The prevailing idea that all fish are “cold-blooded” is largely accurate, but it is not absolute. The term “cold-blooded” is the common name for an ectotherm, an organism whose body temperature is regulated primarily by the external environment. While the vast majority of the world’s fish species fit this description, a select group of highly active, predatory fish have evolved specialized physiological adaptations that allow them to generate and retain their own metabolic heat, meaning not all fish are ectotherms. This small number of exceptions challenges the general rule.
What It Means to Be Ectothermic
The default condition for nearly all fish is to be ectothermic, which means their body temperature closely mirrors the ambient water temperature surrounding them. Water is an excellent conductor of heat, which means any warmth generated internally by the fish’s metabolism is quickly lost to the colder environment, primarily through the gills. For this reason, ectothermic fish do not expend significant energy trying to maintain a body temperature different from the water.
This reliance on external conditions makes ectotherms highly efficient in terms of food energy, as they require far less fuel than “warm-blooded” animals of the same size. However, their activity level is directly tied to the temperature of the water, making them sluggish and less mobile in colder conditions. The performance of their muscles and organs slows down significantly when the environment cools.
The Phenomenon of Regional Endothermy
The exceptions to the ectothermic rule are fish that exhibit regional endothermy, a form of internal temperature regulation where only specific parts of the body are kept warmer than the surrounding water. This partial temperature control is found in groups of fast-swimming, large pelagic predators, including certain species of tuna, such as the Atlantic bluefin, and lamnid sharks, like the Great White and Shortfin Mako. These fish are able to elevate the temperature of their locomotor muscles, eyes, or brain.
Maintaining warmer swimming muscles allows these predators to sustain higher cruising speeds and greater burst performance regardless of the water temperature. The evolutionary advantage of this warmed muscle tissue is the ability to hunt effectively across a wider range of the water column. For instance, warming the eye and brain tissue, known as orbital endothermy, improves sensory perception and reaction time in the colder, darker depths where some prey live.
Physiological Mechanisms for Heat Retention
The ability of these fish to conserve their internally generated heat relies on a specialized anatomical structure called the Rete Mirabile, which translates from Latin as the “wonderful net.” This structure is a dense bundle of parallel arteries and veins that are positioned close together, acting as a countercurrent heat exchanger. Metabolic heat is a byproduct of the fish’s active red muscle tissue, which is often centrally located near the vertebral column.
As warm, deoxygenated venous blood flows away from the active muscles, it passes through the Rete Mirabile immediately adjacent to cold, oxygenated arterial blood flowing toward the muscles. Heat is efficiently transferred from the outgoing warm venous blood to the incoming cold arterial blood before the venous blood reaches the surface of the body, where it would otherwise lose its heat to the water via the gills. This highly efficient heat exchange, which can approach 99% efficiency in fish like the bluefin tuna, effectively traps the heat deep within the body, allowing the muscles and organs to operate at temperatures significantly higher than the surrounding ocean.