How DNA Testing Can Tell You What Type of Fish You’re Really Eating?

The menu says Cod, but it’s actually Pollock. The 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!

Through the innovative application 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 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 cataloguing the complete assortment of species in a given ecosystem. Analysing 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 laboratory, 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 penetrate 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 scientific 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.

As part of broader DNA barcoding projects, other scientists have analysed the sequence of base pairs at that same genetic location in thousands of pieces of fish tissue that can definitively linked to species. Thus, by comparing the genetic sequence in the mystery fish tissue to databases of other species’ known genetic sequences, scientists can determine if, say, the Cod you thought you were buying was actually Pollock.

Figuring out which species a piece of fish truly belongs to has significance that goes far beyond gastronomy. For one, cheaper fish species are most often substituted for more expensive ones. The fact that inexpensive fish is so commonly passed off as a pricier variety, while the reverse occurs much more rarely, indicates that intentional mislabelling by sellers is at play, rather than innocent misidentification.

Additionally, species that are dangerously overfished and are on the verge of ecological collapse are sometimes substituted for more environmentally-benign varieties. Customers that make the effort to choose sustainable types of seafood, in these cases, are thwarted by mislabelling.

Eating different species can also have vastly different effects on your own health. For one, different fish species can have different fat and calorie contents, so mislabelling can lead the nutrition-conscious astray. Moreover, certain species feature in “do not eat” lists for sensitive groups of people (such as pregnant women) because of their high mercury content. One study carried out in the US found several instances of Tilefish being sold as Red Snapper. Perhaps even worse, 94% of the white tuna also tested in the study was actually a fish called Escolar, which has been found to contain a toxin, gempylotoxin, that when ingested, even in small quantities, can cause severe diarrhoea.

So, what to do? Testing the fish’s DNA at home is completely beyond most people’s capabilities. So to avoid being duped, it is recommended to ask about provenance, scrutinizing price – if a fish is being sold far below market value, it’s probably mislabelled as a different species – and buying fish supplied from reputable fish specialists.

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