The Heat Is On: Are Fish Cold Blooded?

are fish cold blooded

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Today we’re going to answer a question that many of our readers are interested to learn more about. Are fish cold blooded?

The world of fish is an amazing and diverse place, but there’s a common misconception that we often hear repeated; that all fish are cold-blooded.

The truth is, the vast majority of fish are indeed cold-blooded – their circulatory system is based on the absorption of dissolved oxygen from the water around them, so their body temperature depends entirely on the temperature of their environment. 

However there are some which are warm-blooded – they have an internal temperature that fluctuates depending on the concentration of dissolved oxygen in their environment, and therefore have an entirely different circulatory system than cold-blooded fish.

Warm-blooded fish are called poikilotherms.

What Does Cold Blooded Mean?

Cold-blooded means that fish have no internal body temperature. The circulatory system of cold-blooded fish relies on a constant supply of oxygen from the water around them.

The human body, on the other hand, has an internal temperature that varies depending on how much oxygen is in the air we breathe and how active we are.

So what’s the difference between cold-blooded and warm-blooded? 

Cold-blooded animals are those which rely on dissolved oxygen in their water to carry out chemical reactions that keep their bodies running.

They, therefore, have a constant body temperature because they don’t produce heat, and rely on their environment to provide all the heat they need.

Warm-blooded animals, on the other hand, create heat as part of their metabolism and therefore have an internal temperature that varies depending on how much oxygen is in the air we breathe and how active we are.

Fish don’t have any special adaptations which allow them to get around without a circulatory system – these are present in only the cold-blooded fish which live outside water.

But the cold-blooded fish we do see in the wild have a few different adaptations, usually involving very shiny coloration to reflect a lot of light, or special internal organs to give them the heat they need.

Other Types of Fish

Other than those which are both cold and warm-blooded, there is one hot-blooded type: the crocodile fish.

Unlike other fish, crocodile fish don’t rely on dissolved oxygen from their environment to keep their bodies going – instead, they use a metabolic process called fermentation.

This makes them an exception to the rule that all fish are cold-blooded, though they are still poikilotherms.

Are All Fishes Cold-Blooded?

Just like us, there are several different types of fishes found in the wild. There are cold-blooded fish that live in the oceans and rivers.

There are also warm-blooded fish that live in freshwater, and some warm-blooded fish that inhabit both fresh and saltwater environments.

Warm-blooded fish such as Opah(moonfish) and Humpback Moray are found in deep ocean waters.

Most of the fish that live on coral reefs, near the surface of the ocean, are also warm-blooded. These include livebearers, such as damselfish, parrotfish, and butterfly fish.

Many tropical freshwater species of fish that inhabit rivers and lakes over a large area but do not enter salt water are cold-blooded. These include jewelfish and black bass.

Fish of the same species which inhabit different waters may be warm-blooded or cold-blooded.

There is no universal difference in their characteristics such as size, feeding habits, and color; this is why the terms “cold-blooded” and “warm-blooded” are misused sometimes to describe a fish’s environment.

Are Any Fish Warm-Blooded?

As mentioned earlier, the vast majority of fish are cold-blooded, but some species are warm-blooded.

These fish have a slower metabolism than other fish, and therefore need an internal temperature that is lower than the external temperature of their environment.

In general, warm-blooded tropical freshwater species are smaller in size than others of the same species because they have less heat produced by their metabolism so they do not need as much food or water.

They tend to be dull in colors because they use less pigment to absorb the light and reflect it away from their body.

Most of the warm-blooded fish are bony fish, and most of them live in tropical freshwater areas. Some are native to Africa, but they also inhabit the waters of South America, Asia, and Australia.

They can be found in hot, shallow water bodies where temperatures range between 23-30 degrees Celsius.

The Main Characteristics Of Warm-Blooded Fish Are:

  • Smaller sizes in comparison with other types of fish of the same species. 
  • Diminished pigmentation and unpigmented body parts. 
  • A higher concentration of myoglobin in the muscles than in other fish. 
  • A lower blood plasma glucose level than cold-blooded fish, especially during the summer months. 
  • Transpiration through the gills is high due to the high metabolic rate.
  • A slow metabolism.
  • Always living in a warm freshwater environment.
  • Foraging for food at night or in the dark, since they are nocturnal animals.
  • Slower growth and reproduction rates than cold-blooded fish.

Fishes that can both live in fresh and saltwater are called euryhaline fish, and they are also warm-blooded. These include mullet, pufferfish, and flounder.

Can Fish Be Both?

Some fish species can be both cold-blooded and warm-blooded – either depending on whether they live in water with a higher or lower concentration of dissolved oxygen.

They are known as poikilotherms. They do not have a constant internal temperature.

About 50 fish species are considered to be both cold-blooded and warm-blooded, but most of them have at least one characteristic of each.

For example, tuna are warm-blooded when they are in tropical waters and cold-blooded when they live in colder waters.

Final Thoughts

Fish are cold-blooded by default. There are only a few, mostly tropical freshwater species that are warm-blooded.

Warm-blooded fish have a slow metabolism and take longer to grow and reproduce, but they can live in water with a lower concentration of dissolved oxygen.

Depending on their environment, some fish have different adaptations that allow them to live in both fresh and saltwater.

Many fish live in deep, warm waters but some occupy shallow waters and coral reefs near the surface of the ocean.

They are not warm-blooded; some are reef fish and others are mollusks or crustaceans.