The long, slender, and legless forms of eels and snakes often cause confusion, leading many to assume a close biological relationship. Both share an elongated, serpentine body shape adapted for movement through dense environments, whether water or ground cover. Despite this superficial similarity, eels and snakes belong to vastly different biological classes. Their profound differences in anatomy, physiology, and evolutionary origins reveal that they are not closely related, representing a striking example of convergent evolution.
Evolutionary Relationship
Eels are fish, classified within the class Actinopterygii (ray-finned fishes) and belonging to the order Anguilliformes. This places them in the same broad biological group as salmon or tuna, distinguished by their bony skeletons. Snakes are reptiles, belonging to the class Reptilia and the order Squamata, which also includes lizards. This fundamental separation means their last common ancestor lived hundreds of millions of years ago, before vertebrates fully colonized land.
The resemblance between these two distant groups is a case of convergent evolution. This process occurs when unrelated species independently evolve similar traits while adapting to comparable ecological challenges. For both eels and snakes, the long, limbless body plan is an efficient adaptation for locomotion. This streamlined form allows eels to navigate and burrow through aquatic sediment and snakes to undulate through dense terrestrial vegetation.
Key Physical Distinctions
A key distinction lies in their respiratory systems, reflecting their classification as fish versus reptiles. Eels breathe underwater using gills, which extract dissolved oxygen from water flowing over them. The gill openings in many eels, such as morays, are reduced to small lateral slits or pores. They often must actively pump water by gulping since they lack the large, bony gill cover (operculum) found in most other bony fish. Snakes are air-breathing reptiles and possess lungs, requiring them to regularly surface for atmospheric oxygen, even in fully aquatic species like sea snakes.
The integument, or skin, of the two animals is profoundly different, reflecting their need for aquatic or terrestrial protection. An eel’s skin is smooth and covered in a thick layer of protective mucus, which helps prevent infection and reduces friction when swimming. While some eel species possess minute, embedded scales, these are cycloid and covered by the skin, giving the animal a slimy appearance. A snake’s body is covered in large, dry, overlapping scales composed of hard beta-keratin, the same durable protein found in human fingernails.
Another easily observable difference is the presence of fins on the eel, which are entirely absent on the snake. Eels possess continuous dorsal, caudal, and anal fins that run along the back, around the tail tip, and along the underside. They use these fins for propulsion and stabilization in the water column. Snakes possess no external fin structures, relying entirely on muscle contraction and body shape for movement, even sea snakes which have a paddle-like tail for swimming.
Habitat and Lifestyle Differences
The aquatic nature of eels dictates their movement, which involves whole-body undulation to generate thrust in water, resulting in a fluid, ribbon-like motion. Eels are found in both marine and freshwater environments. Many species spend a large portion of their lives burrowing into soft substrate to ambush prey. Their diet is broad, consisting of various aquatic organisms like fish, invertebrates, and crustaceans.
Snakes utilize their specialized anatomy for movement over solid ground, employing several distinct methods of terrestrial locomotion. The most common is lateral undulation, where the body pushes off objects like rocks and vegetation to propel itself forward. Other methods include concertina movement, used in tight spaces, and rectilinear motion, where large ventral scales grip the substrate and are sequentially pulled forward by muscles. Even sea snakes, which have adapted to an ocean lifestyle, must surface to breathe and primarily hunt aquatic prey near the water surface or in shallow reefs.
Specialized Internal Systems
The internal skeletal structure highlights their deep evolutionary divergence regarding their elongated shape. A snake’s skeletal system features a highly articulated vertebral column with hundreds of vertebrae, each connected to a pair of mobile ribs. These ribs protect internal organs and play a direct role in terrestrial movement, allowing the snake to propel its body forward during rectilinear movement. Eels, as fish, possess a comparably flexible spine but lack a sternum and have simpler rib structures, which do not serve the same locomotor purpose.
The sensory organs also reflect the animals’ adapted environments. Eels possess the lateral line system, a unique feature of aquatic vertebrates that snakes lack. This system consists of mechanoreceptive organs called neuromasts, which detect minute water movements, vibrations, and pressure changes. Snakes, living in a chemical-rich, terrestrial environment, rely on advanced chemosensory organs, such as the vomeronasal or Jacobson’s organ, located on the roof of the mouth. They use their forked tongue to collect scent particles from the air, delivering them to this organ for a form of “stereo-smelling” used for tracking prey and mates.
The extreme length of a snake’s body required a radical rearrangement of its internal organs to fit the narrow space. Paired organs like the kidneys and gonads are often elongated and staggered, positioned one in front of the other instead of side-by-side. Furthermore, the lungs are asymmetrical. The right lung is typically the main functional respiratory organ, while the left lung is often greatly reduced, vestigial, or entirely absent, a biological compromise to accommodate the snake’s narrow, cylindrical form.