The eel is an aquatic vertebrate known for its serpentine appearance and mysterious life history. Despite resembling a snake, the eel is entirely a fish, belonging to the ancient superorder of bony fish called Elopomorpha. With over 800 known species inhabiting waters across the globe, their unique anatomy and complex behaviors make them remarkable inhabitants of both marine and freshwater ecosystems.
Defining True Eels
True eels are classified under the order Anguilliformes, a group of elongated, ray-finned fishes. A primary defining characteristic is the absence of pelvic fins, which are paired appendages found in most other fish species. Instead of separate fins, the dorsal, caudal (tail), and anal fins are fused into one continuous ribbon of soft rays. This structure facilitates their characteristic undulating, serpentine movement through the water.
The skin of a true eel is notably smooth and often covered in a thick layer of protective mucus, which contributes to their slippery reputation. While many marine species appear scaleless, freshwater eels possess minute, cycloid scales deeply embedded within the skin. The jaw structure is built for predation, featuring strong jaws often lined with numerous small, conical teeth designed for gripping prey. These anatomical details collectively distinguish the true eel from other fish that have independently evolved a similar body shape.
The Unique Life Cycle
The freshwater eels, belonging to the family Anguillidae, exhibit a rare life strategy known as catadromy, meaning they live in fresh water but migrate to the sea to reproduce. The American and European eels, for example, undertake immense migrations to the Sargasso Sea to spawn and die. The eggs hatch into the first larval stage, called the leptocephalus, a transparent, leaf-like creature that drifts on ocean currents, such as the Gulf Stream, for up to two years.
This larva then transforms into the glass eel stage, which is transparent and cylindrical, allowing it to actively swim towards continental coastlines and estuaries. Entering fresh or brackish water, the young eel gains pigmentation and becomes an elver, then transforms into the yellow eel stage, lasting anywhere from 5 to 20 years in rivers and lakes. The final metamorphosis is into the silver eel, involving a physiological overhaul for the return journey to the spawning grounds. This process includes the enlargement of the eyes, widening of the pectoral fins, and the cessation of feeding as the digestive tract shuts down.
Diversity in Habitat and Diet
While the freshwater eels are known for their spectacular migrations, the vast majority of eel species are exclusively marine, including Moray eels and Conger eels. These eels occupy a wide variety of habitats, ranging from deep-sea trenches to the shallow, rocky crevices and coral reefs of tropical oceans. Their elongated bodies are perfectly suited for burrowing into sandy or muddy substrates, often emerging only at night to hunt.
Eels function as opportunistic predators, consuming a diet primarily composed of fish, crustaceans, and other invertebrates. The Moray eel possesses a specialized feeding mechanism: a second set of pharyngeal jaws in its throat. When a Moray captures prey with its main oral jaws, the pharyngeal jaws shoot forward, grip the prey, and then retract to pull the meal down the eel’s long throat. This unique adaptation allows the Moray to effectively swallow large, struggling prey in confined spaces where the suction feeding used by many other fish is ineffective.
Distinguishing Eels from Look-Alikes
The common name “eel” is often applied to other serpentine aquatic creatures that are not true eels from the order Anguilliformes. The most frequently mistaken animal is the electric eel, which is actually a type of knifefish belonging to the order Gymnotiformes. The electric eel is a freshwater fish that breathes air, lacks the fused fin structure of true eels, and uses specialized organs called electrocytes to generate powerful electric shocks for hunting and defense.
True eels are bony fish, possessing gills and fins, distinguishing them entirely from reptiles such as sea snakes. Sea snakes are air-breathing reptiles that reproduce differently and shed their skin, making the resemblance purely superficial. The presence of the fused dorsal, caudal, and anal fins is the most reliable external feature to confirm that an animal is a member of the true eel lineage.