The life cycle of the American eel is highly complex and cloaked in mystery. The American eel inhabits diverse habitats as well as transforms through several metamorphoses throughout its life cycle. The eel transforms physically and behaves differently during each stage of its life as it adapts to the diverse ecosystems it inhabits. Furthermore, the American eel journeys great distances over its lifetime, over 2000 miles from the Sargasso Sea as hatched larvae to the freshwater rivers and lakes of North American, and then back to the Sargasso Sea as mature adults.
Although mating, birth, and death of the American eel is believed to take place in the Sargasso Sea, there is no witness to these events.
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The life cycle of the American eel can be organized into six stages. The first stage is the when the egg is fertilized. Mature eels return to the place of their birth, migrating from rivers all along the Atlantic coast to the Sargasso Sea in order to spawn. The Sargasso Sea covers two million square miles in the Atlantic Ocean and contains enormous mats of Saragassum weed floating on the surface of the warm waters north of Bermuda in the Atlantic Ocean.
The Sargasso Sea is devoid of land boundaries. Instead, the North Atlantic, Gulf Stream, Canary, and
North Atlantic Equatorial ocean currents
form its borders
After making the1000 mile journey from North America, to the Sargasso, mature eels mate. Next, the female eels lay between 10-20 million eggs.
These eggs are then fertilized by the mature male eels.
Once fertilized the buoyant eel eggs float to the surface of the Sargasso where between February and March the eel hatch from the drifting egg to become larvae called leptocephali.
Figure 5. Life Cycle of the American Eel
The second stage of the American eel’s life cycle is the hatched larvae called the leptocephali. The newly hatched eel larvae look like a flat clear willow leaf. These little marine creatures ride the Gulf Stream currents for about a year as zooplankton to the coastal waters of North American. Protected by their transparency, leptocephali, grow and feed on plankton of the rich ocean waters during the journey west.
The third stage of the eel’s life cycle is the “Glass eel” stage. This is the next metamorphosis. After the leptocephi have fed and grown for about a year drifting on the Gulf current to North America, they begin arriving at the shores of Connecticut in late winter and mid-March. The glass eels begin to enter the brackish waters of the Long Island Sound estuary in addition to tidal creeks and inlets of Connecticut and Rhode Island
At this stage,
the Glass eels look like thin, transparent worms with a tiny black dot for an eye, literally as clear as glass. They are about 2-3 inches long and have grown two small fins on each side of their bodies. Glass eels are known to enter tidal creek by the 10s of thousands.
In the fourth stage of their life, eels are called elvers. At this stage, eels grow to about 4 inches in length. As elvers, the young eels begin to migrate into the brackish waters of coastal estuaries as well as fresh water streams and ponds. During this time, their bodies under go another physical change. When elvers enter the fresh water, their bodies develop grey to greenish brown pigmentation and darken as they move inland. Elvers eat almost anything in their habitat, burrowing themselves in the mud during the day and hunting almost anything at night.
The fifth stage of the eel’s life cycle is called the yellow eel. This is the juvenile stage of the eel’s life. When the elver reaches its inland destination, it metamorphoses to a yellow eel. During this phase, eels are dark olive green or brownish. They are bottom dwellers, living in fresh water rivers, lakes, and streams, sleeping in the mud, silt, or sand during the day. Eels are carnivores feeding on insects, mollusks, worms, smaller fish, crustaceans, and dead animal matter at night. Female eels grow to four to five feet in length which is twice the size of adult male eels.
The sixth stage of the eel’s life cycle is called Silver eel. In this stage, the eel metamorphoses again, physically maturing to spawn and prepare for the migratory journey to the salty spawning grounds of the Sargasso Sea. Silver eels are mature adults with an average life span of five years but many live to 15-20 years. In preparation for their journey to the Sargasso, the Silver eel’s body coloration changes to a dark grey with a white underside which offers predatory protection in the open ocean. Additionally, their eyes and pectoral fins enlarge. Furthermore, the eel stops eating when it is in the silver stage because its digestive system stops functioning. The sexually mature Silver eels leave inland fresh water rivers in the fall (September and October), and begin their journey to the Sargasso Sea. James Prosek, author of “Eels”, describes their yearly journeys to the Sargasso as, “the greatest unseen migrations of any creature on the planet.”
Moreover, once in the Sargasso, the entire eel population mate and spawn together. This is the first and last time the American eel will spawn. They spawn just once in their life time and then die. The life of the American eel begins and
ends in the Sargasso Sea.
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Figure 6. Map of American eel migration routes
Source: Parks Canada
Reproduced with the permission of ©Her Majesty the Queen in Right of Canada, 2017.