Watch out for bullfrogs

It is the major invasive amphibian species in the world.

Watch out for bullfrogs. Scientists from the University of São Paulo (USP) and the University of Campinas (Unicamp), in Brazil, performed the most extensive genetic analysis of American bullfrog populations (Aquarana catesbeiana) existing in the country. And the conclusion shows that there are two populations of this species in it, both present in frogs or amphifarms and invading native ecosystems. The bullfrog is considered the world’s major invasive amphibian species. This study, which was supported by FAPESP, was published in the journal Scientific reports.

“We confirm the existence of at least two distinct populations. One of them probably descended from the first animals introduced to Brazil. This population is spread practically throughout the southern and southeastern part of the country. The other is practically limited to the state of Minas Gerais, but exists in smaller amounts in other states,” commented Gabriel Jorgevich-Cohen, first author of the work, which was done as part of his master’s degree at USP’s Institute of Biosciences (IB). scholarship of the National Council for Scientific and Technological Development (CNPq).

A nightmare

Introduced to the country for the first time in 1935 in Rio de Janeiro for meat production, this native North American species began to be cultivated practically in the south and southeast of the country. But it also spreads in nature, generating impacts on local ecosystems, mainly through diseases against which native species have no protection.

“Our results show that invasive frogs and captive frogs are genetically indistinguishable, which reinforces the importance of preventing frog escapes,” says Taran Grant, a FAPESP-supported professor at IB-USP who coordinated the study. Watch out for bullfrogs.

If there was greater genetic diversity in populations, it would be possible to know with greater precision the origins of each animal. Thus, theoretically, analysis of wild-caught bullfrogs could indicate a region or even a frog farm from which it or a close ancestor may have escaped, facilitating inspections. However, among the introduced populations of this species and already studied in other countries, the Brazilian one is the one with the least diversity.

State policy

The researchers analyzed specific genes from animals captured at 38 sites in seven of the nine states where bullfrogs live in the wild. 324 samples were analyzed, both from these “wild” specimens and from those present in anfigranjas. The conclusion shows that the vast majority are part of the same population, descendants of the first batch from North America that arrived in Rio de Janeiro in 1935 and then spread throughout the country, encouraged by state policies.

According to the analyses, the other population originated from a batch of animals that arrived in Brazil in the 1970s, in the state of Minas Gerais, as a result of a later public policy of that state, which imported the matrices, possibly from the United States. In addition to living in the eastern part of this country and in southern Canada, the species is also native to northern Mexico.


“The results of the genetic analyzes are consistent with these two best-documented introductions, although reference is made to others in the 1980s and 2000s, apart from isolated initiatives by some producers. If there were other introgression events, there are three hypotheses along these lines: they were animals of the same origin as those that were here, there was admixture to the extent that pre-existing populations merged, or we simply did not sample from these specimens,” explains Jorgevich-Cohen, who is currently pursuing his PhD at the University of Zurich, Switzerland.

In Brazil, frog farming peaked in the 1980s with about 2,000 operating amphifarms. Due to a number of factors, including a lack of private investment and public incentives, over the next decade this activity declined, with many properties abandoned and the animals dispersed into the wild.

“The species breeds easily, lays a lot of eggs and grows quite quickly, reaching 15 centimeters. Likewise, it is quite resistant to disease and can coexist with infections caused by fungi or viruses that are responsible for the global decline of amphibians, without necessarily hindering their development,” reports Luis Felipe Toledo, professor at the Institute of biology ( IB ) from Unicamp, supported by FAPESP and one of the co-authors of the study.

American invasion

These characteristics, desirable for any species intended for breeding, become numerous ecological problems when the animals in question invade natural areas. In the case of the bullfrog, impacts include competition for resources – food, for example – with native species. The North American species is also a voracious predator: it can eat not only other frogs, but also snakes, birds and even small mammals.

With their deep croaking, bullfrogs also interfere with the reproduction of native amphibians. “These changes can have important effects on reproduction, as most anuran species [sapos y ranas] they rely on acoustic communication to find, evaluate and select their mates,” says Grant. Watch out for bullfrogs.

But the most serious environmental problem, or at least the most documented to date, is disease transmission. “When it spread through the Atlantic Forest, from Rio de Janeiro to Rio Grande do Sul [en la frontera sur de Brasil], the bullfrog has caused various impacts on local fauna. However, the main one is due to the fact that it carries the chytrid fungus (Batrachochytrium dendrobatidis) and ranavirustwo pathogens against which native amphibians do not have the resistance that this species possesses and which have even caused species extinction,” commented Toledo.

chytrid fungus

The chytrid fungus causes chytridomycosis by establishing itself in the skin of amphibians and interfering with the gas exchange it carries out; and can cause cardiac arrest followed by death. This pathogen has decimated the populations of at least 501 amphibian species worldwide.

The ranavirus It has also been linked to population declines in these animals and has already been found in the Atlantic Forest. According to current Brazilian legislation, if the chytrid fungus or ranavirus in a hatchery, it is mandatory to notify the Ministry of Agriculture, Livestock and Supply (MAPA, in Portuguese) and carry out the so-called sanitary vacuum: all animals must be slaughtered and the site disinfected before resuming breeding. However, this does not happen.


“Virtually all the frogs we visited were found to have a chytrid fungus. There is an intensive transit of animals in Brazil, with producers exchanging specimens with each other with the false idea that this will increase the genetic diversity of the stock,” says the researcher, who works together with MAPA and state secretariats to improve legislation and try to control the species in the country.

What this study shows is that the practice of exchanging animals between amphifarms has only consolidated the same populations in the country, maintained with low genetic diversity. Which does not necessarily hinder economic activity. Brazil currently produces 400 tons of bullfrog meat per year. The production is exclusively for delivery to the domestic market.

“The interest in the prevention of diseases caused by the chytrid fungus and ranavirus it’s still very much in its infancy as many of the manufacturers don’t even manage to sell what they produce. Inspections need to be greatly improved. An alternative way out could be the development of the sector. For the large refrigerators interested in the product, the sanitary requirements will necessarily be greater, both for them and for the consumers,” concludes Toledo. Watch out for bullfrogs.

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