A hungry guide caught a foot-long catfish in a Suriname river in 2010 and began to gut it. But the fish didn’t turn into a fillet—rather, it turned out to be an undiscovered species. The guide was part of a Conservation International-led survey of a tropical forest. Scientists rescued the specimen just in time when they noticed its exceptionally long spines, likely meant to fend off giant piranhas. Unlike any catfish in reference books, the as yet unnamed fish was one of 46 candidates for new species status found within three weeks. “They’re all pieces of a big puzzle,” says expedition member Philip Willink. “The more we have, the better our understanding of how the whole world works.”
In the 1730s Swedish botanist Carl Linnaeus set out rules for classifying species, the most basic biological category: a group of living things that look alike and normally mate only with each other. Since then, scientists have cataloged more than 1.7 million. But there’s still a lot of work to be done. Estimates of the total number of species range from 3 million to 100 million; one new study puts the figure at 8.7 million, give or take a million. The numbers at right, compiled from many surveys and statistical projections, offer the best guess for species in selected categories.
As these examples illustrate, most vertebrates have been identified, but many invertebrates—especially insects—remain for future generations of explorers to discover.