The answer is straightforward: yes, the vast majority of fish commonly called eels are considered “cold-blooded.” Eels are slender, elongated fish belonging to the order Anguilliformes, which includes familiar species like moray eels and freshwater eels. Like most fish, these true eels are ectotherms, meaning they primarily rely on external sources to regulate their body temperature. This physiological classification dictates nearly every aspect of their thermal biology and behavior in aquatic environments. We must look closely at the scientific definitions of thermoregulation to understand the underlying mechanisms.
Understanding Ectothermy
The traditional term “cold-blooded” is a generalized phrase that scientists replace with more precise terminology. The primary scientific classification is ectothermy, which describes organisms that gain most of their body heat from their surrounding environment. This stands in contrast to endothermy, the strategy used by mammals and birds, where the body internally generates the majority of its heat through metabolic activity.
Ectotherms are often also described as poikilotherms, a term defining animals whose internal body temperature fluctuates significantly with the ambient temperature. An ectotherm’s low metabolic rate does not produce sufficient heat to maintain a constant internal temperature against a changing environment. This is the opposite of homeotherms, like humans, who possess physiological adaptations to keep their body temperature within a narrow, stable range. Therefore, eels are ectotherms and poikilotherms, with their body temperature closely mirroring the water around them.
Thermoregulation in True Eels
The thermoregulatory strategy of true eels is defined by their low metabolic rate, which limits their capacity for internal heat production. Their body structure reinforces this reliance on the environment, as their long, serpentine shape gives them a high surface area-to-volume ratio. This means that any heat generated metabolically is quickly lost to the surrounding water, making it impossible to maintain a temperature gradient between their internal organs and the water.
Because they lack internal control, eels rely heavily on behavioral thermoregulation to keep their body temperature within an optimal range for physiological function. American eels, for instance, prefer a water temperature around \(16.7^{\circ}\text{C}\) when given a choice in a thermal gradient. They actively move between different water depths, currents, or microclimates to find these preferred thermal zones.
This behavioral reliance means that temperature directly affects their activity and feeding. When water temperatures drop below \(14^{\circ}\text{C}\), the foraging activity of eels significantly decreases. In colder climates, such as the northern range of the American eel, they enter a state of metabolic torpor during the winter months when the water temperature falls to \(5^{\circ}\text{C}\) or less. This torpid state is characterized by a decrease in oxygen consumption rates and a cessation of feeding. This allows them to conserve energy and survive prolonged cold until the water warms again, a shutdown dictated by their ectothermic nature.
Clarifying the Electric Eel
The common name “electric eel” often causes confusion regarding the thermoregulatory status of the entire group. It is important to know that the electric eel is not a true eel, despite its striking physical resemblance and common moniker. True eels belong to the order Anguilliformes, while the electric eel is classified in the order Gymnotiformes, a group of neotropical knifefishes more closely related to carp and catfish.
This distinction is based on evolutionary history, but the thermoregulatory answer remains the same for both. Even the electric eel, famous for its ability to generate powerful electrical discharges of up to 860 volts, is ectothermic. Its unique electrical capacity is a specialized adaptation for defense, predation, and navigation, having no connection to its ability to regulate body heat. Therefore, regardless of whether you refer to a true eel or the electric eel, both fish species follow the same ectothermic strategy for survival.