Eels are thought to have originated in a small sea hundreds of millions of years ago, before the continents drifted apart.

At that time, it’s speculated the eels could not find enough food in their spawning areas, and began making their journeys to freshwater streams.

Today, it just takes them longer to make the trip, which starts in the Sargasso Sea, an area of relatively still water covered by seaweed that floats over its surface.

Adult eels spawn in the winter and early spring in the Sargasso. After spawning, the adults die. The eggs hatch into larvae, which are transported by ocean currents, mostly in a northwesterly direction, although some end up in rivers in South and Central America, and the Gulf Coast.

During the trip, the larvae develop into transparent “glass” eels, which are just a few inches long, and so clear it is possible to read type through their bodies. Reaching coastal waters can take anywhere from a few months (in the case of the Mid-Atlantic) to two or three years for Canada and other far-flung parts of its range.

As the glass eels begin moving up the estuaries, they become pigmented with gray or brown tint as they reach a size of about 6 inches, and are known as “elvers.” Elvers become yellow eels—because of their yellow-green tint. They spend most of their lives as yellow eels, and early in that stage, eels—in an unusual trait—select their sex. Where populations are dense, males predominate, but where numbers are less dense, the population is overwhelmingly female.

Females tend to continue their migrations up freshwater rivers in the spring, while males may remain in brackish coastal waters for almost all of their lives.

When eels mature, which generally takes eight years or less in warm climates, but 25 years or more in colder areas, they turn a bronze black sheen, and are known as silver eels.

Their skin thickens, they store body fat and absorb their own digestive systems to make the long swim back to the Sargasso, a journey that begins in the autumn.