Eels are a group of ray-finned fish that resemble snakes in appearance. They are characterized by elongated bodies, smooth skin, and a continuous, ribbon-like fin that runs along their back. They belong to the order Anguilliformes, comprising around 1,000 species.
These fish are primarily marine, though some species migrate between freshwater and saltwater. They are harvested in countries like Japan, where they have been an important food source for centuries.
Their size varies considerably among the species. For example, the longest eel, the slender giant moray (Strophidon sathete), gets as long as 13 ft (4 m), whereas the smallest species (as of 2016), Snyder’s moray (Anarchias leucurus), attains a maximum length of around 4.5 in (11.5 cm).[1]
The heaviest eel, the European conger (Conger conger), has been reported to weigh as much as 240 lb (110 kg). However, depending on the species, eels typically weigh between 1 oz (30 g) and 55 lb (25 kg).[2]
These fish have an elongated, snake-like body with tiny, embedded scales that are almost invisible. Their skin is often covered with a thick, slimy mucus that reduces drag. Being ray-finned fish, they possess an internal skeleton made primarily of bone and not cartilage.
Their dorsal and anal fins have merged with the caudal fin, forming a continuous ribbon-like structure that runs along the body. They lack pelvic fins, and in many species, the pectoral fins are also absent.
According to Eschmeyer’s Catalog of Fishes, all species of eels are classified into 5 suborders and 21 extant families.
Fossil records reveal that these fish originated around the Late Cretaceous Period. These earliest fossils had primitive features, such as pelvic fins, which the modern eels lack.
Most eels are marine and inhabit shallow coastal waters, often seeking shelter among coral reefs or within rocky crevices. However, members of the genus Anguilla (such as the European and American eels) and the freshwater moray (Gymnothorax polyuranodon) spend most of their adult lives in freshwater environments, like rivers, streams, and lakes. The rice-paddy eel (Pisodonophis boro) spawns in freshwater, migrating from saline water to rice paddies during the rainy season.
Some species, such as Ilyophis arx, have been observed at depths as low as 10,100 ft (3,083 m).[3]
Most eels are carnivorous predators and feed primarily on animal matter. Their diet includes snails, bivalves, squids, crabs, shrimps, lobsters, aquatic worms, and juvenile fish. They may occasionally scavenge on detritus if primary food sources are scarce.
Under a controlled environment, the juveniles of some species, such as the American conger (Conger oceanicus), were found to eat other juveniles of the same species (cannibalism).[4]
Their lifespan varies with species. For example, American eels typically live for at least 5 years, whereas some European eels have been observed living over 80 years.[5][6]
The males and the females release sperm and eggs simultaneously into the water, which eventually fertilize externally. The fertilized eggs hatch into flat, transparent larvae called leptocephali. These larvae drift with water currents, feeding on marine snow, which is small particles of detritus falling from the upper levels of the water column. In 6 to 12 months, they metamorphose into glass eels. These then become pigmented, develop into elvers, and migrate towards shallow coastal waters in large numbers.
The elvers gradually become brownish-yellow (yellow eels) and eventually transition to sexually mature silver eels.
In the ocean, eels fall prey to sharks, groupers, barracudas, tuna, and snappers. When in freshwater habitats, juveniles are preyed upon by fish, including pikes and bass, as well as birds (in shallow water), such as herons, cormorants, and egrets. Some mammals, like otters and raccoons, also feed on them.
Small crabs, jellyfish, and shrimp may occasionally feed on eel eggs.
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