There's All This Fish DNA in the Water

And now scientists are using it to take a census of the sea.

The Open Sea exhibit at the Monterey Bay Aquarium (

Fish are, in a lot of ways, like humans. They're amazingly diverse. They have well-defined senses of taste and smell. They often travel in groups. They work really well with olive oil.

One other way fish are like us? They're also shedding cells, constantly. But while humans' cells generally slough off into air, the fishy ones end up in water. This leads to what one scientist refers to, accurately if somewhat horrifyingly, as "a soup of cells" in the sea. The ingredients of said soup? "Skin, damaged tissues, and ... body wastes."

The mixture is technically called eDNA, and you'd be forgiven for thinking that the "e" stands for "ewwwww." (It actually stands for "environmental.") And while you may prefer not to think about the stuff when planning your next beach vacation, eDNA could prove to be wonderfully useful. For science! 

Traditionally, when researchers have needed to take censuses of the marine life populating particular areas, they've had to do it the old-fashioned way: by going out and sampling, netting animals or simply cataloguing them in their habitats. This is inefficient for obvious reasons. And eDNA—all that coded genetic material, just rolling in the deep—could obviate the need for it. 

A researcher gathers water from the Aquarium's Kelp Forest exhibit (University of Washington)

To test that potential, researchers at the University of Washington recently put eDNA sampling techniques to use. For that, they turned to a nicely concentrated source of that maritime genetic material: the enormous Open Sea Tank at the Monterey Bay Aquarium, which houses 13,000 live fish within 1.2 million gallons of water. From those contents, they extracted just two pint-sized glasses of water. (The researchers said, in retrospect, that their sample could have been smaller.) To zero in on vertebrate DNA, the researchers used a set of "primers"—molecular probes—that were designed by another group the year before they conducted their study.

Then, they cross-referenced the findings of the primer-aided census with the known inventory of the Aquarium tank. The scientists were able to determine eight of the bony fishes that call the tank home, with tuna and sardines making up the greatest amount of biomass. While these results tracked with the Aquarium's records, the primers weren't fully effective: The researchers were unable to detect DNA from turtles and cartilaginous fish like rays and sharks. 

When it came to the bony fishes, however, the primer was even more sensitive than the researchers had anticipated: It detected, among the other fish, the presence of menhaden from the Atlantic—a species decidedly not in the Aquarium's manifest of tank-dwellers. The researchers were temporarily baffled, until they remembered that Atlantic-caught menhaden are thrown into the tank as food for the other species.

University of Washington professor Ryan Kelly and his co-authors published the results of their findings in a paper in the January 15 issue of the journal PLoS One. And they think the Aquarium research will be the first step in a larger exploration of eDNA's life-tracking possibilities. Now that they know the primer technique can be used for species identification, the next step will be applying it to open-ocean surveys of marine life—otherwise known as putting the world's "soup of cells" to good use.

University of Washington via The Smithsonian