clam, oyster
鮚 centers on shellfish with hard shells, specifically clams and oysters. The meanings are closely related as types of bivalve seafood.
鮚 combines 魚 (fish) with 吉, which likely acts as a phonetic component. The character is used for shellfish, though the exact historical development is uncertain.
The left side is 魚 (fish), and the right side is 吉 (good luck). Imagine a lucky fish that brings you clams and oysters from the sea.
For キツ, picture a clam snapping shut with a sharp 'kitsu!' sound. The kitsu sound cues キツ while the clam protects its pearl.