Sea urchin roe (uni, ウニ) is a traditional dish on New Year’s morning, perhaps a result of a felicitous combination of extravagant spending on food and cold weather allowing its transport. But - kanji time! The three kanji pairs representing uni each have a different nuance. (Note: all are pronounced “uni” and that the katakana is most commonly used in any case).
「海胆」- literally, “sea bile” - the roe. This is the part we eat. It is yummy.
「海栗」- literally, “sea chestnut” (kinda evocative!) This refers to the animal itself.
「雲丹」- literally, “vermilion cloud.” In the Edo era, transporting fresh uni was impracticable, so it was often pickled, resulting in a vermilion-tinted cloud of Echinoidea roe.
“Uni don” 「うに丼」is a popular but barbarous combination of warmed uni atop a bed of rice. Avoid it. The best uni is plucked directly from the rocks and eaten with a slight bathing of sea water (that’s how I do it).
Mention this the next time you visit your local fishmonger to elicit a startled reaction. - William