Author: Victoria Swyers
Man-made reservoirs throughout the Southeast have become infested with the invasive aquatic plant, hydrilla, (Hydrilla verticillata), which hosts a new species of toxic cyanobacteria. Waterfowl and birds of prey, most notably the American coot (Fulica americana), and bald eagle, (Haliaeetus leucocephalus), can consume the neurotoxin and die—within this unsuitable toxic habitat.
Dr. Susan Wilde, associate professor of aquatic science in the Warnell School of Forestry, and multidisciplinary collaborators recently characterized a new toxin that is made by the novel cyanobacteria that she also described. Wilde got to name the cyanobacteria. She chose Aetokthonos hydrillicola, which is Greek and Latin for “eagle killer, living on hydrilla.”
Aetokthonos hydrillicola afflicts waterfowl and its predators with the fatal disease Avian Vacuolar Myelinopathy (AVM), which affects them neurologically.
“When coots are sick with AVM, they just kind of stumble around. Eagles have these droopy wings,” Wilde said. “They can’t hold themselves up right.”
Eventually, the neurotoxin causes the birds to become unresponsive, and results in lesions in the myelin sheath, or white matter, of the brain.
Hydrilla is an invasive aquatic plant that is rapidly infiltrating man-made reservoirs. The most common way invasive aquatic plants are introduced to new environments is via boats, which may have pieces of plant matter stuck to their propellers or sides. Once introduced to the waterway, the invasive plant expands through branching rhizomatous growth, both vertically and horizontally through the water table.
However, the plant isn’t the main culprit in harming wildlife. “The leaves [of Hydrilla] are coated with this novel species of cyanobacteria,” Wilde said.
According to Wilde, the disease is difficult to document because dead birds are often scavenged. Tracking the extent of the cyanotoxin within aquatic food webs will be the next step for researchers.
“We have been sampling sites that we know are new infestations of hydrilla and are writing a proposal to do even more samples, but so far we’ve documented this species of cyanobacteria growing on hydrilla as far north as Virginia, as far west as Texas and down into Florida,” Wilde said.
Wilde wants to use a new grant to sample untested watersheds. The only way to confirm whether a bird has AVM is by examining the brain lesions, which requires dissection before the brain tissues break down.
Other species with documented cases of (A)VM include other waterfowl, grass carp, tadpoles, herbivorous turtles and snails.
“[Infected] tadpoles start to swim in circles, or they’re non-responsive to probing, or they’ll spend all their time in the corner,” Wilde said.
Each becomes neurologically impaired after exposure.
“I think this is important for a lot of different aquatic taxa; it could be causing some sub-lethal effects. While not all may be killed outright, they are another source of stress for aquatic organisms that are living in these man-made reservoirs with invasive plants,” Wilde said.
Wilde also hopes to bring more attention to the issue, so that when people see sick birds, they know who to call.
Wilde and her associate researchers began testing the effects of Aetokthonos hydrillicola on macroinvertebrates and larval zebrafish to avoid having to always test vertebrates like— chickens and mallards. Meanwhile, State and federal biologists from Arkansas, North Carolina, South Carolina, Virginia, Texas, Florida, and Georgia have collaborated in determining the extent of the spread of cyanobacterium.
Wilde wants to learn how we can manage hydrilla in the sites where it does occur, in the hopes that efficient management will prevent further wildlife death.
“[My goal is to be able to] do something about this within the environment,” Wilde said.