A biologist walks into a sushi bar and orders some tuna. What does he get? Escolar, a nasty fish with buttery flesh that can cause bizarre episodes of diarrhea, accompanied by a waxy intestinal discharge.

It’s not a joke. It happened five times to the same scientists during a brief research project. The results of that study were published Wednesday in PLOS One.

“A piece of tuna sushi has the potential to be an endangered species, a fraud or a health hazard,” wrote the authors. “All three of these cases were uncovered in this study.”

The team of researchers from Columbia University and the American Museum of Natural History ordered tuna from 31 sushi restaurants and then used genetic tests to determine the species of fishes in those dishes. More than half of those eateries misrepresented, or couldn’t clarify the type of fish they were mongering. Several were selling endangered southern bluefin tuna.