蟹 is the literary/formal character for crab. In everyday speech, 螃蟹 (pángxiè) is far more common. 蟹 alone appears mostly in compound words: 蟹黄 (crab roe), 蟹肉 (crab meat), 大闸蟹 (hairy crab).
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The character has 虫 (insect/creature) at the bottom — crabs are classified alongside insects and creepy-crawlies in Chinese radical logic.
