crab
蟹 centers on the crab as an animal, with no major abstract extensions beyond the literal creature.
蟹 combines 虫 (insect/creature) with 解, which likely contributes the sound and may relate to the crab's sideways movement or shell-shedding idea, though the exact semantic link is uncertain.
The top 解 suggests something coming apart or being solved, and the bottom 虫 is a creature. Picture a crab that can solve the puzzle of escaping its shell, or one that moves sideways as if dismantling its own path.
For カイ, imagine a crab scuttling sideways along a dock, and someone shouts 'Kai!' as it nips at their toe: Kai -> カイ.
crab
horseshoe crab
red king crab (Paralithodes camtschaticus)
snow crab (Chionoecetes opilio); queen crab
crab-canning boat
Japanese freshwater crab (Geothelphusa dehaani)
Dungeness crab (Metacarcinus magister)
fire crab (Pyromaia tuberculata)
Chinese mitten crab (Eriocheir sinensis)
Chinese mitten crab (Eriocheir sinensis)
soldier crab (Mictyris brevidactylus)
Mediterranean green crab (Carcinus aestuarii)
Zosimus aeneus (species of poisonous crab)
Japanese mud crab (Macrophthalmus japonicus)
Geothelphusa shokitai (crab species)
striped shore crab (Pachygrapsus crassipes)
Cancer (4th zodiacal sign); the Crab
Heike crab
swimming crab (Ovalipes punctatus); sand crab
Sesarmops intermedium (species of grapsoid crab)