Buried in an article about scientists studying effects of artificial light on oysters, is this pearl of wisdom:

How oysters see is a bit of a mystery. While related bivalves, such as scallops, have eye-like organs, oysters likely use patches of specialized cells on their skin to detect light, though scientists have yet to identify the cells or figure out exactly how they might work.

Seems important, yet research only started about ten years ago? Hard to believe. It brings to mind how the first robots in the 1950s used cells to detect light.