Recently I was asked by a fellow reefer "Do clams have eyes? If not how do they know something is close to retract there mantles so quickly?" It was refreshing to be asked a question I had not heard before in my 20+ years in the hobby nor did I know the answer. The question kept returning to my thoughts throughout that day so I began researching the topic and not only do tridacnid clams have eyes, their eyes even contain zooxznthellae.
Tridacna clams have hundreds of eyes along the edges of their siphonal tissue (mantle). T. crocea and T. maxima can also have eyes on top of raised tubercles scattered over the mantle surface. These eyes are used mostly to detect shadows, which warn the clam of potential predators. The eyes are also sensitive to green, blue, and ultraviolet light. This helps the clam to position itself toward the light to expose as much zooxanthellae as possible. The eyes may also function to detect excessive amounts of potentially harmful UV wavelengths.
So next time your gazing upon that awesome clam at the local fish store remember it may very well be looking right back at you.