noun
shellfish
Refers to the living animal, especially edible bivalves and mollusks.
この辺りの海では、いろいろな貝がとれる。
Various kinds of shellfish can be caught in the sea around here.
海岸で美しい貝を拾った。
I picked up a beautiful seashell on the beach.
貝殻 specifically means the empty shell, while 貝 can refer to the living shellfish or the shell itself.
The kanji 貝 is a pictograph of a shell, originally used for cowrie shells that served as currency in ancient China. The reading かい is native Japanese.