If you lean over the side of a boat and scoop up some water with a jug, you have just taken a census of the ocean. That water contains traces of the animals that swim below your boat—flecks of skin and scales, fragments of mucus and waste, tiny cells released from their bodies. All of these specks contain DNA. And by sequencing that DNA gathered from the environment—which is known as environmental DNA, or eDNA—scientists can work out exactly what’s living in a patch of water, without ever having to find, spot, or identify a single creature.
And that helps, even when the creature in question is 18 meters long.
In August 2007, an oil worker named Soren Stig looked over the side of a rig in the middle of the Persian Gulf, and saw a horde of giants. They were whale sharks—the world’s largest fish, with a cavernous mouth and a back that looks like a field of stars. They live throughout the tropical oceans and often gather en masse to feed on plankton and fish eggs. But scientists believed that they were rare in the Persian Gulf, unable to tolerate its superlatively hot waters. And yet, when Stig looked into those waters, he saw huge numbers of them.
He took a photo and uploaded them to an online database of whale-shark sightings. Within a few years, the Qatar Whale Shark Project was born, and scientists were studying the animals with help from the oil workers. By towing nets amid the feeding shoals, they showed that the sharks had gathered to eat the eggs of spawning mackerel tuna. By fitting the sharks with satellite tags and photographing them, they could identify and track individuals—more than 300 to date.