Readings & Meanings:
I gather seashells on the shore.
The point is, this heap of seashells is the remains of an ancient people's trash site.
I almost died of seafood poisoning.
貝 is its own radical, Kai-hen. It's from a pictograph of a clamlike shellfish. The original pictograph split the box in half vertically, to more resemble a bivalve.
As we've seen in all the kanji we've already looked at with Kai-hen, this radical almost always imparts the meaning of "goods, wealth, money, commodities," because shells used to be used as currency.