蛎 (lì) is a character that appears almost exclusively as part of the compound 牡蛎 (mǔlì, 'oyster'). It is rarely used alone in modern Chinese. When you see 蛎 in writing, it's nearly always in 牡蛎 or related terms like 蛎壳 (oyster shell). You don't need to memorize it as a standalone word — just recognize it as the second syllable of 'oyster'.