crab
蠏 centers on the crab, a creature with a hard shell and sideways walk. The meaning is straightforward: crab.
蠏 is a variant form of 蟹, both meaning crab. It combines the insect radical 虫 with 解, which likely provides the sound and may relate to the crab's segmented, 'dismantled' appearance.
The insect radical 虫 suggests a small creature, and the right side 解 (unravel, solve) looks like a crab's many legs and claws. Picture a crab as an insect-like creature that looks like it's been taken apart and put back together.
For カイ, imagine a crab walking sideways on a sandy beach, and you shout 'Hi!' to it: 'Hi' sounds like カイ, and the crab waves a claw back.