Scallops are marine bivalves with an unexpectedly sophisticated visual system. Unlike many other bivalves that remain largely stationary, scallops are mobile and possess unique adaptations for sensing their environment. Their distinctive visual capabilities set them apart in the underwater world. These creatures navigate their surroundings and detect threats using a surprisingly complex array of eyes.
The Many Eyes of the Scallop
Scallops possess numerous small, strikingly blue eyes lining the edge of their mantle, the soft tissue just inside their shell. The number of eyes can vary significantly, with some species having up to 200 individual eyes, each about one millimeter in size. These bead-like structures are clearly visible and are arranged along the mantle, acting as a kind of underwater necklace.
How Scallop Eyes See
Scallop eyes function in a manner different from human eyes, which use a lens to focus light. Instead, scallop eyes operate more like a reflecting telescope. Light enters the eye, passes through a pupil, a lens, and two retinas before reaching a concave mirror at the back. This mirror, composed of millions of perfectly square guanine crystals arranged in 20 to 30 layers, reflects light onto the retinas.
The scallop eye features two distinct retinas: a proximal retina, closer to the mirror, and a distal retina, further away. The distal retina is responsible for forming more focused images, enabling the scallop to discern shapes. The proximal retina, meanwhile, is more sensitive to light levels and becomes particularly important in dimmer conditions. This dual-retina system allows scallops to process different aspects of the visual information, providing both sharp images and sensitivity to light changes.
The Purpose of Scallop Vision
Scallop vision is primarily adapted for detecting changes in light and shadow, which is crucial for their survival. They use their eyes to sense the movement of potential predators, such as sea stars, allowing them to initiate an escape response. Upon detecting a threat, scallops can rapidly clap their shells together, creating a jet propulsion that allows them to swim swiftly and erratically away.
Their visual system also assists scallops in detecting subtle environmental cues. This includes changes in water currents or the type of seabed, which can influence their movement and where they settle. While scallops have a relatively simple nervous system, their eyes provide sufficient information for these vital behaviors.