Anguillidae

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Anguillidae: Freshwater Eels

The Anguillidae are a family of ray-finned fish known as freshwater eels. All living species belong to the genus Anguilla. They are long, snake-like fish with a continuous fringe of dorsal, caudal, and anal fins, and they lack pelvic fins.

They are catadromous: adults live in rivers, lakes, and estuaries, but migrate to the ocean to spawn. Their life starts in the ocean as eggs, which hatch into leptocephalus larvae. The larvae drift with the currents for months or years, then become glass eels, grow into elvers, become yellow eels, and finally turn into silver eels before returning to the sea to reproduce. They are semelparous, meaning they typically die after spawning, though timing depends on conditions. Migrations in temperate species often last many months; tropical species usually migrate for shorter periods. Some eels, like the European eel, can travel thousands of kilometers.

Anguillid eels have a global distribution, living in more than 150 countries, mostly in tropical and temperate waters. Many species are threatened by habitat loss, barriers to migration (such as dams), pollution, overfishing, and climate change. Notable species include the European eel (A. anguilla), American eel (A. rostrata), Japanese eel (A. japonica), and the New Zealand longfin eel (A. dieffenbachii). Because most eels used for food are not bred in captivity, farming relies on wild juveniles, which adds pressure on wild populations. For this reason, Seafood Watch advises consumers to avoid anguillid eels.

In appearance, adult eels are brown to olive and camouflaged for river bottoms. They have small embedded scales, a strong lateral line for sensing water movement, and highly developed senses of smell. They can burrow into mud and often move with a powerful, snake-like swimming motion. They can exchange gases through their gills and even through their skin when out of water. They use long migrations between fresh water and the sea to reach their breeding grounds, and some species may navigate using the earth’s magnetic field.

Fossil evidence helps date the origin of freshwater eels. The oldest anguillid fossil is Anguilla ignota, from about 43.8 million years ago, with broader origins estimated between 83 and 43.8 million years ago. Modern freshwater eels remain a subject of study as scientists work to understand their complex life cycles and migrations.