oyster
蛎 centers on the oyster, a shellfish creature. The meaning is straightforward and specific to this animal.
蛎 combines 虫 (insect/creature) with 厉, which likely contributes the sound. The character denotes a type of shellfish, the oyster.
The left side 虫 suggests a creature, and the right side 厉 looks like a shell with a rough edge. Picture an oyster clinging to a rock: 蛎 is the oyster.
For レイ, imagine a ray of sunlight shining on an oyster bed: ray -> レイ, and the oyster opens in the light.