One of the world's largest, most endangered, and most mysterious freshwater fish has yielded a new surprise: a likely new species—and possibly several more—have been lurking in the backwaters of the Amazon.

New research published by National Geographic explorer Donald J. Stewart and colleagues L. Cynthia Watson and Annette M. Kretzer in the journal Copeia this week reveals strong genetic evidence for an unknown new species of arapaima that was found at several locations in southwestern Guyana.

Long, narrow giants, arapaimas live in tropical South America. They can grow up to 10 feet long and weigh 440 pounds. They breathe air through a primitive lung, and tend to live in oxygen-poor backwaters. (See photos of arapaimas and other megafish.)

Stewart, who is also a biology professor at the SUNY College of Environmental Science and Forestry in New York, says the team sampled hundreds of the giant fish in the Essequibo and Branco River basins in Guyana. The Branco is part of the Amazon system while the Essequibo drains separately into the Atlantic. The team found two sets of fish with highly distinct genetic markers at three locations in the Essequibo.

The genetic markers indicate the fish have not bred across the two groups for a long time and they are likely so different that they represent distinct species, says Stewart. At least one is therefore new to science.

"If you have two types of fish swimming along together but not interbreeding that’s pretty good evidence they are new species," Stewart explains. "But we still have to work out the details."

A Complicated History

Arapaimas have long been a poorly studied group, in part because they are hard to catch and they live in remote areas. Scientists in the early and mid 19th century thought there were several distinct species in the Amazon, but then a British scientist published a paper in 1868 arguing that all arapaimas belong to a single species, with regional differences.

That was the prevailing view until 2013, when Stewart published a paper proving that another species existed, based on physical characteristics. That specimen, collected in 2001 in Amazonas State, Brazil, was named Arapaima leptosoma, referring to the fact that it has a slimmer body.

But now at least one more species must be added to the arapaima family tree, in addition to the historic species Arapaima arapaima. In fact, Stewart suspects there may be even more distinct species in other parts of the animal's range, and he intends to continue the work of finding them.

"I think we’ll have many more species before we’re done," he says.

Saving the Species?

The classification isn't just academic, though. Arapaimas are highly endangered, thanks to pollution and disruption of their habitat and centuries of fishing pressure. Their meat was historically prized by indigenous peoples, some of whom used it for ceremonial purposes. A moratorium on their capture was enacted in 2002, but illegal poaching continues.

Scientists aren't sure how many arapaimas are left but they think it is probably around 5,000 in the Essequibo, up from a low of around 800 in 2012. But just how many remain in each species is unknown, making the conservation status of some potentially dire.

"We need to pay attention to this diversity so we don’t eliminate one of them," says Stewart.

Fisherman With No Fish Joshua Jackson meets a family on Apo Island in the Philippines. They live off the fish they catch, but the fish are disappearing.

As an example, if one of the species spawns at an older age fishermen might end up wiping that animal out without realizing it.

Further, "it’s hard to argue for conservation if you don’t know it’s there," says Stewart. "The more of these we can recognize the more arguments we can make for getting the resources to protect them."

The species has long been cherished in the Amazon as a symbol of the river. "Some people consider it a flagship species that we need to conserve, kind of like a tiger or an elephant,” says Stewart.