Barcoding Guts

The European eel is one slippery invertebrate — not just physically, but scientifically too. For a long time, fishermen and researchers only ever saw adult eels. Scientists had no idea where they came from or what their larvae look like. In 2006, they finally found the larvae, but they still don't know much about them. We don't know what they do with their time, where they go, or what they eat. But a recent study using DNA barcoding has tackled at least one of those questions: What do these eel larvae eat?

The eels live mainly in freshwater, but they migrate to the oceans to reproduce. They travel to the Sargasso Sea, which is actually a region in the middle of the Atlantic Ocean (named after the Sargassum seaweed that floats on the surface for miles). The open sea is bounded by currents and for a long time was thought to be extremely low in nutrients with very little life. Yet somehow these eels spawn there, and their larvae find something to eat.

Until recently, nobody knew exactly what they were eating. Guts are hard to analyze, everything is all mashed up and unrecognizable. But with DNA barcoding, you just throw the guts into a test tube and voilà! You get to know what's in their bellies! Okay, it's a little more complicated than that.

The mashed up food from the larvae's stomach has a whole lot of DNA in it, unique to whatever the little eels ate. Instead of sequencing all of it, the researchers picked a segment —-- in this case the 18S rRNA region — that is slightly different in every species (just like a barcode). They used a chemical snipper to remove that region from all the rest of the DNA sequence and replicated those segments to later determine which species were a match.

What they found was a whole lot of lunch — these little larvae were eating 75 different species, mostly plankton, from copepods to ctenophores to polychaetes. But the researchers also found fungi and Streptophyta, a land plant. Turns out that the Sargasso Sea is not so lifeless after all.

These European eels are of particularly importance because they were recently added to the endangered species list by the International Union for the Conservation of Nature (IUCN). Their populations have suffered mainly because of overfishing and habitat loss. Knowing what they eat could be key to farming the eels and allowing the wild populations to recover.

Gut analysis is just one of the many ways DNA barcoding can tell us more about what animals are up to. I just hope that no one barcodes the contents of my stomach any time soon — or at least doesn't report the results to my parents.

--Rose Eveleth

Image Credit: Ron Offermans (via http://wikimedia.org)