Fish in Focus:   American eel, Anguilla rostrata


Garold W. Sneegas and Aquatic Kansas Images

In ancient times, noticing that eels did not lay eggs or give birth, people speculated that the eels spontaneously generated from bits of skin rubbed off an eel's body, even from such materials like horsehairs, rotting vegetation, and water insects. It was during the latter part of the 18th century that scientists correctly concluded that eels must lay their eggs at sea, observing that adults left freshwater to head out to sea, and young eels were seen to return. What happened in-between, however, was a mystery.

Where did the adult eels spawn? What did their young look like? In 1892, scientists announced that a peculiar willow leaf-shaped sea creature, known as leptocephalus, was in fact the larval stage of the eel. But the birthplace of these larvae remained a mystery until 1920, when a Danish scientist named Johannes Schmidt, in a series of painstaking investigations, traced the eels' birthplace to an area in the Atlantic called the Sargasso Sea. Although it is now well-established that eels spawn there, no adult eel has ever been found in that region, and nothing is known about the spawning behavior and the fate of the adult eels.

The spawning waters in the Sargasso Sea actually hosts two kinds of eels. European eels Anguilla anguilla larvae ride the North American current on a 3-year journey to Europe, where they lead a life cycle very much like their American cousins Anguilla rostrata. Except for some minor differences between American and European eels, such as slightly different number of vertebrae in the backbone, as well as some genetic differences, there is some uncertainty over whether they should be considered the same or separate species.

The American eel larvae spends up to a year in the ocean, drifting in the ocean currents. Many are swept by the Gulf Stream to eventually reach the US and Canadian Atlantic coastline, typically arriving in the fall of the year they were born. Some larvae are drawn by currents towards the Gulf of Mexico. American eels have also been found as far north as Greenland, and as far south as the West Indies.

Near the coast, the larvae undergo a transformation, becoming worm-like "glass eels" about two inches long. Pigmentation follows not long after, and they begin resembling miniature versions of the adults we're familiar with. Many young eels begin a long trek inland, through rivers and streams, and have been observed overcoming land obstacles by wriggling over wet grass and wet rocks. Some eels however, remain in estuaries and salt marshes. Eels have been known to travel quite far to inland waters, some reaching the upper Mississippi river and the Great Lakes. This period is the the longest phase of the eel's life cycle--the yellow eel phase--where they will spend their time feeding and growing.

Yellow eels, or immature adult eels, remain in this stage for five to twenty years. They are found in a wide range of habitats: rivers and streams, lakes and ponds, estuaries and salt marshes. Here, they appear snake-like, yellowish to olive-brown above with lighter-colored bellies. They secrete copious amounts of a protective slimy mucous from their thick skin--hence the expression "slippery as an eel".  Eels are nocturnal predators, taking small fish, insects, worms, crayfish, and snails, and prefer to burrow in bottom sediment during the day. In winter, eels hibernate, burrowing into the bottom mud. Females can reach two to three feet in length, and weigh three to four pounds. Males tend to be smaller, around a foot and a half in length. Land-locked eels, however, can each five to six feet and weigh ten to fifteen pounds.

The final phase of the eel's life is a series of dramatic transformations, and a long journey to the waters from where they were born. It's not known what triggers the change, there seems to be no correlation with age and sex. The change and migration usually occurs in the late summer and fall, as early as August in the north and as late as December in the south. The eels begin heading downstream towards the sea, with the peak migration occurring from September to November. Once at sea, they seem to disappear, and no one knows their migratory route or how they navigate to their breeding waters.

Major physical changes occur during the migratory phase these changes begin while the eels are still in their freshwater growing habitats. The color of their skin changes, taking on a metallic cast, with black or brown at the top, and silver below. At this stage, they are known as silver eels. The sexual organs begin to enlarge and mature. By now, they have already accumulated large reserves of fat--an essential source of fuel on their migratory journey. They have also stopped feeding and their gut has degenerated. Their body begins to adjust to a saltwater environment. Eventually, the eyes become larger, with retinal pigments sensitive to red light in freshwater being replaced by pigments sensitive to blue light that permeates the upper ocean. The gas bladder changes, allowing the eel to swim at deeper depths and pressures.

The final days of the eel's life, upon reaching the spawning waters of the Sargasso Sea, have never been observed in nature. But in laboratory tests, advanced maturation has been induced by hormone treatments. With no more need for muscle and fat, the female directs all her bodily resources to the generation of eggs--her ovaries grow to occupy almost her entire body cavity where she could carry as many as a million eggs. Males remain more active, and it is therefore speculated that they might take the initiative in courtship. The eels presumably die after spawning silver eels have never been observed returning to the coast. Left behind is a new generation, in the form of willow leaf-shaped eel larvae, the leptocephali.

In ancient Rome, eels were a popular food. Today, eels are a gourmet food item in Europe and Japan. Yellow eels are caught in baited traps and trot lines. Silver eels are caught as they migrate downstream, in stationery nets. Because of their high fat content, silver eels are more valuable. Eels are generally shipped live to Europe, since chefs prefer to work with live eels over frozen ones. Elvers have also been collected and shipped live to Japan where they are raised to market size.  However, American eels have fallen in popularity in favor of native eels due to difficulty in raising them in captivity. Besides their use in the food industry, the tanned eel skin makes a high quality leather.

References:

1 Fishes of the Western North Atlantic Part Nine: Orders Anguilliformes and Saccopharyngiformes Edited by Eugenia B. Bohlke, Published by Sears Foundation for Marine Research, Yale University, 1989. - Chapter Order Anguilliformes: Family Anguillidae, Freshwater eels by David G. Smith, Smithsonian Institution.

2 Canadian Ministry of Fisheries and Oceans eel page
http://www.dfo-mpo.gc.ca/COMMUNIC/ss-marin/eel/eel.htm  

3 Government of Nova Scotia, Canada, eel web page
http://www.gov.ns.ca/fish/inland/species/eel.htm

The above was written by Shireen Gonzaga for Earth and Sky
http://www.earthsky.com

This article appears at:
http://www.earthsky.com/1999/esmi990513.html

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