Eels are elongated, snake-like fish belonging to the order Anguilliformes. The simple answer to whether they have fins is yes, but they are highly modified compared to most other bony fish, which causes confusion about their anatomy. The eel’s unique body plan has evolved for a specialized lifestyle of burrowing and navigating tight spaces. This adaptation has resulted in a streamlined form where some fins are merged and others are absent.
Anatomy of Eel Fins: The Continuous Structure
Eels possess fins, but their dorsal, caudal, and anal fins are fused together, forming a single, long, ribbon-like structure. This structure runs along the back, around the tail tip, and along the underside of the body. This continuous median fin is a defining characteristic of true eels, replacing the distinct, separate caudal fin seen on most fish with a flexible, undulating fringe.
The dorsal and anal fin sections are exceptionally long-based, sometimes containing up to 650 soft rays. This extensive fin surface is used for propulsion and stability during their wave-like swimming motion. Eels also have a pair of pectoral fins, located just behind the head near the gill openings.
Pectoral fins are present in many species, such as freshwater and conger eels, but they are often small and can be entirely absent in certain families, notably moray eels. When present, they are used for subtle steering, maneuvering, and maintaining position rather than for the primary propulsion. The eel’s smooth, scaleless skin, often covered in a thick, protective mucus, contributes to the sleek, streamlined body form that facilitates movement through dense environments.
The Absence of Pelvic Fins
The misconception that eels lack fins entirely stems from a specific anatomical absence: they do not possess pelvic fins, also known as ventral fins. The loss of these paired fins is a defining feature of the entire Anguilliformes order. Most bony fish use these fins, positioned on the belly before the anal fin, for stability, braking, and making fine adjustments to their pitch and roll.
Eels lack both the pelvic fins and the skeletal support structure, known as the pelvic girdle, that anchors them in other fish. This absence is a significant evolutionary adaptation linked to their elongated bodies and lifestyle. The smooth, uninterrupted length of the body is better suited for burrowing into sand or mud and maneuvering through narrow crevices.
The evolutionary history of eels shows that their earliest fossil ancestors, dating back to the Cretaceous Period, possessed pelvic fins. The subsequent loss of these fins became a hallmark of the modern eel lineage. The “limbless” nature of the eel is reflected in an older scientific grouping name for the order, “Apodes,” which literally means “without feet.”
Anguilliform Locomotion: How Eels Move
The unique fin structure and elongated body are perfectly adapted for the serpentine movement known as anguilliform locomotion. This method involves generating a wave of muscular contraction that travels down the entire length of the body, from head to tail. The eel’s highly flexible backbone, which can contain hundreds of vertebrae, allows the body to bend into a deep, continuous S-shape.
As this wave passes backward, the sideways motion of the body pushes against the water, generating a propulsive force that moves the eel forward. The continuous dorsal and anal fin acts as an extended paddle, working in concert with the main body musculature. This results in an efficient, undulating movement that is effective for slow to moderate speeds and for navigating complex habitats.
Unlike fish that rely on a powerful flick of a separate, rigid caudal fin for thrust, the eel distributes the propulsive effort along its whole length. This method allows for exceptional maneuverability, enabling the eel to swim backward by simply reversing the direction of the traveling wave along its body. The combination of the long, continuous fin and the flexible, muscular body makes anguilliform locomotion a specialized and effective way to move through both open water and confined spaces.