shellfish
貝 centers on shellfish: the hard-shelled sea creatures themselves, and by extension the shells that were historically used as currency.
貝 is a pictograph of a shellfish with its shell and soft body. The modern form retains the shell-like shape with a closed top and dangling lower parts.
The top box is the hard shell, and the two strokes below are the soft body peeking out. Picture a clam opening slightly: 貝 is a shellfish.
For バイ, imagine buying a shellfish at the market: buy -> バイ, and you get a fresh clam.
shellfish
seashell; shell
shell heap; shell mound; kitchen midden
shellfish
Korean mussel (Mytilus coruscus)
adductor muscle
chambered nautilus (esp. species Nautilus pompilius)
(edible) mussel (esp. the Mediterranean mussel or blue mussel)
bivalve
conch (esp. Charonia tritonis); trumpet shell
blood clam (Scapharca broughtonii); ark shell
snail; spiral shell
univalve
cowrie; cowry
Japanese scallop (Patinopecten yessoensis); Yesso scallop
scale insect
cockscomb pearl mussel (Cristaria plicata)
basket clam (Corbiculidae spp.); freshwater clam
whelk (esp. Neptunea and Buccinum spp.)
to blow a conch; to blow a trumpet shell; to blow the war horn