The Guinness Book of Records points out in an article on its website that the smallest vertebrate animal known in the world is the fish Photocorynus spiniceps, a species of teleost (bony fish) whose male specimens measure between 6.2 and 7.3 millimeters. This brand is based on a scientific study published in 2005 by Professor Theodore Pietsch in 2005.
This tiny anglerfish found in Philippine waters now has a small competitor, the Brachycephalus pulex known in Brazil (where the species was discovered, in 2011) as sapinho-pulga or flea toad. The first data on this amphibian indicated that its adult male specimens (also in this case the males are smaller than the females) measure between 6.45 mm and 7.90 mm. An article now published in the journal Zoologica Scripta indicates that they could even be smaller in size, becoming the smallest existing amphibians and vertebrates. The authors of the study (Wendy H. Bolaños, Iuri Ribeiro Dias, Mirco Solé, all of them from the State University of Santa Cruz, Ilhéus, Bahia) title their article with a question: Getting closer to amphibians: What is the smallest vertebrate of the world? (Zooming in on amphibians: Which is the smallest vertebrate in the world?
Researchers first discovered the tiny amphibian in 2011, with specimens only known from hills in the southern part of Bahia, Brazil. At that time, researchers also believed that it was a toad, hence its name, but now they consider the name frog more appropriate.
For this new study, the researchers ventured to the site where the original specimens were found and captured (and later released) several specimens for study and measurement, checking for signs of maturity; they wanted to make sure those they measured were adults. To do this, they had to check both their gonads and their vocal clefts. Once they confirmed that a specimen was as large as it would become, they measured its body length.
Measurements of 46 specimens revealed that the average male body length was just over 7 millimeters, making them smaller than a pea, or put another way, two of them could rest comfortably on a pinky nail. Females were, on average, 1 millimeter longer. The researchers noted that the smallest specimen they found was only 6.45 millimeters long, which they say is about 30% smaller than the smallest frog previously observed.
The discovery represents the smallest vertebrate observed (along with the fish mentioned at the beginning). All in all, experts suggest that there may be smaller creatures that have not yet been found or studied. They also point out that there are probably limits: They point out that most small frogs have strange quirks, such as fewer fingers than other frogs or insufficiently formed ears, signs that suggest smaller frogs would have trouble surviving.