Larger species have a large gem inside of them, called a gemheart. Greatshells are usually wild and hunted for their gemhearts, edible meat and carapace, although some species are domesticated.
Their blood is violet, and has a musty, moldy smell.
They have no tongue, and a complex set of jaws. Chasmfiends, for example, have some flat platelike teeth for crushing and destroying shells and other, smaller mandibles for ripping off flesh or shoving it deeper into the throat.