Mantis Shrimp, more commonly known as Shako (シャコ,/蝦蛄) in Japan, or Oratosquilla oratoria, is a kind of shrimp commonly prepared as nigiri in Edomae sushi restaurants as they were caught in Tokyo Bay.
It is only in season briefly during the change over from spring to summer. It is not served raw, but lightly boiled before being allowed to marinade in the boiling liquid. If lucky, you may be able to eat Mantis Shrimp with their roe/eggs (Katsubushi/かつぶし) attached to them, a type of nigiri only very few sushi restaurants still serve.
After marinating, be sure to drain the Shako to ensure that there is no excess liquid, which would make it hard to make nigiri with.
Edomae-style preparation of Mantis Shrimp (Shako/シャコ/蝦蛄):
Start by adding mirin to a pot of water.
Next, add soy sauce.
Bring the mixture to a boil to evaporate away the alcohol. Skim off any foam that forms on the surface.
Once boiling, add the Mantis shrimp in and return the pot to a boil. Once boiling again, immediately turn off the heat.
The residual heat will be enough to cook the Shako and it will continue to marinade in the sauce. Once the pot has reached room temperature, the Shako will be fully cooked and completely marinated.
Drain the Shako well. To begin peeling, first start by cutting off the head.
Next, with the Shako lying on it’s back and using a pair of scissors, make two incisions down the sides of the body of the Shako, breaking the shell.
Gently peel back the shell to expose the flesh.
The flesh can now be gently removed from the shell and used to make nigiri.