Dams & Crayfish Genetics

In Alabama, crayfish isolated by dams change genetically

a dark green crayfish is held in someone's hands
Faxonius erichsonianus, one of the crayfish species collected in this study, in Cedar Creek, Franklin County, Alabama. Photo by Zanethia Barnett, USFS.

In Alabama, crayfishes are being separated and genetically changed, which increases the risk of local extinction. This work is not done by a mad scientist, but by dams with their reservoirs and unnatural pools of water.

A novel study published in the journal Freshwater Biology by USDA Forest Service scientists Zanethia Barnett and Susan Adams, along with colleagues from the University of Mississippi, covers this phenomenon.

“Crayfishes play an important role in these streams,” says Barnett. “They eat detritus, serve as prey for numerous species, create burrows, make nutrients available by moving soil that wouldn’t be seen or used by other species.” It is these integral crayfishes that are being separated and genetically changed.

The researchers collected two different crayfishes, Faxonius erichsonianus and F. validus, from five streams and forty-one sites in the Southern Appalachian region of Alabama. More than 320 specimens were caught, and their DNA was extracted and sequenced.

The scientists used these data to test the hypothesis that crayfish of the same species, separated by dams, were more likely to have significant genetic variation than crayfishes that were not separated.

a small stream disappears as it travels under the berm of an earthen dam
Little Bear Creek Dam Spillway in Franklin County, Alabama. This structure is a dam on one of the streams sampled by the team led by Barnett. It serves as an example of an anthropogenic barrier that can isolate species of crayfish. Photo by Zanethia Barnett, USFS.

Their genetic results provide evidence that dams genetically fragmented populations of these two species – due to the restriction of movement and inability for genetic exchange between species above and below the dams. The analysis identified high genetic connectivity and significant mobility of individuals in streams with no barrier.

The scientists detected significant genetic differences between individuals of the same species that had been separated by a dam for 36, 40, and 104 years. The magnitude of these differences depended on the size and age of the dam and reservoir. These findings suggest that greater dam size and time since construction can result in more significant genetic differences in crayfishes.

Isn’t greater genetic diversity supposed to be beneficial? Yes, but not when the population is abruptly isolated. “Because of the dam, the crayfishes’ don’t have access to the entire stream gene pool, reducing the genes available to them when mating and producing offspring,” says Barnett.

Crayfishes in dammed streams were more genetically diverse compared to their relatives in streams without a dam. In streams with a dam, however, upstream and downstream groups are more prone to extinction because of their limited mating potential.

two people stand in a river
Electrofishing to collect crayfish in Little Bear Creek in Franklin County, Alabama. Pictured are J. Gordon McWhirter and Mickey Bland, SRS technicians. Photo courtesy of Earl Choice, Hampton School District 2, South Carolina.

As with natural selection, some species are better suited to deal with change than others. Faxonius erichsonianus is better able to handle dam isolation than F. validus, which could be attributed to the former species being a more adaptive crayfish by having a broad geographic range, higher tolerance of impounded habitats, and a comparatively larger population size.

Not all hope is lost regarding genetic effects from the damming of a river or stream. There are numerous conservation strategies to combat dam-instigated isolation of species. These strategies include relocating individuals in a population, implementing ‘ladders’ for organisms to cross a dam boundary, or removing the dam.

Learn more about genetic variability of crayfishes from dams.

For more information, email Zanethia Barnett at zanethia.c.barnett@usda.gov.

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