Common eel  (Anguilla anguilla)

The common eel has a fascinating life-cycle; it is a 'catadromous' species, breeding in the sea and migrating to freshwater in order to grow before returning to the sea to spawn (4). It is thought that all European eels spawn in the Sargasso Sea. The larvae, which look like curled leaves and are known as 'leptocephalli', drift in the plankton for up to three years (2), and are carried by the Gulf Stream towards the coasts of Europe (3). They then undergo metamorphosis into young eels; at this stage they are known as 'glass eels' because they are transparent (2). They become darker in colour and start to migrate up freshwater streams in large numbers; they are known as 'elvers' at this time and measure around 50 mm in length (2). The eels, now called 'brown' or 'yellow eels' grow in freshwater (5), with males and females spending 6 to 12 and 9 to 20 years in freshwater, respectively (3). Towards the end of this time, they become sexually mature; they turn a silvery colour and migrate back towards the sea on dark, moonless and stormy nights; during this time they are known as 'silver eels' (5). Upon returning to the sea, the common eel lives in mud, crevices, and under stones (3). Spawning occurs during winter and early spring in the Sargasso Sea (3). This is a very long-lived species with a maximum life span of 85 years (3).

This eel is predated upon by birds, including cormorants and gulls, as well as a number of species of fish (3). Remarkably, they can survive out of water for several hours on damp nights; they may travel overland on dark rainy nights (6).