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Conclusion: the animal migration series
There would be very few of us, I'd venture to say, that would think of "eels" when we ponder animal migration, but the migration and life cycle of the American and European eel is a fascinating episode in animal migration and survival.
There are 18 known species of eel. The American eel (Anguilla rostrata), has the broadest diversity of habitat of any other fish species in the world. It is also called the yellow eel, the silver eel, the green eel, the black eel, the Boston eel, the freshwater eel, the glass eel, the common eel, the snakefish, and the Atlantic eel. These eels are catadromous, meaning they live in fresh water but reproduce in salt water. They begin, and end their lives in the Saragasso Sea.
Much of the life cycle of this fish of the deep is shrouded in mystery. The first known eel research goes back to Aristotle. He stated that eels "...seem to come from the entrails of the Earth." What we do know about this species of fish is that it spends the greater part of its life in freshwater and estuaries, the wide mouth of a river into which the tide flows from the sea), and then migrate back to the sea to breed. Eel spawning grounds were discovered in the early 1900s through extensive and exhaustive trawling surveys of the Atlantic Ocean beginning from Greenland to Puerto Rico, and the English Channel to Chesapeake Bay. After many years of study at sea, Johannes Schmidt determined that the spawning grounds of American and European eels were located in the Sargasso Sea, near the center of the North Atlantic, about midway between Bermuda and Puerto Rico, east. At this writing the actual mating behavior of either the American or European eel has never been observed. For the adult eels, the Sargasso Sea is their final resting place, as they will never return to their freshwater home.
Most of the eels' lives is spent in what is referred to as the "yellow eel" phase. From this phase they reach sexual maturity and begin the transformation to the final stage of life known as the "silver eel" phase. During this phase their physiology undergoes dramatic change in preparation for the long journey back to the spawning grounds in the Sargasso Sea.
American eels are shaped probably close to how most of us would imagine. Their dorsal fins are continuous and their anal and caudal fins form a fringe, lining the posterior end of the body; small pectoral fins are navigational aids that enable them to maneuver on the river bottom. Eels possess two lateral sensory lines that run along each flank of their bodies, and they can move just as easily backward as forward.
The tiny, transparent juveniles, as well as the larger adults, are capable of accomplishing impressive feats of navigation which rivals other fish species like the Atlantic Blue-Fin tuna.
Sources: SeaGrant Maryland the Littoral Society
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