Confessions first – I love sushi. But getting hedonistic pleasure from biting into a firm, cold piece of fish paired with salty-sweet vinegared rice doesn’t mean that I also want to be chowing down on an endangered animal. Unfortunately, according to a study published in PloS ONE last week, there’s a good chance that the fish inside my tuna roll came from an endangered or threatened species.
There are eight species of tuna in the genus Thunnus, any of which may be served up as sushi: yellowfin, bigeye, longtail, blackfin, albacore, as well as the threatened Pacific bluefin and northern bluefin, and the critically endangered southern bluefin. Conservation organizations urge consumers to avoid eating any kind of bluefin tuna, but unless your ahi is a slice of the light-fleshed albacore tuna, it’s hard to tell which species you’re getting on your plate. Tuna species look different from one another when they’re whole fish, but slice them into filets and they all look pretty much alike. People may want to avoid eating endangered types of tuna, but they can’t do it effectively unless they can identify the fish in the restaurant or the market. (They could, of course, give up eating all tuna. But few people are willing to push their convictions that far.)
Jacob Lowenstein and his colleagues at the American Museum of Natural History tackled this problem by developing a DNA barcode system for all eight species of tuna. By looking for small, species-specific differences in the DNA sequence for a single protein – an important mitochondrial enzyme called cytochrome c oxidase – they developed a key that could match the DNA from a piece of tuna with the species of fish it came from. They tested the accuracy of their method by collecting samples from the places where most people meet tuna – sushi restaurants.