Sea urchin tastes the best during midsummer. They are ready to mate and the gonads are the biggest during this time. Interestingly, both the male and female gonads are used for sushi, or any other cuisine.
There are only slight differences in color tone between the male and female gonads and as for the flavor, there is not much of a difference between the two. The orange color of the sea urchin can change according to what the sea urchin fed on (usually seaweed), so it is very difficult to distinguish whether it is a male or female gonad.
A good sea urchin is difficult to choose. Unlike a fish, it must be opened and tasted, since some of them are bitter and unpleasant. This is why they are first opened and tasted by the fish market, and then sold on small trays. Sushi restaurants usually do not buy them alive and do not place them in water tanks for the chance of some being not presentable.
Sea urchin has a strong ocean scent, and it is very creamy and rich. It is prepared as a ship roll or gunkan miaki. However some people prefer not to use nori for the ship roll and ask to be rolled with paper thin Japanese cucumber strips. The main reason for this is that seaweed has another kind of different ocean scent. Quail egg is often placed inside the ship roll,but it is not recommended for it kills the flavors.