What is a species?

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A frog that looks like it's made of tempered chocolate. A rainbow-colored fish that dwells in the ocean's "twilight zone." A hairy sloth with a coconut-shaped head.

These are just a few of the hundreds of newfound species that scientists described in 2022. The animals join a growing list of more than 1.25 million species that have been scientifically described and cataloged since the 18th century.

But what defines an organism as a species that's new to science? And what exactly is a species, for that matter?

Biologists have wrestled with the concept for about as long as the field of biology has existed. Renowned naturalist Charles Darwin wrote in 1859: "No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species."

Fast-forward to the present, and the debate hasn't changed much. "There are many definitions, and none of them applies broadly to all life on the planet," said Bruno de Medeiros, assistant curator of insects at the Field Museum in Chicago.

And yet, recognizing and distinguishing between species is vital - and not only for biologists. It's also necessary for cultivating the food we eat, treating diseases caused by different pathogens, and conserving endangered animals, plants and habitats.