The oyster has two rows of interlocking, short tentacles along the outer edges of the mantle. Like the tentacles of other intertidal critters, they can sense temperature changes, floating debris, and chemicals in the water. They also serve as rudimentary eyes, sensing changes in the light level.
When I poke at an oyster with my handy wooden chopstick, it slams the shells shut immediately. I don't actually have to touch it; as long as the chopstick is within a few millimetres, the oyster reacts. Whether it "sees" the chopstick's shadow, or "smells" the wood, I can't tell. It doesn't seem to mind crabs and hermits wandering all over it, unless a careless foot pokes right inside the mouth.
Pigment cells are concentrated along the free edge of the mantle and in the tentacles in a band varying in color from light brown to jet black. (NOAA)
This one has an attractive marbled pattern. Not all oysters, not even all same-species, same-environment oysters have matching patterns or colours.