My students and I encountered a mystery. We were partnering with the Missouri Department of Conservation to monitor for fish presence and abundance in the Missouri Ozarks. We visited half a dozen rivers and streams across the region and collected a few cups of water from each.

We were testing for DNA in the environment to determine which species had left behind their genetic material and were therefore present in the stream. There was no need to capture or even see the species themselves. We were delighted to detect a variety of species in their appropriate habitats that are considered at risk in Missouri, including the lake sturgeon, crystal darter and Alabama shad. In the surface waters emerging from one large underground spring, we detected DNA from a cave-dwelling fish species that would never have been detected by researchers who caught or looked only for fish in the surface waters.

But there was a problem: We were certain we would find one particular species in every stream: the bleeding shiner (Luxilus zonatus), one of the most common fish in Ozark streams. To have confidence in our results we needed to understand........

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