Eels, with their distinctive elongated bodies, often lead people to question their biological classification. Despite their snake-like appearance, eels are not cartilaginous fish. They belong to the vast group of bony fish, sharing fundamental skeletal characteristics with familiar species like salmon and cod. This classification is based on their internal anatomy, specifically the composition of their skeleton.
Characteristics of Cartilaginous Fish
Cartilaginous fish, known as Chondrichthyes, are distinguished by skeletons made entirely of cartilage rather than bone. This flexible yet strong tissue provides support and allows for efficient movement through water. Unlike bony fish, they lack a swim bladder, often relying instead on a large, oil-rich liver and continuous swimming to avoid sinking.
Another defining feature is the presence of several visible gill slits on the sides of their heads, which open directly to the exterior. They do not possess a bony gill cover (operculum), so water must constantly pass over their gills for respiration. Their skin is also unique, covered in tiny, tooth-like structures called placoid scales or dermal denticles, which give their skin a rough, sandpaper-like texture. Examples include sharks, rays, and skates.
Eels’ Place in the Fish Family
Eels are classified as bony fish (Osteichthyes). Their internal skeleton is primarily composed of bone tissue, a fundamental difference from cartilaginous fish. Their bodies are notably flexible and elongated, a characteristic attributed to a high number of vertebrae in their bony spinal column.
Eels, like most other bony fish, have an operculum, a bony flap that covers and protects their gills. They utilize this structure for efficient respiration. Eels also possess a swim bladder, a gas-filled organ that helps them regulate buoyancy in the water column. This organ allows them to maintain their position without expending constant energy on swimming for lift.
The unique body shape of eels, often described as snake-like, is an adaptation to their environment and lifestyle, allowing them to navigate narrow spaces and burrow into substrates. Despite this distinct morphology, their internal anatomy, including their bony skeleton, operculum, and swim bladder, confirms their identity as bony fish. Some species have small, embedded cycloid scales.
Understanding the Misconception
The misconception that eels are cartilaginous fish often stems from their unusual appearance. Their elongated, snake-like bodies visually set them apart from typical bony fish, leading to an assumption of a more flexible skeletal structure.
Many eel species appear to lack prominent scales or have very small, embedded scales, contributing to their smooth appearance. This contrasts with the obvious scales found on most bony fish. Cartilaginous fish, with their dermal denticles, also have a unique skin texture, and the perceived lack of scales on eels can mistakenly lead people to group them together. However, these superficial characteristics are deceptive, as they do not reflect the internal bony skeleton that defines eels as bony fish.