Bullfrogs are considered the world’s leading invasive amphibian. The American bullfrog (Aquarana catesbeiana) is native to North America and was first brought to Rio de Janeiro in Brazil in 1935 to produce meat. Now, it is farmed throughout the South and Southeast of Brazil, but has also spread into the wild, negatively impacting local ecosystems by competing with native species for food and other resources, feeding on other frogs, as well as snakes, birds, and even some mammals, and spreading fungal and viral diseases against which local species have little or no immunity.
Recently, a team of researchers from the University of São Paulo (USP) and the State University of Campinas (UNICAMP) has conducted the most comprehensive genetic analysis of the American bullfrog, and found that there are two populations of this species in Brazil, living on frog farms or invading local ecosystems.
“We confirmed the existence of at least two different populations of bullfrogs. One probably descends from the first bullfrogs introduced into Brazil. This population is present in practically all the South and Southeast. The other is basically confined to the state of Minas Gerais, but occurs in small numbers in other states,” said study lead author Gabriel Jorgewich-Cohen, a former master student in Biology at the USP, and currently a doctoral fellow at the University of Zurich.
“Our results show that captive and invasive bullfrogs are genetically indistinguishable, reinforcing the importance of preventing escape from frog farms,” added study senior author Taran Grant, a professor of Ecology and Evolutionary Biology at the same university.
By analyzing specific genes in 324 tissue samples from 38 sites where captive and feral bullfrogs are found, the scientists discovered that the vast majority belonged to the same population descending from the animals brought from North America in 1935. The other population descended from a batch of bullfrogs brought in the 1970s to Minas Gerais.
“The results of the genetic analyses match these two more well-documented introductions, although there’s anecdotal evidence of others in the 1980s and 2000s, and isolated initiatives by some producers. If there were other introductions, the animals concerned could have had the same origin or may have interbred and merged with the existing population. Alternatively, we simply didn’t collect samples from these individuals,” Jorgewich-Cohen explained.
Since these bullfrogs can transmit pathogens, such as amphibian chytrid fungi or ranoviruses – which often end up decimating local, immunologically naïve populations – it is highly important to stop farm bullfrogs escaping from farms and to enforce high sanitary standards by both producers and consumers.
The study is published in the journal Nature Scientific Reports.