fresh-water clam
蜆 centers on a specific small freshwater shellfish: the clam. It refers to the creature itself rather than an abstract concept.
蜆 combines 虫 (insect/creature) with 見, which likely contributes the sound. The character was created to name a specific shellfish.
The insect radical 虫 on the left suggests a small creature, and 見 on the right looks like a shell with a seeing eye. Picture spotting a freshwater clam peeking out from the mud.
For ケン, imagine Ken the clam-digger finding a freshwater clam: Ken -> ケン, and he sees the clam with his sharp eyes.
basket clam (Corbiculidae spp.); freshwater clam; freshwater mussel
Corbicula japonica (species of basket clam)
Shijimi clam caught during winter
Sphaerium japonicum (species of fingernail clam)
Corbicula sandai (species of basket clam)
Corbicula leana (species of basket clam)
soup with basket clams (usu. miso soup)
bridal-wreath spirea (Spiraea prunifolia)