Passage IV

NATURAL SCIENCE: This passage is adapted from Consider the Eel by Richard Schweid (©2002 by Richard Schweid).

The known facts, as they are pretty much universally accepted among biologists and naturalists today, are that all the eels in all the rivers of eastern North America and the Caribbean countries, and all the eels in all the rivers of eastern and western Europe, are born in the same area of the Sargasso Sea, a huge area within the Atlantic Ocean, between Bermuda and the Azores, the surface of which is frequently covered with sargassum seaweed. In fact, the word "Sargasso" comes from the Portuguese sargaço, meaning seaweed. The sea is about 2,000 miles long and 1,000 miles wide, set off from the surrounding waters of the Atlantic by strong currents. It includes the area known in popular legend as the Bermuda Triangle.

Eels hatch in the Sargasso as larvae and are carried by the ocean currents to either Europe or the United States, a journey that can cover thousands of miles and take years. Where they end up depends on which of two similar species they belong to. Those that are Anguilla anguilla invariably wind up in European rivers, and those that enter North American rivers always belong to the species Anguilla rostrata. The first person to find eel larvae in the Sargasso Sea was Danish researcher Johannes Schmidt, who published his findings in 1924, after spending 18 years hauling nets in search of eels.

The larvae of both species are shaped like small oval leaves and are called leptocephali. Each leptocephalus begins to assume the form of a tiny eel, called an elver or glass eel, when it gets close to the coasts of either Europe or the Americas. By the time it reaches brackish water, where fresh and salt water mix, it is thin and transparent, hardly bigger than a hair, with a pair of eyes like black dots at one end.

From the estuaries and mouths of rivers, the tiny eels frequently continue upstream, particularly the females, who sometimes go great distances inland. American eels have been found as far up the Mississippi River system as the rivers of Iowa. They keep going upriver until something tells them they've reached home, and then they stop. Whatever it is that signals to eels that they are home is definitive—they settle in and live there for as long as 20 years, growing up to a yard long before beginning their journey back to the Sargasso Sea. Scientists determine an eel's age using a microscope to read the growth rings of its otolith—a small, hard calcium deposit at the base of its skull.

In preparation for the return journey to the Sargasso, sexually mature female eels feed voraciously and change color from the muddy-yellow/green of adult eels, often called yellow eels, to a darker green on top and snow-white on their bellies. At this stage, they are called silver eels. They swim downriver in the fall, on the first leg of their journey to the Sargasso, and when they reach estuarine waters, they rest, completing their final transformation as silver eels. They will have eaten heavily and will be about 28 percent body fat. They will never eat again, and their digestive systems will atrophy. Their pupils will expand and turn blue. They will need a new kind of sight adapted to the depths of the sea, where there is little light. They will also have to go through a drastic adjustment, via osmosis, in their blood chemistry, to prepare for the tremendous change in water pressure, going from some 14 pounds of freshwater pressure per inch of their bodies to over a ton of ocean pressure per inch. Once they are back in the Sargasso Sea, the females produce eggs for the males to fertilize, and then the adults die.

At least that is what today's marine biologists and naturalists tell us, although adult eels have never been seen swimming, reproducing, or dying in the Sargasso. In fact, live adult eels have never been seen there at all. The only two adult eels ever reported in the Sargasso Sea were dead, found in the stomachs of other fish. The eel's migration back to its birthplace and what it actually does when it gets there are assumed to take place far below the water's surface and, as of the year 2001, were still completely unobserved. However, the eel larvae—the leptocephali that Schmidt found in the Sargasso—were so small that it was certain they had been born recently, and nearby. Such small larvae have never been seen elsewhere, and while eels have never been observed reproducing in the Sargasso, they have never been seen doing so anyplace else either. Scientists believe the larvae hatch out of eggs at a depth of 100–300 yards and rise slowly toward the light at the sea's surface.