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« Shirako » is « whiting », or in more prosaic terms, male fish sperm sacs.
It seems to be an acquired taste even for the Japanese.
The most available kind is that of « tara », or cod. Do not confuse it with « tarako », which is the exact opposite as it means female cod roe!
Other kinds, more expensive and tasty, are those of « tai » (seabream ) and « fugu » (globefish).
The best way to enjoy it is either:
as a « tsumami » (snack) with ponzu, momijioroshi (grated daikon with chili pepper) and some finely chopped thin leeks. Fresh seaweed is optional.
As a sushi, either on top of a gunkan. Ask your sushi chef to season it, so as to avoid the chore of dipping it into shoyu, or, if your chef is a real expert, as a nigiri. The last might seem difficult. Actually, there are two tricks to stabilize the « shirako » on the « shari » (rice ball): coat the the « shari » with chopped thin leeks, or put the « shirako » on a « shiso » (perilla/beefsteak plant) leaf, place the « shari » on top, press very lightly and turn it over!