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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, which are part of the Amazon system. They 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.