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How Do Fish Stay Warm in Winter?

It’s the dead of winter, and hopefully you’re warm and bundled up, enjoying some nice hot chocolate inside a warm house. But if you’re a fish, that’s probably not the case for you. Have you ever wondered, what happens to the fish when the water freezes in winter? The answer to this all has to do with evolution and the interesting biology of how fish adapted to suit their environment, with a variety of mechanisms from unique thermoregulation to antifreeze in their blood!

Fish regulate their body temperatures differently

First of all, animals control body temperature in very different ways. Humans are endotherms, which means that we generate heat internally, and our core body temperature remains relatively stable (at around 98° Fahrenheit or 37° Celsius) regardless of the environment. The heat generated comes from our metabolism, when glucose is turned into ATP and heat during cellular respiration. But why do we even need to regulate our body temperature in the first place? Since enzymes and proteins function at a specific temperature, too high of a temperature would cause our organs and other proteins to denature, while too low of a temperature would cause decreased circulation. Our body needs blood circulating in order to get nutrients and remove waste. For humans, it is most convenient to regulate our own body temperatures rather than relying on other mechanisms to keep our proteins running.

However, most fish, unlike humans, are cold-blooded animals. This means that their body temperature is regulated by the environment, while their body temperature can fluctuate significantly depending on the outside temperature, making them a type of ectotherm called a poikilotherm. In other words, fish don’t regulate their own body temperature, it simply conforms to the temperature of the environment. Since fish live in the water, this process makes sense from an energy standpoint. Heat can be lost very quickly when in direct contact with water. You can see this when you dunk your hand in cold water, compared to if you exposed your hand to air at the same temperature. If fish needed to maintain a stable body temperature all the time through metabolism, this would be incredibly difficult, as the heat would be lost immediately in contact with water. 

How do proteins in fish function at different temperatures?

Still, fish, just like humans, are made up of proteins, and proteins need to function at specific temperatures. In response to that, fish have evolved to become eurythermic, which means that they live in a wide range of temperatures, or stenothermic, which means that they live in a narrow range of temperatures. Eurythermic fish have proteins that adapt remarkably to increase their protein synthesis rates at different temperatures. This occurs in response to different environments, as certain proteins can be produced to help the fish survive in a warmer or colder climate. For example, the Senegalnese sole lives in an environment that fluctuates between temperatures of 54° to 79° Fahrenheit (12° to 26° Celsius). In a study published in 2012, it was found that Senegalnese sole fish that had been living in a warm environment of 79° had different amino acid concentrations than fish that had been living in a cold environment of 54°, many of the amino acids functioning to aid in homeostasis and metabolism.

This is how fish antifreeze proteins work! Image Source

Some Arctic and Antarctic fish have antifreeze in their blood!

Meanwhile, many stenothermic fish simply have proteins evolved to function at a different temperature, such as the enzyme trypsin found in Antarctic cod. However, some proteins are necessary for the fish’s existence, so fish have evolved additional mechanisms to prevent these essential proteins from freezing. To illustrate, human blood mostly consists of water and other particles, so blood is estimated to freeze just below the freezing point of water at around 28° Fahrenheit (-2° Celsius). In the arctic and Antarctic, the temperatures there could freeze a human within a matter of minutes. So, why don’t the fish freeze there? The reason is because of… Antifreeze! Yes, fish that live in the arctic and Antarctic have built-in antifreeze proteins in their blood that bind to ice crystals. If the internal fluids of the fish begin to freeze, the proteins bind to the ice crystals to prevent them from growing further. This unique function occurs due to the shape of the protein with repeating amino acids and a helical structure that allows it to bind to a large surface area. The protein then allows the liquid to evaporate through a process known as the Kelvin effect. In the Antarctic, the Notothenioid fish who have these antifreeze proteins make up around 95% of all the fish in the region, while in the arctic, the Arctic cod is one of the most abundant fish in the region. Yet, these fish don’t have any common ancestors, so the proteins for antifreeze evolved independently in the Notothenioid fish and Arctic cod in convergent evolution. All because of how necessary antifreeze proteins are for survival!

With the expansive importance of antifreeze proteins in fish, could there be any implications for humans? One example of the uses for antifreeze in the industry is in food preservation and texture. The process of making ice cream is much more complicated than just putting cream in the freezer. The texture of ice cream can be ruined by large ice crystals, so in order to make ice cream smoother, antifreeze proteins can be added to minimize the growth of ice crystals. Similarly, the qualities of thawed meats can be improved when antifreeze proteins are added, as large ice crystals can damage the meat. But besides food, antifreeze proteins have significant uses in medicine as well. In particular, biofilms formed by bacteria clumping together can cause serious infections that are hard to treat, but it was found that antifreeze proteins were capable of preventing the growth of biofilms by binding to the bacteria. Finally, these proteins can aid in the preservation of cells. Normal freezing procedures can damage cells because of ice crystallization, but with the help of antifreeze proteins, large ice crystals aren’t capable of forming and damaging the cell.

How is climate change impacting thermoregulation in fish?

Because of these evolutionary adaptations, some fish are extremely sensitive to temperature changes. And with our changing climate, that’s a problem. Many fish rely on cooler waters to live and spawn. As we’ve seen within the past 20 years, the rise in water temperatures have caused fish to move North in search of cooler waters. This is significant because habitat loss is the number one reason for the extinction of freshwater fish, and as fish are forced to move, more and more fish species will go extinct. In addition, algae blooms can kill fish by producing toxins and taking up all the oxygen in the water, and because of the rising temperatures, more and more algae blooms are being observed as the warmer temperatures are the perfect environment for cyanobacteria. But the harmful effects of climate change don’t just end there. The rising carbon dioxide levels is also causing ocean acidification, as the increased carbon dioxide emissions in our atmosphere is being taken up by the ocean, causing a decrease in pH. This means that calcium carbonate can’t form, harming marine species with shells made from calcium carbonate like coral reefs, oysters, and sea snails. The loss of coral reefs means the loss of habitat for thousands of fish species as well, as coral reefs provide a rich environment for protection and nutrients. Climate change is a huge issue that has disastrous effects, affecting all wildlife on Earth, including fish, as their environments and thermoregulation depend on it.

Have humans evolved to adapt to different temperatures and climates?

Humans live everywhere, and in fact, there are humans on every continent. Therefore, it would make sense for some humans to have evolutionary adaptations to be better suited for the climate of the region, and that’s certainly been the case. For example, researchers Nina Jablonski and George Chaplin found that a map of the skin colors across the world directly correlated with the amount of UV radiation that the area receives. The pigmentation in skin is caused by melanin, and melanin serves to protect cells from damage from UV radiation. However, because UV radiation also provides vitamin D (which is a critical nutrient for human health), as humans began to migrate towards colder climates with less UV, skin color became lighter to allow more UV radiation to penetrate the skin in order to get vitamin D. Humans also have other slight adaptations that allow us to be better suited for the climate we live in. In animal evolution, Allen’s Rule and Bergmann’s Rule states that in colder climates, bodies are larger and rounder with shorter appendages in order to retain body heat, while in colder climates, bodies are smaller and more linear with longer appendages to expel heat. While we see that in animals, such as when we compare the bodies of polar bears to giraffes, scientists have long debated whether these rules apply to humans as well. Research data has generally supported this theory, as it was found by a study in 2013 that humans do follow Allen’s Rule and Bergmann’s Rule when there are significant differences in the climate. With these adaptations, humans in colder climates are better able to preserve body heat because of the volume to surface area ratio and increased heat production from more cells.

In conclusion…

So in the end, how do fish stay warm in winter? The short answer is, fish don’t actually need to stay “warm,” at least in human standards. Humans have different thermoregulation mechanisms than fish, since we have to maintain our own core body temperature. In comparison, fish take on the temperature of their outside environment, but in order to keep their proteins functioning, some fish have evolved proteins that can function at specific temperatures, or proteins that can adapt in response to the environment. In particular, fish that live in extremely cold climates have antifreeze proteins to prevent ice crystals from forming in their blood, and this allows them to live in places where any other animal would immediately freeze to death. However, with the rise in temperatures due to climate change, this has caused serious disruptions in ocean environments, leading to the extinction of many fish species. Fish are critical to our ecosystem, and while their amazing evolutionary adaptations have allowed fish species to live all across the world, it’s important to help preserve our environments to help fish continue to thrive.

Fun Fact: If 98.6° Fahrenheit seems slightly high as the average temperature of humans, you’re right! This temperature was established as the average by a German physician in the 1800s, and today, the average temperature might actually be lower, closer to around 98°. That’s why it’s more important to consider the individual person’s normal temperature to determine whether or not they have a fever, since 98.6° could be normal for someone but quite high for someone else.

Posted in Research & STEM

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