The smallest vertebrates so far discovered are the flea-toads, which live at the limit for how small any vertebrate can possibly get.
I’ve shared news of several species of extremely tiny frogs and toads, commonly known as flea-toads or toadlets (read more here and here). These tiny amphibians are remarkable because they are much smaller than a human fingernail.
“These are small toads with all the characteristics of large toads except for their size,” said Luís Felipe Toledo, who is the recently published study’s corresponding author and a professor in the Biology Institute of the University of Campinas (IB-UNICAMP).
The most recently discovered flea-toad to be described was given the scientific name, Brachycephalus dacnis, in honor of Project Dacnis, a conservation, research and education NGO that maintains private areas of the Atlantic Rainforest, including the one in Ubatuba, on the coast of Brazil’s São Paulo state, where a Project Dacnis team discovered a new flea-toad, which was classified into the genus, Brachycephalus.
“This genus is different,” Professor Toledo explained. “During its evolution, it underwent what we biologists call ‘miniaturization’, which involves loss, reduction and/or fusion of bones, as well as fewer digits and absence of other parts of its anatomy.”
This new species of flea-toad is not the first, nor the second, to be discovered and scientifically described. In fact, it is the seventh such species of flea-toad described in the genus Brachycephalus so far.
This genus is primarily known for its small but brightly colored amphibian species that are highly poisonous, most notably, the bright orange pumpkin toadlets, B. rotenbergae, B. ephippium and B. pitanga (more here).
Pumpkin toadlets are somewhat larger than their cousins, the flea-toads, but curiously, pumpkin toadlets lack some anatomical features that are present in other frogs and toads. For example, they lack a tympanic middle ear bone, which renders unable to hear their own vocalizations.
The other group of closely related Brachycephalus species are the flea-toads, which are even smaller than pumpkin toadlets and are cryptically colored in shades of brownish-yellow so they can blend in with the leaf litter where they live.
How did the researchers find such a tiny creature?
According to Professor Toledo, researchers first became aware that there might be a new species to find after hearing the adults’ distinctive vocalizations whilst conducting sampling surveys between June 2021 and May 2022 in Ubatuba, on the coast of São Paulo state, in an area of the Atlantic Rainforest (Figure 6b).
“Edelcio [Muscat] and his team at Projeto Dacnis, a private nature reserve, found the new species (Brachycephalus dacnis) in the same spot as its sister species (already known Brachycephalus hermogenesi), after he recognized 2 different sounds,” Professor Toledo described in email. “Then, he sent me the specimens, sounds, and we conducted DNA analyses, high resolution CT-Scans, anatomical studies, and confirmed the finding.”
Figure 6: Geographic distribution, advertisement call sonograms and phylogeny. … [+]
The DNA analysis revealed that B. dacnis is closely related to another, similarly colored and marked yellowish-brown flea-toad, B. hermogenesi (Figure 6c). Both species are cryptic, closely matching the leaf litter where they live, and both are found in the same geographic area (Figure 6a). In fact, so close is the resemblance between these two flea-toad species that it’s possible that B. dacnis may have been accidentally discovered previously.
“There may have been specimens belonging to the new species among those that served as a basis for describing B. hermogenesi in 1998,” Professor Toledo noted.
Is this new flea-toad species smaller than a previously discovered and described flea-toad, B. pulex (read more here)?
“One of the individuals measured 6.95 mm, which is currently the second smallest adult vertebrate ever described, only larger than another individual of a different congeneric [belonging to the same genus B. pulex] species,” Professor Toledo and collaborators wrote in the paper describing the new species.
Nevertheless, both Brachycephalus species are extremely small.
“There is only one individual of B. pulex that is smaller than the smallest individual of B. dacnis,” Professor Toledo replied in email. “Therefore, they are pretty much the same size in general.”
What was the inspiration for the common name, “flea-toad”?
“We know that they JUMP outstandingly far — that’s where the name ‘flea’ comes from, besides being small,” Professor Toledo replied in email.
How small can these — or any — vertebrates possibly get? Have flea-toads reached the evolutionary limit for how small vertebrates can possibly get?
“YES ! that’s the limit…. if someone in the future finds an even smaller frog, or vertebrate, that would be just few decimals of millimeters smaller (an insignificant difference),” Professor Toledo responded in email.
How do these tiny animals defend themselves from predators? Are they poisonous?
“[T]hey open their mouths to threaten their predators!” Professor Toledo responded in email.
“[But there’s] no evidence of poison,” Professor Toledo added.
Newly described flea-toad, Brachycephalus dacnis, in a defensive position (Credit: Lucas Botelho.)
What else is known about flea-toads?
“As we just discovered this species, we don’t know many things yet,” Professor Toledo said. “For example, who are the predators? how females choose males? do they hybridize with the sister species? how do such small internal organs function? (heart, liver, lungs, etc)?”
What is the most surprising or interesting aspect of flea-toads?
“Size, of course, but besides that, in my opinion, it will be their internal anatomy,” Professor Toledo replied. “For example, by examining another Brachycephalus species (B. rotenbergae), we found that they have a ‘fish like heart’ with only two cavities, instead of three, as in most of amphibians — and that’s information from one of the largest Brachycephalus species — so, how it would be in such a smaller species?”
Professor Toledo and collaborators also discovered that, unlike many frogs and toads, the flea-toads show direct development — bypassing the tadpole stage and emerging from their eggs as fully formed miniature versions of the adults. Additionally, they also discovered that flea-toads only produce two eggs in a cycle, unlike many other frogs and toads that produce dozens, hundreds or sometimes thousands of eggs at a time.
Since there are seven species of flea-toads already described, how many more might be out there awaiting discovery?
“The diversity of these miniature frogs may be far greater than we think,” Professor Toledo said. “Hence the importance of describing as many traits and features as possible, to expedite the description process and get to work on conservation as quickly as possible.”
Source:
Luís Felipe Toledo, Lucas Machado Botelho, Andres Santiago Carrasco-Medina, Jaimi A. Gray, Julia R. Ernetti, Joana Moura Gama, Mariana Lucio Lyra, David C. Blackburn, Ivan Nunes, Edelcio Muscat (2024). Among the world’s smallest vertebrates: a new miniaturized flea-toad (Brachycephalidae) from the Atlantic rainforest, PeerJ 12:e18265 | doi:10.7717/peerj.18265
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