No, an eel is not a mammal. This elongated aquatic creature belongs to the class Actinopterygii, which is the scientific classification for ray-finned fish. The confusion over its identity arises from its distinctly serpentine shape, which appears unlike the typical fish most people imagine.
The Defining Features of Mammals
To be classified within the class Mammalia, an animal must possess specific characteristics. One of the most defining traits is endothermy, the ability to regulate an internal body temperature independently of the external environment. Mammals also universally possess hair or fur at some stage of their lives, providing insulation and sensory function.
Female mammals are uniquely defined by the presence of mammary glands, which produce milk to nourish their young after birth. Furthermore, with the exception of the egg-laying monotremes, most mammals are viviparous, meaning they give birth to live, relatively well-developed young. These four non-negotiable biological requirements—endothermy, hair/fur, mammary glands, and live birth—immediately exclude eels from the mammalian class.
Eels: Classified as Fish
Eels are classified as fish, specifically belonging to the order Anguilliformes within the class Actinopterygii. Their respiratory system relies on gills, which extract dissolved oxygen from the water, a stark contrast to the lungs used by all mammals. Eels are also ectotherms, meaning their internal body temperature fluctuates with the surrounding water, a fundamental difference from mammalian internal temperature regulation.
Their skeletal structure includes a vertebral column, but their fins are supported by bony rays, a feature that characterizes all ray-finned fish. Eels lack pelvic fins entirely, and their dorsal, caudal, and anal fins are often fused into a single continuous ribbon along the body’s length. As with other fish, eels possess a lateral line system, a specialized sensory organ that detects movement, vibration, and pressure changes in the water.
Why Eels Often Cause Confusion
The reason for the public’s confusion is the eel’s elongated, snake-like body, which is highly uncharacteristic of most familiar fish species. Their lack of prominent, easily recognizable pectoral and pelvic fins also contributes to this non-fish-like appearance. The long, sinuous body shape is an adaptation for burrowing and navigating tight spaces, allowing them to swim by generating propulsive waves along their entire length.
Many eel species also have scales that are either greatly reduced in size, very small, or deeply embedded within the skin, giving them a smooth, almost scaleless appearance. Additionally, the life cycle of a true eel involves a unique larval stage called the leptocephalus, which is flat, transparent, and ribbon-like. This unusual development stage is so different from the fry of typical fish that it historically led scientists to believe the larvae were a separate species entirely, further complicating the understanding of their true biological identity.