When you hear the word “eel,” what comes to mind?

For many, eels are repulsive, slimy creatures to be avoided. But to fishermen and crabbers, eels are an important commodity, either as bait or exported as a delicacy.

Whatever you may think of them, eels are extraordinary fish, undergoing a variety of growth phases in very different aquatic habitats.

For such lowly creatures, eels were once surrounded by myth and mystery. Tribes in the Philippines once thought that eels were the souls of the dead. Some Europeans believed that skin rubbed with eel oil caused one to see fairies. Because no one had ever seen a female eel with eggs, the Greeks once believed that baby eels were produced by spontaneous generation. This last myth was dispelled in 1922 when a scientist, Johannes Schmidt, discovered the spawning ground for both American and European eels in the Sargasso Sea, near Bermuda.

This discovery led to a greater understanding of the fascinating life cycle of eels. The American eel (Anguilla rostrata) is a catadromous fish, meaning that it lives most of its life in fresh and estuarine water, and migrates into saltwater to spawn. Very few fish match the eel’s ability to exist in such a broad diversity of habitats

Prior to their fall migration, adult eels undergo changes that adapt them for life in the ocean. These changes include an accumulation of fat and the degeneration of the gut. Spawning migration takes place between August and December, with eels being most active during the night. American eels spawn in the western Sargasso Sea between January and March. A female eel may lay between 400,000 and 2,500,000 eggs. Parent eels die after spawning.

The eggs hatch and the young, which look like transparent ribbons, are seized by ocean currents and transported to the mouth of the Chesapeake Bay and other inlets of the Atlantic Ocean. This journey can take months or even years. After their first year, the young change, taking on a tiny, unpigmented form, known as glass eels.

As glass eels move into coastal areas they become pigmented. The body turns dark brown and they are now known as elvers. The elvers continue the journey, wriggling upstream into brackish and freshwater streams, rivers and creeks. Elvers are found in the Chesapeake Bay in April and May.

When elvers stop migrating, they undergo a period of growth and are known as yellow eels. Yellow eels remain in the Bay and its tributaries from eight to 24 years. Eels, while in estuaries and rivers, have small home ranges in which they feed. Yellow eels may migrate into estuaries during the spring, move into rivers in summer and autumn and hibernate in the mud during the winter. They are more active at night, and feed voraciously on a variety of fish, insects, crayfish and worms.

About one year before reaching sexual maturity, the eel undergoes its last transformation before spawning, resulting in the silver eel phase.

Commercially, there are two eel industries; a bait market for crab trotliners and a live eel market for human consumption.

Other factors can affect the eel population. Because of their bottom-feeding behavior, contaminants in bottom sediments and low, dissolved-oxygen levels can negatively impact eels. Like many migratory fish, eels must also overcome dams and other obstructions that block their movement into important habitat.

Addressing threats to the American eel has taken several forms, from research and monitoring to increasing eel access to former habitat and spawning grounds. Conservation organizations have targeted projects removing dams that interrupt historic eel migration routes.

State and federal natural resource agencies are working to mitigate adverse impacts to eels through measures specified during the licensing or relicensing of hydropower projects. These might include eel ladders to safely pass eels upstream or possible stocking.

Even if you are not a fan of the American eel, there is another reason for ensuring healthy eel populations. Recent research has shown that eels are critical for the reproduction of the Eastern elliptio, a common freshwater mussel. The larval stage of freshwater mussels needs to live for a time on a fish “host” before it drops off and begins to grow on its own. Eels are the main host for the Eastern elliptio.

The hope is that restoring eels will boost the mussel population and ultimately improve water quality. Like oysters, referred to as the kidneys of the Chesapeake Bay for their power to filter Bay water, freshwater mussels are the kidneys for our streams and rivers.