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沙 (sand) supplies the sound directly: shā → shā (no shift).
沙 itself is
氵 (water) +
少 (few), depicting fine water-washed grains. In
鲨 the role is primarily phonetic, but a faint semantic flavour also applies — sharks were once called
沙鱼 (sand-fish) for their rough, sandpapery skin. So this is a rare case where the phonetic doubles as an etymological hint.
