The American eel is catadromous, meaning that it spends the majority of its life in freshwater but migrates to the sea to spawn and die (3) (5) (8) (10). Although very little is known about the spawning period of the American eel, it is thought to occur in the autumn, with larger females releasing somewhere between 15 and 30 million eggs to be fertilised by the male (3) (4) (8). The buoyant eggs of the American eel hatch into transparent, leaf- or ribbon-like larvae (4) (8), known as ‘leptocephali’, which drift with ocean currents for around a year before reaching the Atlantic coast (2) (3) (4) (5) (8).
Once they have reached a length of around 6 to 6.5 centimetres, the larvae transform into the first juvenile phase and will actively migrate into coastal estuaries in late winter and early spring (2) (5). Eels in the first juvenile phase are known as ‘glass eels’, as although they are shaped like adults, they lack pigmentation and are much smaller (4) (5). The glass eels feed very little, but within a few weeks of leaving the ocean they enter the second juvenile phase and begin to grow and feed, at which time they are called ‘elvers’ (4) (5).
Elvers resemble the adult eels and are grey to greenish-brown in colour (4) (5). The elvers generally migrate upstream to freshwater, although it is thought to be mainly the females that migrate far inland, while the males stay in brackish and estuarine areas (2) (3) (5) (8). The American eel is able to absorb oxygen through its skin as well as its gills, which enables it to travel over land to reach isolated water bodies (3) (4). The elvers in particular are well known for their ability to negotiate obstacles such as waterfalls and rapids (10), and they are even able to traverse vertical objects such aslow dams and canal locks (3) (5), as long as the surface is damp and textured (5).
The elvers develop into immature adults, known as ‘yellow eels’ (2) (4). Generally, the yellow eels live in estuaries, rivers and streams for 4 to 10 years before reaching sexual maturity, although some may not mature for 20 years or more (2) (8) (10). The American eel reaches maturity once it grows to a certain size, and faster-growing individuals mature earlier (2). It is thought that yellow eels remaining in estuarine areas around the coast typically mature earlier than those in freshwater (4).
The sexually mature adult American eel undergoes a number of significant changes, including enlargement of the eyes and pectoral fins (2). It also becomes characteristically silvery and is henceforth referred to as a ‘silver eel’ (2) (4). The mature adult eel ceases feeding and begins its migration back to the spawning grounds in the Sargasso Sea (2) (4) (10). The American eel is thought to die after it has spawned (2) (4).
The American eel may live for more than 50 years in the wild (3).
The American eel is an opportunistic carnivore, feeding on a wide range of prey species (3) (5) (9). Adult American eels typically feed on small fish and invertebrates, including crustaceans, insects, worms and molluscs. Elvers and yellow eels feed mainly on aquatic insects and their larvae, as well as small crustaceans (5). The American eel may sometimes also take frogs and carrion (3).