Eels are fascinating aquatic creatures, often recognized for their elongated, snake-like bodies. Despite this serpentine appearance, eels are unequivocally fish. Belonging to the order Anguilliformes, they possess all the fundamental biological characteristics that classify them within the diverse group of ray-finned fish. This article explores the biological reasons why eels are fish, clarifying their unique features and dispelling common misunderstandings.
Common Misconceptions About Eels
Eels are often mistaken for snakes due to their striking physical resemblance. They possess long, slender bodies that undulate through water, mimicking a snake’s slithering motion. Many species also appear to lack prominent fins and have smooth, slimy skin, unlike the scaly appearance of most fish.
However, eels are aquatic organisms adapted to life underwater, distinguishing them from reptiles. Snakes are air-breathing animals with lungs and dry, keratinous scales. While some eels can survive briefly out of water, their biology aligns with fish.
Defining Features That Make Eels Fish
Eels breathe underwater using gills, a defining characteristic of fish. Like other bony fish, they draw water into their mouths and pass it over specialized gill filaments, extracting oxygen into their bloodstream. Some species, like moray eels, constantly open and close their mouths to force water over their gills, but this mechanism remains a form of aquatic respiration.
Eels possess fins essential for fish locomotion. They have a continuous dorsal, anal, and caudal (tail) fin fused into a single, ribbon-like structure along much of their body. While pelvic fins are typically absent, pectoral fins are usually present behind their heads, aiding in maneuvering. These fin structures are important for their unique swimming style.
Eels also possess a bony skeletal structure, including a vertebral column adapted for fish. Their vertebral column has a high number of vertebrae, providing flexibility for undulating movement and navigating tight spaces. This internal framework supports their muscular system, enabling “anguilliform” swimming.
Most eel species have scales, though they often appear scaleless. These scales are tiny, deeply embedded within their skin, and covered by a protective mucus layer, giving them a smooth feel. Eel reproduction involves spawning eggs directly into the water, where external fertilization typically occurs, and their life cycle often includes a distinct larval stage known as a leptocephalus. This aquatic reproductive strategy is fundamentally different from terrestrial egg-laying or live birth seen in reptiles.
Diversity Within the Eel Family
True eels belong to the order Anguilliformes, a diverse group with approximately 1,000 species across 20 families. This highlights the wide array of forms and adaptations within these fish. Notable families include Anguillidae (freshwater eels) and Muraenidae (moray eels).
Eels inhabit a wide range of aquatic environments. Freshwater eels, like the American eel, live in rivers and lakes but migrate to oceanic spawning grounds. Marine eels, such as moray and conger eels, primarily reside in saltwater habitats. Their varied habitats and behaviors, from hiding in coral reefs to dwelling around shipwrecks, underscore their successful integration into aquatic ecosystems.