oyster
蚵 centers on the oyster, a shellfish creature. The meaning is specific to this animal.
蚵 combines 虫 (insect/creature) with 可, which likely contributes the sound. The character is used for oyster, a shellfish.
The insect radical 虫 on the left suggests a small creature, and 可 on the right looks like a shell opening. Picture an oyster as a bug-like shellfish with a hinged shell.
For カ, imagine a car (カ) shaped like an oyster, with the 虫 side as the wheels and 可 as the body. Car -> カ.
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