蛤 combines 虫 (insect/creature) with 合, which likely provides the sound コウ. The character denotes a clam, a shellfish categorized under the broad 'creature' radical.
Meaning mnemonic
A creature (虫) that fits together (合) like two halves of a shell: that's a clam.
Reading mnemonic
For コウ, imagine a clam shell closing with a hollow 'koh' sound: koh -> コウ, as the two halves come together.
