The head turns black within half a day. There is copper in the blood, and once it meets air the colour goes. That single fact decides everything else about this shrimp: it cannot be shipped, so it is eaten where it is landed, and almost nowhere else.

The proper name is kurozako-ebi. It lives in water around five degrees, two hundred to two hundred and fifty metres down, and it is found in the Japan Sea and nowhere on the Pacific side of the country. Nothing about it is farmed or held in tanks.

Preparation is minimal. Rinse it, leave the head and the tail on, peel the shell off the body between them, and eat it with soy sauce. The flesh has more spring to it than you expect and pushes back against the teeth, and the sweetness is stronger than that of the sweet shrimp people know from elsewhere.

The trawlers work it with offshore bottom nets from September through to May, and the season is reckoned best in spring, when the females are carrying green eggs under the tail. Sakai, Tottori and Ajiro are the ports. So an animal that a great many people would happily pay for has stayed a local dish, not through any decision or protection, but because of a chemical reaction that starts the moment it is landed.