Most fish are “cold-blooded,” meaning their internal body temperature largely matches their surrounding aquatic environment. While many fish follow this rule, there are exceptions that employ strategies to manage their body temperature.
Understanding Ectothermy
The scientific term for animals that rely on external heat sources to regulate their body temperature is “ectothermy.” Fish are primarily ectotherms, absorbing heat directly from the water through their skin and gills. Their body temperature, therefore, fluctuates with the ambient water temperature.
Another related term is “poikilothermy,” which describes animals whose internal temperature varies considerably rather than remaining constant. Most fish are poikilothermic, and their metabolic rate is directly influenced by the water temperature. This means that warmer water generally increases their metabolic processes, while colder water slows them down. Unlike endotherms, such as mammals and birds, fish do not expend significant metabolic energy to generate and maintain a constant internal body temperature. This energy efficiency can be advantageous, allowing ectothermic fish to allocate more resources towards growth and reproduction, especially in environments where food might be scarce.
How Fish Regulate Temperature
Even as ectotherms, fish employ strategies to manage their body temperature. A common approach is behavioral thermoregulation, where fish actively seek out areas within their habitat that offer more suitable temperatures. For instance, they might move to warmer, shallower waters during colder periods or descend to cooler depths when water temperatures rise. Some fish also utilize specific microclimates, such as shaded areas or spots near thermal vents, to find their preferred temperature range.
Fish can also adjust their activity levels in response to temperature fluctuations. They may become more active when water temperatures are optimal for their metabolism and less active during periods of extreme heat or cold. This allows them to conserve energy when conditions are unfavorable. While most fish rely on these behavioral adaptations, some also exhibit physiological adjustments, such as regulating blood flow to their gills or making metabolic changes to cope with varying temperatures.
Fish with Specialized Heat Retention
While the majority of fish are ectothermic, some species have evolved mechanisms to retain metabolic heat, allowing parts of their bodies to be warmer than the surrounding water. This is known as regional endothermy. Examples include predators like tuna, certain sharks (such as the great white and mako), and swordfish. These adaptations provide advantages, particularly for fast-swimming species.
These fish often possess a network of blood vessels called a “rete mirabile,” which translates to “wonderful net.” This structure functions as a countercurrent heat exchanger, transferring heat from warm blood flowing away from active muscles to cooler blood returning from the gills. This system helps to retain heat within the core muscles, eyes, and brain, allowing these tissues to operate at elevated temperatures. For example, a bluefin tuna can elevate its muscle temperature by as much as 21 degrees Celsius above the ambient water temperature.
Maintaining warmer eyes and brains can lead to faster neural processing and improved vision, offering a hunting advantage in colder or deeper waters. A warmer stomach and viscera can also accelerate the digestion and absorption of food. While these fish can warm specific body regions, they are not “warm-blooded” like mammals or birds, as their body temperature is not uniformly regulated and can still fluctuate with environmental changes. The opah, or moonfish, is an example that circulates heated blood throughout its entire body, making it a truly warm-bodied fish.