うに

• 個こN5

  個 counts うに as an individual sea urchin or serving of roe, the most common everyday choice.

• 匹ひきN3sense 1

  匹 is sometimes used for うに when emphasizing it as a live sea creature rather than a food item.

• バフンウニ

  green sea urchin

• うにUsage28%
  gikun (meaning-based reading) or jukujikun

  Usually written in kana; the kanji forms are rare.

• ウニUsage65%
  kana-only spelling

  Katakana spelling is common in menus and food contexts.

• 雲丹うにUsage6%
  rarely-used kanji formgikun (meaning-based reading) or jukujikun

  Rare kanji form used specifically for the food sense (seasoned sea urchin eggs).

• 海う胆にUsage1%
  rarely-used kanji formgikun (meaning-based reading) or jukujikun

  Rare kanji form; not commonly used.

• 海う栗に
  rarely-used kanji formgikun (meaning-based reading) or jukujikun

  Rare kanji form; not commonly used.

The origin is uncertain. The word is native Japanese, and the kanji forms are jukujikun (special readings) assigned based on meaning.