clam
蜃 centers on a large bivalve mollusk: a clam. The character is used for the mythical clam-like creature said to create mirages.
蜃 combines 虫 (insect/creature) with 辰, which likely contributes the sound. The character originally referred to a large clam or a mythical clam-like creature.
Inside the shell-like enclosure 辰, there is a 虫 (creature). Picture a clam as a shell with a soft creature inside.
For シン, imagine a shiny clam shell: shiny -> シン, and the clam's shell gleams.
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