species of tortoise found near the mouth of the Yellow River
蟕 refers to a specific species of tortoise found near the mouth of the Yellow River. The meaning is narrow and tied to that particular animal.
蟕 combines 虫 (insect/reptile) with 觜, which likely contributes the sound. The character was created to name a particular tortoise species.
The insect radical 虫 on the left suggests a small creature, and the right side 觜 looks like a complex shell pattern. Together they form the tortoise 蟕.
For スイ, imagine a tortoise swimming (スイミング) near the river mouth: swimming -> スイ, and the tortoise glides through the water.
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