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Genetic Profiling of Northern Snakehead Fish May Help Scientists Track Their Spread in U.S. Waters
By Rita Zeidner
Special to Inside Smithsonian Research
Most people have difficulty identifying different species of fish laid side-by-side in the fishmarket's display case, let alone distinguishing different siblings of the same species.
| But Thomas Orrell, a research associate in the Division of Fishes at the Smithsonian's National Museum of Natural History, is using DNA technology to understand the population structure of the northern snakehead, Channa argus, that has been introduced into several waterways in the United States—most notably the Potomac River in Virginia and Maryland near Washington, D.C. |  Thomas Orrell holds a northern snakehead specimen at the National Museum of Natural History. (Photo by John Steiner) |
His mission: to determine whether individual northern snakehead fish caught at various points in and around the Potomac are genetically related and if there is evidence that they are breeding in the Potomac. Orrell is also using snakeheads caught in Massachusetts and Pennsylvania to examine the species' genetic diversity.
Orrell has coordinated with local state and federal fisheries agencies to have the fish delivered to him frozen—the best way to preserve the genetic material. Eventually, the specimens will be become part of the fish collection—the world's largest—at the Natural History Museum. With 3.5 million specimens, it is a center of collections-based taxonomic research on fishes.
Concern
The northern snakehead, native to Northern China and Korea, has the potential to expand its range throughout most of the United States and into Canada, Orrell says. Proliferation of the snakehead in the United States is cause for alarm for several reasons. One is that these voracious eaters compete with native species for food.
As juveniles, they seek zooplankton, insect larvae, small crustaceans and the fry, or young, of other fishes. As adults (they can grow to nearly 5 feet and weigh as much as 13.5 pounds), they feed on other fishes, larger crustaceans, frogs, small reptiles and, sometimes, birds and mammals.
Other threats are their ability to live out of water for up to three days—the snakehead can breath oxygen in the air—and their tendency to “walk� over land by wriggling. Both abilities allow them to disperse widely.
“The northern snakehead presents a formidable environmental threat that has the potential to upset the natural balance of freshwater ecosystems in the United States,� Orrell says.
Baseline genetic data
Determining whether individual snakeheads found in various rivers, streams and lakes are genetically related, Orrell says, may provide clues as to how this species was first introduced into the wild in the United States and if it is breeding in the Potomac River.
One juvenile snakehead was found in a Potomac tributary near Alexandria, Va., in September, proof that the fish are breeding, some say. Genetic data can provide additional evidence, even if no fertilized snakehead eggs or other juvenile fish are found.
Orrell is using mitochondrial DNA—the same genetic material used by anthropologists to track ancestral links in humans—to see if wild-caught snakeheads are related. Mitochondrial DNA is considered a valuable tool in genetics research as it is passed on only from the mother to all her offspring. This means that all of the DNA in the mitochondria of a particular fish is a copy of its mother's.
“It is also important to have baseline genetic data so, if the fish starts spreading, we can track it better,� he says.
DNA sequences
“Mitochondrial DNA is particularly useful since some regions of the molecule are less likely than others to change from generation to generation,� Orrell explains.
Orrell is studying DNA extracted from tiny pieces of tissue collected from 29 snakehead fish caught in Maryland, Virginia, Pennsylvania and Massachusetts in the last two years. He is comparing some 1,200 of the more than 16,500 base pairs that, in a unique sequence, make up the snakehead's DNA. Base pairs are the molecules that form DNA strands.
After isolating DNA from each fish, staff at the Genomics Core Facility of the Smithsonian's Laboratories of Analytic Biology helped Orrell make millions of copies of each gene. This allowed him to determine the sequence of nucleotides, the actual genetic code of the DNA, from each sample.
“Once the genetic sequence for each fish is determined, we can compare the sequences of individual fish,� he says. “If a mother has a particular sequence, then all offspring should share that sequence.�
Fish with the same genetic sequence are considered closely related. This information will be useful in determining where the fish was first introduced in the Northeast and if its population is growing.
“If the samples from a particular area are determined to have the same genetic sequence and there is little variation within a particular water system, then this is potential evidence that the fish are breeding,� Orrell explains. A second possibility for this scenario is that someone released a large quantity of sibling snakehead fish in the same area.
Two pathways
Experts agree that snakeheads entered the United States by either one or both of two pathways: release by aquarium owners—overwhelmed by the size of the snakehead as a pet—or release by someone who wants to catch snakeheads regularly in a local pond or stream for food.
Orrell's research may soon provide wildlife managers with a better idea of how and where snakeheads are being introduced. Although having the genetic data will not directly help eradicate the northern snakehead if they do become established, it will help managers track established populations in the event that they expand their range.
Northern snakehead fish illustration by Susan Trammell, courtesy of the U.S. Geological Survey
This article originally appeared in Inside Smithsonian Research, Autumn 2004.
