crucial to maintain a much colder water temperature for koi fish.

Are flying fish cold-blooded?

Flying fish are one of the most famous sea animals, which can fly. But similar to any other fish or vertebrates, flying fish are also cold-blooded.

Are reptiles cold-blooded like fish? What are the differences between these two?

Both fish and reptiles are cold-blooded creatures, though the characteristics and body anatomy of reptiles and fish differ from each other.

Reptiles’ body temperature does not get adjusted or changed as per the temperature of their surrounding.

Fish’s body temperature vary according to their aquatic environment’s temperature.


So, the answer to your query, are fish cold-blooded is a definite yes. Thus, fish require proper care, as their body temperature is subject to change as per the environment.

Any rapid change in their environment can harm their body’s functionality and well-being, making them sick.

It is vital to ensure that you maintain a steady and stable tank water temperature while caring for any fish at your home.