The sushi topping made from sea urchin gonads is uni.
In culinary use, uni refers to the soft, rich gonadal tissue found inside a sea urchin. It is commonly described as “sea urchin roe,” but that phrase is technically imprecise: both male and female gonads are eaten, not just eggs. Regular sea urchins typically have five gonads arranged inside the hard shell.
In Japan, uni is served as sushi, sashimi, or in gunkan-maki, whose raised nori rim keeps the delicate pieces in place. Its texture is creamy and melting, while the flavor can range from sweet and oceanic to intensely briny depending on the species, season, handling, and origin. Japanese markets also import sea urchin from places including the United States and South Korea.
The alternatives are different seafoods: ankimo is monkfish liver, ikura is salmon roe, and unagi is freshwater eel. Uni should therefore not be confused with fish eggs, even though menus and translations sometimes casually call it roe.