August 20, 2013 — The menu says red snapper, but it’s actually tilapia. The white tuna, meanwhile, is really escolar, while the seabass is Antarctic toothfish.
Welcome to the wild world of modern seafood, where not everything is as it seems.
New research is revealing that merchants and fish dealers often mislabel their product as an entirely different species to fetch a better price at market. A study realeased last week by UK researchers found that a number of species in the skate family are sold as “sting ray wings,” while a separate study produced in February by the group Oceana found that, of 1215 seafood samples from 674 restaurants and grocery stores in 21 U.S. states, a full third were mislabeled. In Chicago, New York, and Washington, DC, every single sushi bar that was tested was found to sell at least one mislabeled fish species.
How did the researchers figure all this out? Through the innovative use of DNA barcoding, in which a specific segment of genetic material (analogous to a product’s barcode) in a piece of fish is used to determine exactly which species it truly belongs to. For years, we had no real way of determining the true species of a piece of seafood—a filet of fish, after all, often looks like any other filet—but this new application of an existing scientific technique is rapidly becoming a crucial tool in combating seafood fraud.
Testing a piece of fish to determine its species is fairly straightforward—scientists perfected DNA barcoding years ago, albeit typically as part of other sorts of projects, like cataloging the complete assortment of species in a given ecosystem. Analyzing the DNA in a piece of fish is a relatively similar process.
To start, researchers acquire a piece of fish and freeze it, as fresher and better-preserved tissue samples generally yield more accurate results. Then, in the lab, they slice off a tiny piece of the sample for testing.
To extract and isolate the DNA from the tissue, scientists break open the cells—either physically, by grinding them or shaking them in a test tube filled with tiny beads, or chemically, by exposing them to enzymes that chew through the cell membrane. Next, they remove other components of the cell with various chemicals: proteases digest proteins, while RNAase digests RNA, an alternate form of genetic material that could cause errors in DNA testing if left in place.
Once these and other substances are removed, the remaining sample is put in a centrifuge, which spins it at high speed so that the densest component—in this case, DNA—is isolated at the bottom of the tube in a pellet. A variety of different approaches are currently used to sequence the DNA, but all of them achieve the same end—determining the sequence of base pairs (the building blocks of DNA that are unique to each organism), at one specific location in the fish’s genome. All fish of the same species share the same sequence at that location.
Read the full story at Smithsonian Magazine