Thursday, September 23, 2010

The Remarkable Life Cycle of the Eel

European Eel Elver, Anguilla anguilla
The European Eel, Anguilla anguilla, is a truly remarkable animal. It is an amphihaline fish, spending part of its life in the ocean and part in the fresh water of the streams of continental Europe and western Africa. A. anguilla has been seriously under threat since the 1980's. The reasons for this decline are seen to be manifold according to Feunteun (2002, Ecological Engineering 18 pp. 575-591), namely changes in the Gulf Stream current, obstructions to migration, the effect of fisheries, loss of habitat, parasite infection and poisoning due to pollution. The length of this list is explained by the complex life cycle of A. anguilla.

It begins with the spawning of the leptocephali. It is assumed (Schmidt, 1922 Philosophical Transactions of the Royal Society 211 pp. 179–208) that this is adjacent to the Sargasso Sea in the middle of the North Atlantic. Here the smallest leptocephali have been found, however no mature adults, adults mating or eggs have been found.

The currents carry the leptocephali to European and African coasts where they metamorphose into the glass eel stage. The glass eel is so named as it is translucent and is fished is certain parts of Europe where they are eaten, fried with garlic.
European Eel Elver, Anguilla anguilla
As the silver eels travel from marine to brackish and fresh water, they metamorphose into elvers. As these grow larger, they become a yellow-brown colour and are referred to as yellow eels (Arai et al., 2006 Estuarine, Coastal and Shelf Science 67 pp. 569-578).

These in turn metamorphose into silver eels, which emigrate back to the Atlantic to mate. However nothing is known about this stage of the eels life as it travels deep in the ocean and thus far tracking the eels has proved in the past to be too difficult/expensive (Feunteun, 2002). Yet in 2009 Aarestrup et al. (2009, Science 325 p. 1660) succeeded in tracking A. anguilla for the first 1300 km of its journey from the western coast of Europe using a miniaturized pop-up satellite archival transmitter indicating that the eel's migratory mystery is close to being revealed.

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