loach, lamprey, (kokuji)
鯲 is a Japanese kokuji for a specific freshwater fish: the loach. The lamprey meaning is a secondary application, likely due to a similar elongated body shape.
鯲 is a Japanese kokuji, created by combining 魚 (fish) with 於, which likely contributes a phonetic element related to the reading どじょう. The exact historical development is uncertain.
The left side 魚 means fish, and the right side 於 looks like a wriggling shape. Picture a loach squirming in the mud: 魚 + 於 = loach.
For どじょう, imagine a loach saying 'dojo' as it practices martial arts in the mud: dojo -> どじょう.
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