A friend gave me a few fillets of katsuo (skipjack tuna). I’m sure he didn’t know about the cestode guest lurking within one of them! This is the larva of a certain species of platyhelminth, but completely harmless to humans (you can even eat them without any ill effects). You may also find them in saba (mackerel) and surumeika (flying squid). The dish tataki made with katsuo, a specialty of Tosa, is specifically done with the purpose of getting rid of these creatures (with fire). If you are lucky enough to eat katsuo up north (such as I was served in Iwate) the water temperature is too low for the larvae to survive so the locals always eat their katsuo as-is, without the charring. Incidentally, this particular fillet was shallow-fried in olive oil with salt & pepper and eaten between two slices of toast for breakfast, with no detectable influence of the parasite on the deliciousness of the fish.