From glass eels to silver eels and everything in between The life stages of the American Eel

Imagine yourself hiking next to a babbling creek.  You come to a small waterfall surrounded by rocks.  The rocks glisten from the spray of the falls.  You walk closer and see dozens of small snake like creatures slithering over the wet rocks.  You watch them move from the top of the rock pile to the bottom.  Then they slide back into the creek.

You saw the American eel utilizing one of its unique adaptations.  Their bodies are coated in a mucus layer, providing protection and a way to absorb oxygen through their skin.  This mucus, in combination with their muscular bodies, allows them to move out of water and across land to avoid barriers.  This, and other adaptations, makes the American eel able to live in more diverse habitats compared to most other fish species.

American eels are fish, despite their snake like appearance, and the only species of eel that live in North America.  They are catadromous, migrating from the saltwater of the Sargasso Sea to the freshwater of streams and lakes.  The Sargasso Sea spans a part of the Atlantic Ocean between Bermuda and Puerto Rico.  Once they reach maturity, they journey back there to spawn.

The vastness of the Sargasso Sea makes it tough for researchers to locate and observe eels spawning in the wild.  At this point, observations of spawning eels remain to be made, although one silver eel was tracked to the Sargasso Sea. Researchers believe the eels die right after spawning.  Some mystery surrounds the final life stages of the American eel.

What happens as they grow?

Let us review the known information about the life stages of the eel.  The eel’s life begins in the Sargasso Sea.  First, they resemble a willow leaf.   These small, oblong, transparent fish, called leptocephali, lack the snake like form of adult eels.  They are about one inch long and rely on the ocean currents to bring them to the east coast.  This journey takes about one year.

Lept
The beginning life stage of the American Eel is called a leptocephalus and these leptocephali use the current to travel to the East Coast. Kils at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons
Now they resemble vermicelli or rice noodles.  At two inches long and still transparent, they are called glass eels.  They make their way into estuaries which connect saltwater to freshwater.  Many of them find themselves in water bodies of local New York State parks along the Hudson River.  Once in freshwater, they develop a brown coloration.  This signifies the shift to their next life stage as elvers.

Glass
The American eel in their glass eel life stage as they arrive to the coast. Chris Bowser

As the elvers grow longer over the next few years, they enter their yellow eel stage.  They live in this stage right before they reach full maturity.  Their size varies based on sex.  Males can grow to two feet long whereas females can reach sizes of four feet.  Their size in each life stage is based on their surrounding environment.  They become silver eels when they reach full maturity to start their migration.

This silver eel stage happens to be the most understudied of all the life stages.  There is no set age that eels are known to reach full maturity and age cannot be determined from external characteristics. Researchers look to study silver eels right before they begin their migration.

What kind of research?

Sarah’s motivation to study silver eels stemmed from her previous experiences working with them in their other life stages.  Her work with eels started with a summer project at Bard College, eight years ago.  After graduation she continued to work with glass eels, elvers, and yellow eels as a Student Conservation Association (SCA) intern at the Hudson River National Estuarine Research Reserve and Estuary Program.  Studying silver eels seemed like the next logical and exciting step for her. Sarah Mount at the SUNY College of Environmental Science and Forestry conducted research on yellow and silver eels.  Her research led to a model that sorts yellow and silver eels into different maturity classes.  The model relies on external characteristics such as the length, weight, eye diameter, pectoral fin length, head length, head width, and body depth of the eels to differentiate maturity classes. This means that future researchers can utilize this model to study the relative age of eels with a capture and release method that does not harm the fish.

With the guidance of Karin Limburg at SUNY College of Environmental Science and Forestry, she developed her research ideas into a master’s level study.  With the help of colleagues at the Hudson River Research Reserve, she spent two summers and two autumns collecting yellow and silver eels from the streams of the Hudson River estuary.

Interns
SCA interns at the Hudson River Research Reserve help Sarah set up a silver eel fyke net. Chris Bowser

Silver eels migrate at night during rain events in the autumn.  To catch them, Sarah set up a fyke net the day before a predicted rain storm.  This v-shaped net spanned the width of the stream and was removed the next morning.

The final life stages of the American eel still remain a mystery.  Sarah Mount’s research begins to solve it both for future research and for herself.  Her model will help future researchers understand when eels reach their full maturity to begin their migration.  When asked about her next steps she said, “Now the only missing piece left is the ocean, I’ve got to get out to the Sargasso Sea sometime.”

Post by Brianna Rosamilia,  Master of Science candidate in Environmental Interpretation at SUNY College of Environmental Science and Forestry

 

2 thoughts on “From glass eels to silver eels and everything in between The life stages of the American Eel”

    1. Hi Jinky – both male and female eels head out to the Sargasso Sea once they reach adulthood. Here the adult eels spawn (eggs and sperm released or deposited into water) and are thought to die after spawning. Scientist still do not know what happens to the adult eels after they spawn.
      And thanks for reading Nature Times!

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