Eels are recognizable by their elongated, serpentine bodies. This shape is the result of a highly successful evolutionary path, allowing them to colonize virtually every aquatic environment on the planet. The term “eel” is a broad designation encompassing hundreds of species that share this fundamental body plan. To understand the variety, one must examine the formal scientific classification that separates the true eels from other aquatic animals that share a similar look.
The Scientific Classification of True Eels
The approximately 800 to 1,000 species of true eels belong exclusively to the Order Anguilliformes, which is organized into about 20 distinct families. All members of the Anguilliformes are united by specific characteristics that distinguish them from other fish. They all lack pelvic fins entirely, and in most species, the dorsal, anal, and caudal fins are fused into a single continuous ribbon along the body. True eels also possess a unique life-stage known as the leptocephalus larva. This larval form is transparent and leaf-like, drifting in ocean currents for months or even years before undergoing metamorphosis into a miniature eel.
The Most Common Marine Eels
The most famous true eels are the Moray Eels (family Muraenidae), a diverse group with over 200 species found predominantly in tropical and temperate reef environments. Morays are known for their thick, scaleless skin, often covered in a protective layer of mucus, and their generally nocturnal, crevice-dwelling behavior. A remarkable adaptation is their unique feeding mechanism, which involves a second set of jaws located in the throat called pharyngeal jaws. These pharyngeal jaws shoot forward to grab the food and pull it back into the throat, compensating for the eel’s inability to use suction to swallow.
Conger Eels (family Congridae) are marine eels often found on continental shelves and slopes worldwide. They tend to have smoother bodies and are sometimes commercially important as food fish. Within this family are the garden eels (Heterocongrinae), which anchor their tails in the sandy bottom and sway in the current, creating large colonies on the seafloor.
The family Ophichthidae, known as Snake Eels or Worm Eels, represents a specialized group adapted for burrowing. Many of the approximately 350 species have reduced or entirely absent fins, allowing them to move efficiently through soft substrates. A defining characteristic is their rigid, pointed tail tip, which lacks a caudal fin and is used to rapidly burrow backward into the sand or mud for protection.
Specialized and Freshwater Eels
The True Freshwater Eels (family Anguillidae), represented by the genus Anguilla, exhibit a unique life cycle described as catadromous. The adults live and grow in freshwater rivers and lakes for years, but they must migrate thousands of miles to the ocean to spawn and then die. Their larvae then drift back to the continents, developing through the transparent glass eel stage before entering the rivers as elvers.
In the deep ocean, the Saccopharyngiformes, including the Gulper Eels, are a highly modified lineage considered part of the Anguilliformes order. These fish have evolved features to survive in the nutrient-scarce abyssal zone, including a massive, unhinged mouth that can stretch to swallow prey larger than the eel itself. The Gulper Eel’s body is characterized by a reduced skeleton and musculature, a small head, and often a bioluminescent organ at the tip of its long, whip-like tail, which is used to lure prey in the darkness.
The Cutthroat Eels (Synaphobranchidae) are benthic dwellers found on the continental slopes and abyssal plains worldwide. They gain their common name from a distinctive feature where the gill openings are set low on the body and often join together on the ventral surface. They are adapted for the dark, high-pressure environment.
Animals Commonly Misidentified as Eels
The term “eel” is frequently used to describe several unrelated aquatic animals. The most famous example is the Electric Eel (Electrophorus), a South American freshwater fish that belongs to the Order Gymnotiformes, making it a type of knifefish more closely related to carp and catfish than to true eels. Its powerful electric generation capability is not a trait of the Anguilliformes.
Other commonly mislabeled creatures include Lampreys and Hagfish, which are not bony fish, but are instead jawless fish belonging to the infraphylum Agnatha. Lampreys (Order Petromyzontiformes) have a sucker-like mouth and a cartilaginous skeleton, separating them from the bony, jawed true eels. Hagfish (Class Myxini), often called “slime eels,” also lack jaws and vertebrae, placing them far outside the true eel lineage.