An incredible twitter thread has recently gone viral, highlighting how the fish we think we are buying may not necessarily be the fish we end up eating. Dr Jennifer McDonald, a professor of biology at Fanshawe College in Canada, had her students sequence the DNA of fish samples from restaurants and markets, only to discover that the labels on the fish rarely correspond to the true species.

The 16 students in Dr McDonald’s class performed a test called polymerase chain reaction (PCR), which is used to find a small amount of DNA in a sample. They were able to extract DNA from 10 of the 13 samples, nine of which were fish. More on the non-fish one later.

From those nine, only two were actually what they said they were: an Icelandic cod that had a marine stewardship certification (MSC) and a smoked rainbow trout. For the frauds, there was tilapia that was substituted for both red snapper and red tuna. Atlantic salmon turned out to be rainbow trout and an alleged sample of rainbow trout was coho salmon. Pacific cod was actually Atlantic cod. Surprisingly, one sample of white tuna was actually a yellowfin tuna, certainly an improvement.

The other sample of white tuna was escolar, a tasty fish but one that has some dangerous side effects. The fish cannot metabolize compounds known as esters, which give it an oily texture. Too much of this fish in humans can cause – brace yourself – explosive, oily, orange diarrhea.

If you thought that was the worst of it, please leave now. The tenth sample was a bit of a mystery. It was purchased as salmon from the seafood department, but the number of base pairs they were able to extract was much lower compared to the other fish. Dr McDonald tried to clean the sample to no avail. It turns out, the belief it was a fish was a red herring. Do you want to know what was in that sample? According to her tests, it was body louse.

Clearly, it is concerning to know all this, but you don't have to go off fish altogether. Dr McDonald has a simple piece of advice for how to eat what you are actually buying: “Make sure it's from a certified sustainable fishery. They know what they're fishing, and know what they're doing.”

Hopefully, the people who sell these mislabeled fishes take a long look at themselves and consider doing some sole searching!