Common eel  (Anguilla anguilla)

Classified as Critically Endangered (CR) on the IUCN Red List (1).

The common or European eel has a very unusual and fascinating life cycle. Adults have long, narrow bodies, with a continuous dorsal, anal and tail-fin (2). The skin is slimy, the lower jaw is longer than the upper jaw, and the scales are tiny or absent (2). The colour of adults depends on their age; they are often brown, black or olive-green with yellowish bellies. Some adults may be silvery (known as 'silver eels'); the lifecycle stages differ greatly in appearance (2).

Found in the rivers of the North Atlantic, Baltic and Mediterranean Seas; it also occurs along European coasts from the Black Sea to the White Sea in Russia. Spawning takes place in the Sargasso Sea in the western Atlantic (3).

Part of the common eel’s life cycle is spent in the sea, and part in freshwater rivers. It is often common on the shore (2).

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).

The population of the common eel is threatened at present, and eel stocks have declined in recent years. However, there is currently very little scientific knowledge of this species, which would aid its management. The threats facing the species are unknown, however, pollution, overfishing, habitat degradation, parasite infection and changes in climate have all been forwarded as potential causes of the decline (7).

The European Union is currently funding research that aims to halt the decline of the common eel population (7).

• Catadromous: migratory behaviour of some fish that spend most of their lives in freshwater, but migrate to the sea to breed.
• Larvae: stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
• Metamorphosis: an abrupt physical change from the larval to the adult form.
• Plankton: aquatic organisms that drift with water movements; may be either phytoplankton (plants), or zooplankton (animals).

1. IUCN Red List (November, 2008)
   http://www.iucnredlist.org
2. Fish, J.D. and Fish, S. (1996) A Student's Guide to the Seashore. Second Edition.Cambridge University Press, Cambridge.
3. FishBase (January, 2003)
   http://www.fishbase.org/Summary/SpeciesSummary.cfm?ID=35&genusname=Anguilla&speciesname=anguilla
4. Strathclyde University (January, 2003)
   http://www.strath.ac.uk/Departments/Civeng/research/cemr/eel_study.htm
5. Silver eel: Integrative Zoology, University of Leiden (January, 2003)
   http://www.fishbiology.net/silvereel.html
6. Burnie, D. (2001) Animal.Dorling Kindersley, London.
7. Estimation of the Reproductive Capacity of European Eel (EELREP) (January, 2003)
   http://www.fishbiology.net/eelrep.html