Stroud™ Scientists at Work
March of the Ancient Vertebrates: Eels and Lampreys in White Clay Creek . . . and the Technologies to Understand Them
Lampreys are not eels. That fact needs to be made clear at the outset because most people think they are — including this writer before he was set straight in a recent interview by William H. Eldridge, Ph.D., head of the Stroud™ Water Research Center’s Fish Molecular Ecology Group.
Indeed, eels and lampreys represent entirely separate families of the animal kingdom with vastly different evolutionary and life histories. Lampreys are the oldest vertebrates on Earth and have existed essentially unchanged for more than 400 million years. Eels have been around only for the past 60 million years or so. There are both freshwater and saltwater species of lamprey: brook lampreys live entirely in fresh water, while sea lampreys are born in streams, and most (but not all) of them subsequently make their way to the ocean. All Atlantic eels spawn in the Sargasso Sea, from which they disperse to streams and rivers in Europe and the Americas where they spend most of their lives.
One thing eels and lampreys do have in common, however, is that they live in large numbers in White Clay Creek. Both the American brook lamprey and the sea lamprey can be found there, as can the only species of eel that exists on the East Coast. Scientists have long known that brook lampreys live in the White Clay. But no one knew that sea lampreys (which are virtually indistinguishable from brook lampreys for most of their lives) were also there — until a couple of years ago when Willy Eldridge established their presence with the help of a pioneering technology. A second technological innovation is enabling him to expand his understanding of the roles that both eels and lampreys play in the ecosystem of the stream and its watershed. The two new technologies, which are behind these and other breakthroughs in scientific knowledge, have deceptively simple names: Barcodes and EZ Pass.
“Eels are the coolest fish in White Clay Creek,” said Eldridge early in our conversation, and soon he was talking animatedly about this “mysterious and economically important fish” that lives in streams and rivers from Canada to Central America and travels thousands of miles to the Sargasso Sea to spawn. The larvae then ride the Gulf Stream west for a year or more, undergoing at least two metamorphoses along the way. Most of the young eels ultimately peel off into streams up and down the coast — where they live entirely in fresh water until, several years later, they return to their birthplace to give birth and die.
Eels are an important source of both aquatic diversity and human food. Yet, despite people’s fascination with their origins in the Sargasso Sea, they have received surprisingly sparse attention from the scientific world. Consequently, little is known about their numbers and activities or the roles they play in a stream’s ecosystem. (In fact, there was much historical debate about whether eels were even fish until Carol Mondini, an Italian scientist, found their gonads in 1777. Ninety-nine years later, in an effort to understand their life cycle, a young Austrian medical student spent several weeks dissecting hundreds of eels in a vain search for their male sex organs. In the first published paper of his career, Sigmund Freud conceded that he had been unable to find them and decided to focus on other things.)
Because so much about eels remains unknown, there are few effective conservation and management policies in place to protect them. In recent years, in response to an observed decline in eel numbers in the Delaware River watershed, a movement arose to have them listed under the Endangered Species Act. But eels are panmictic — that is, they constitute a single population across a vast geographic range, mate indiscriminately, and disperse randomly from the Sargasso Sea. Consequently, their movements transcend the boundaries of political planning units, and scientists have had a hard time understanding their distribution, behavior, and activities with much precision. As a result, the petition failed.
To monitor the activities of eels in White Clay Creek, Eldridge has issued them EZ passes — small glass-encased capsules that he inserts into their abdomens — which give each fish a unique code that can be read by antennae that will be located at nine sites along the creek. This allows researchers to follow the movements of eels up- and downstream, as each antenna records their position in real time, much as the EZ pass system tracks individual cars on the Pennsylvania Turnpike.
The EZ Pass — or Passive Integrated Transponder — allows Eldridge to study the activities of individual fish, something that had been impossible when he could only track populations in the aggregate. As a result, he will be able to pinpoint life histories and to analyze the consequences of short- and long-term movements in the stream. For the first time, researchers will know what individual fish are doing in particular places over recorded time, knowledge that will enable them to more fully understand the dynamics of the life cycles of eels and their responses to environmental stresses and human impacts.
How Eldridge discovered that sea lampreys were in the stream at all is the result of a second technological innovation, this one a revolution in molecular genetics. The process, called barcoding, enables researchers to determine an organism’s species from a microscopic sample of its DNA that carries a unique sequence of genes — much the way each product in a grocery store has its own unduplicated barcode. With the help of this new technology, Eldridge has to date identified 24 species of fish in White Clay Creek, including eels and both species of lamprey.
One of the questions he is asking is how the fish get up the creek in the first place, since seven dams lie between the source of the White Clay and the Delaware estuary. Sea lampreys are anadromous — like salmon, they reproduce in fresh water, and most of their progeny ultimately make their way downstream to the sea. Eels are catadromous — after spawning in the Sargasso Sea, they spend most of their lives in fresh water. The dams present no small obstacle both to fish trying to get up the White Clay and those trying to get down, and Eldridge is trying to determine how they do so.
Here’s what he does know: eels can walk on land by absorbing oxygen from the air for short periods of time, while lampreys latch onto rocks with their suction-cup mouths. In fact, the etymological root of the word lamprey is “stone licker,” a phrase that undoubtedly adds little to the fish’s human appeal. Imagine, then, an improbable parade of lampreys executing gymnastic mouth flips from rock to rock, while eels are actually getting out of the water and walking around the dams. Sometimes the natural world can rival any circus.
Mostly it’s a nocturnal march, for eels travel only at night — a characteristic that serendipitously helped researchers answer a completely unrelated question from another major research project. In their work on the Critical Zone Observatory, the multiyear collaboration with the University of Delaware focused on climate change, Center scientists noted nightly spikes in stream turbidity that they could not explain. It turned out to be caused by eels, who were churning the water every night as they searched the stream bottoms for food.
Unlike with eels, scientists have showered attention on sea lampreys, primarily with the aim of exterminating them in the Great Lakes, where they have been mauling other fish for almost a century. In their adult stage, sea lampreys are large and aggressive parasites who use their mouths filled with concentric rings of teeth — and a tongue that is well supplied with teeth of its own — to latch onto other fish and suck out their blood and body fluids. But brook lampreys are not parasites at all, and sea lampreys are not the vicious predators in stream habitats that they are in the ocean. In fact, both brook and sea lampreys spend most of their lives as immature ammocoetes, feeding on organic matter they filter through their mouths. It is only near the end of their lives that adult sea lampreys swim to the ocean, and it is there, and in large harbors and the Great Lakes, that they have earned their reputation as “nature’s vampires.”
There remains much to learn about eels and lampreys in White Clay Creek. The eel population, long a source of a thriving commercial food industry along the Delaware River and an important part of the White Clay ecosystem, continues to decline. While eels have clearly been overfished, both for bait and to provide food for Asian markets, scientists still don’t fully comprehend the impact on the eel and lamprey populations of pollution, sedimentation, or sudden changes in temperature or water levels. Equipped with barcodes and EZ pass technology, Willy Eldridge is determined to change that.