Researchers from the University of São Paulo (USP) and the State University of Campinas (UNICAMP) have conducted the most comprehensive genetic analysis ever performed on the American bullfrog (Aquarana catesbeiana) in Brazil, concluding that there are two populations of the species here, living in frog farms or invading local ecosystems.
Bullfrogs are considered the world’s leading invasive amphibian. An article on the study is published in Scientific reports.
“We have confirmed the existence of at least two different populations of bullfrogs. One is probably descended from the first bullfrogs introduced to Brazil. This population is present in practically all of the South and Southeast. The other is mostly confined to the state of Minas Gerais, but occurs in small numbers in other states,” said Gabriel Jorgewich-Cohen, first author of the paper.
The study was part of his master’s research at the Institute of Biosciences (IB-USP) with a scholarship from the Brazilian National Council for Scientific and Technological Development (CNPq). A. catesbeiana is native to North America and was brought to Rio de Janeiro in 1935 to produce meat. It is now cultivated throughout the south and southeast and has also spread in the wild, with adverse effects on local ecosystems, such as diseases against which Brazil’s native species have no defense . “Our results show that captive and invasive bullfrogs are genetically indistinguishable, reinforcing the importance of preventing escape from frog farms,” said Taran Grant, FAPESP-supported IB-USP professor and lead researcher. of the study. If the populations were genetically different, it would be possible to know the origin of each animal. In theory, analysis of a captured bullfrog could point to an area or frog farm from which it or a close relative escaped, so that surveillance and law enforcement would be possible. Compared to introduced populations of the species studied in other countries, however, the Brazilian groups have the least diversity.
The researchers analyzed specific genes in 324 tissue samples. The specimens came from 38 sites in seven of the nine Brazilian states where captive and wild bullfrogs are found. They concluded that the vast majority belonged to the same population, which descends from animals first brought from North America to Rio de Janeiro in 1935, after which bullfrogs spread across the country in response to the enticements offered. under state policy. The other population descends from a batch of animals brought in the 1970s to Minas Gerais as part of a public policy later implemented in the state. These included breeding pairs likely imported from the United States. The species is native to the eastern United States, as well as northern Mexico and southern Canada. “The results of genetic analyzes are consistent with these two more documented introductions, although there is anecdotal evidence from others in the 1980s and 2000s, and isolated initiatives by some growers. If there were other introductions, the animals involved could have had the same origin or interbred and merged with the existing population. Alternatively, we just didn’t take samples from those people,” said Jorgewich-Cohen, currently a PhD student at the University of Zurich in Switzerland.
Bullfrog farming reached its peak in the 1980s in Brazil. Some 2,000 farms were producing at that time. Activity declined over the following decades due to a number of factors, such as lack of private investment and public incentives. Many farms were abandoned and animals escaped into the wild. “The species reproduces easily, lays many eggs and grows rapidly until individuals reach 15 cm. In addition, it is very resistant to diseases and can coexist with fungi and viruses that have led to an overall decline in the populations of other amphibians, without necessarily seeing its development altered,” said Luís Felipe Toledo, the other co- author of the article. . Toledo is a professor at the Biology Institute of the State University of Campinas (IB-UNICAMP) and is supported by FAPESP.
American invasion
These characteristics are desirable in any breeding species but become a major environmental problem when the animals in question invade wild areas. In the case of A. catesbeiana, adverse effects include competition with native species for food and other resources. The North American species is also a voracious predator, feeding on other frogs as well as snakes, birds, and even mammals, and its loud croaking interferes with the reproduction of native amphibians. “These alterations can have a significant impact on reproduction since most anurans [frogs and toads] rely on acoustic communication to locate, assess and choose mates,” Grant said. The most serious environmental problem, or at least the best documented to date, is disease transmission. “Having spread through the Atlantic Rainforest biome from Rio de Janeiro to Rio Grande do Sul [Brazil’s southernmost state], bullfrogs affect native wildlife in a variety of ways. The main problem is that they are carriers of the amphibian chytrid fungus [Batrachochytrium dendrobatidis] and ranaviruses. Native amphibians lack resistance to these two pathogens, which have even led to species extinction,” Toledo said.
The chytrid fungus causes chytridiomycosis, an infectious disease that penetrates the skin of adult amphibians, which become unable to breathe and die of cardiac arrest. It decimated the populations of at least 501 amphibian species worldwide. Ranavirus is also associated with declining populations of these animals and has been detected in the Atlantic Rainforest.
Brazilian law requires anyone detecting chytrid fungus or ranavirus in farm animals to notify the Ministry of Agriculture, Livestock and Food Supply (MAPA) and perform a “stamping down” (or depopulation), destroying all animals and disinfecting the facility before starting a new breeding cycle, but that’s not what happens. “We detected chytrid fungi in almost all the frog farms we visited. There is a lot of movement of bullfrogs across the country. Producers trade animals based on the misconception that it increases their genetic diversity,” said Toledo, who works with MAPA and state departments of agriculture to try to improve legislation and control the breeding and marketing of bullfrogs. The study shows that frog farms have simply consolidated the same populations with low genetic diversity by exchanging animals. The practice does not necessarily have economic or financial disadvantages. Brazil currently produces 400 metric tons of frog meat per year, all of which is sold domestically.
“Interest in preventing diseases caused by chytrid fungus and ranavirus is very nascent. Many producers cannot sell all the meat they produce. Monitoring and inspection needs to be greatly improved. An alternative strategy would be to grow the industry if big meat packers were interested in the product, in which case high sanitary standards should be applied by both producers and consumers,” Toledo said.
Reference: Jorgewich-Cohen G, Toledo LF, Grant T. Genetic structure of American bullfrog populations in Brazil. Scientific representatives. 2022;12(1):9927. doi: 10.1038/s41598-022-13870-2.