Wani Sashimi

The dish grew straight out of geography. The Sanin uplands sit days of overland carrying from the Sea of Japan, and before refrigeration almost no sea fish could survive the haul to the mountains in edible condition; ordinary fish spoil within about three days. Shark was the exception, and the reason is its body chemistry. Sharks are cartilaginous fish that regulate their internal salt balance by holding large amounts of urea and trimethylamine oxide in their flesh, and these nitrogen compounds strongly retard the bacterial spoilage and histamine formation that ruin other fish, so shark flesh stayed safe to eat raw for around two weeks rather than a few days. That window was long enough to move the fish inland by foot and packhorse from the coast, and so in country with no fresh sea fish of its own, shark became the festive raw fish reserved for the New Year and for celebrations. The word wani for shark is old: it is how shark is written in the Inaba white-hare myth of the eighth-century Kojiki, evidence that the usage long predates the dish.

Cut fresh, the flesh is pale ivory, lean, and firm with a clean, faintly sweet taste and only the gentlest hint of the sea. The texture is springy and a little resilient under the tooth, softer than tuna, holding the soy and the heat of ginger or wasabi well. As the fish sits over days, a mild ammonia scent rises and the flavor turns more pronounced; eaten at its freshest, that note is barely there, and the meat reads simply as a clean, soft white sashimi.

The whole tradition rests on shark biology. Unlike bony fish, sharks and rays keep their blood roughly as salty as seawater by loading their tissues with urea and trimethylamine oxide rather than excreting salt, so raw shark flesh can hold on the order of two percent urea by weight together with high trimethylamine oxide. These nitrogen compounds, and the mildly alkaline conditions they create, suppress the growth of spoilage bacteria and slow the formation of histamine, the compound behind scombroid poisoning in other fish, while trimethylamine oxide also helps resist the fat oxidation that turns fish rancid. The practical effect is a sea fish that stays edible for around two weeks instead of a few days, which is exactly why it could be carried into the mountains and still eaten raw. The same chemistry sets the limit: after death, enzymes and bacteria slowly break the urea down into ammonia, so an older shark smells faintly of it, and the fish is at its best eaten early. Soy sauce, ginger, and wasabi are not incidental; their pungency masks any residual ammonia and their components help blunt it, the conventional seasoning for urea-bearing fish.