Part IThe thought that life is through with the body once sexual reproduction has been accomplished is repugnant to us as men. I shall have more to say of this later. Yet now I should like to say that, repugnant or not, this would be no surprise to a salmon. For in salmon, and eels, and many such creatures, it is all too clear that reproduction is the last act of life, and that the preparation to reproduce is simultaneously the preparation to die. I should like to speak of one such animal, the lamprey.
So-called sea lampreys are probably not familiar to some of you, but they are quite familiar to us along the coasts. That's because of their life cycle. Lampreys have the general shape of eels, and are frequently called lamprey eels, but they are not eels nor are they even fish. They belong to a small group of the most primitive of living vertebrates, the jawless vertebrates or Agnatha.
They have no jaws, just a sucker disc with a kind of coarse rasp on it. When they get a chance, they attach themselves to a fish by that sucker disc and just begin to rasp their way in. If it is a big enough fish and the fish holds out, the lamprey may end up completely inside of it.
A lot of this has been going on in the Great Lakes, as many of you perhaps know, because the digging of a canal persuaded the lampreys, instead of going down to the sea as they had done heretofore, to go into the Great Lakes. For a while they had almost cleaned out the whole Great Lakes fishery.
The lamprey begins its life as a wormlike larva, with no eyes, buried in the mud or sand of a swift-flowing stream. It stays that way for perhaps two or three years. Then it goes through a first metamorphosis, in the course of which, among other things, it acquires eyes.
With that it gets itself out from the mud and sand and starts migrating downstream, usually to the sea, where it grows up. At sexual maturity it goes through a second metamorphosis.
There are a lot of changes, but one of the most striking is a complete disintegration of the digestive system. That animal will never eat again; it loses its entire apparatus for consuming food. Then it starts its journey upstream.
I got my lampreys in the Exeter River in New Hampshire. A hydroelectric development and a dam had been built across the river. The good people of Exeter had been throwing bottles and tin cans into the water below the dam for generations. There wasn't much water and it was pretty perilous, but there were those lampreys still coming up with the first warm days of spring. How they got themselves over the dam I do not know.
I suspect they took to the shore, because one of these animals on a sexual migration has only one thing on its mind, and that is to get up into its spawning ground. There it makes a nest of round stones, the females lay their eggs in the nest, the males shed their sperm over the eggs, and with that they're through. All the adult lampreys then die; there is nothing more left in life for them.
The freshwater eels have a life cycle that's just the reverse of that of the lamprey. It was discovered by a great Danish oceanographer, Johannes Schmidt, many years ago. It had been a great mystery until then, where the eels reproduce. The eels of the shores of the Atlantic are of two different species, European and American. All of them come together to spawn in overlapping areas of the Sargasso Sea, the region of the South Atlantic that includes Bermuda. It represents the deepest and saltiest part of the Atlantic Ocean.
Having made that enormous journey, the adult eels spawn and die. Then the baby eels make their way back alone. We have no idea how they get back. It takes the American eels about 15 months to come back to our shores, metamorphose, and head upstream. It takes the European eels three years to get back home. There is no record as yet of any baby eel ever getting mixed up and going to the wrong place.
When they get into fresh water, they live there for five to fifteen years, growing up. Then at sexual maturity they go through a second metamorphosis. There are a lot of changes: the eyes blow up to twice their former diameter, four times their former area. This animal is getting ready for a deep sea journey. Among other things, there is a complete collapse of the digestive system. Before beginning this enormous journey that will take the adults to the Sargasso Sea, those animals have had their last meal. They will never eat again.