A brand-new species of frog has been discovered in the middle of America's largest metropolis, New York City.

Not only had the green, black-spotted Atlantic Coast leopard frog gone unnoticed by the eight million residents of New York City, but it appears to be widespread along the Atlantic Coast of the United States, researchers reported this week in the scientific journal PLOS ONE.

"This discovery clearly demonstrates that human knowledge of the natural world remains incomplete even in the best-known locales," wrote lead author Jeremy Feinberg, a PhD student at Rutgers University in New Jersey, and his colleagues.

The paper noted that New York is "one of the most developed, heavily settled and well-inventoried places on Earth" and that "novel and undescribed vertebrate species are unexpected here."

In fact, the last new amphibian discovered in New York or New England was the Fowler's toad in 1882, the paper said.

Jeremy Feinberg

Rutgers University PhD student Jeremy Feinberg was studying frogs in the marshes near the Statue of Liberty on Staten Island when he noticed one that had an unusual call. (Courtesy Jeremy Feinberg/Rutgers University)

The new frog looks similar to two known species, but was identified from its distinctive calls and genetic testing.

The frog has been given the scientific name Rana kauffeldi, after Carl Kauffeld, an amphibian expert who claimed in the 1930s that a third species of leopard frog existed in the northeastern U.S. His claim was generally rejected by other biologists in subsequent decades.

In 2008, Feinberg was studying frogs in marshes on Staten Island, near the Statue of Liberty, when he noticed one that had an unusual call — a single "chuck," instead of the "pulsed ak-ak-ak" and "snore-like calls" of the two known species.

The researchers confirmed the existence of a new leopard frog species in 2012, after comparing the frog's DNA to that of the two known local species. But at that time, they didn't have enough information to "formally describe" or name it.

This week's report gives a full description of the frog's physical characteristics, its behaviour, including its call, and its range.

Based on recordings from people who have contacted the researchers since first hearing about the new species, the frog lives in seven U.S. states as far south as North Carolina, said Matt Schlesinger, chief zoologist of the New York Natural Heritage Program and co-author of the paper. DNA from the species has also been collected in New York, New Jersey and Connecticut.

The researchers have now released recordings of the frog's call to the North American Amphibian Monitoring Program. The U.S. government-run program relies on volunteers to listen for frog and toad calls to track their populations.

The Atlantic Coast leopard frog was never described before largely because it is a "cryptic" species — one that has a close physical resemblance to another species but is genetically different. The researchers think that other cryptic species could still be found in "well-catalogued" areas like big cities.