Scallop Eyes Mirror Reflecting Telescopes

Three of the eyes of Pecten maximus. The scallop has a unique layered mirrored structure at the back of each eye. | Dan-Eric Nilsson, Lund University

The visual systems of scallops boast a sophisticated arrangement of up to 200 eyes, each of which is strikingly similar to a reflecting telescope or a large radio telescope, say researchers studying the features.

The scallop's eye could inspire new optical devices such as medical diagnostic cameras or wide-view surveillance imaging, just as compound eyes of thousands of small lenses joined together in lobsters and other animals have informed telescope design, Benjamin Palmer of the Weizmann Institute of Science in Rehovot, Israel and colleagues note in the December 1 issue of Science.

Most animals use lenses to focus light onto their retina, a light-sensitive layer of tissue coating the inner portion of the eye. However certain marine organisms, including the Pecten scallop analyzed in the study and deep-sea fish like the brownsnout spookfish, have evolved mirrors to create images. Lenses and mirrors treat light differently — while lenses manipulate light using a process called refraction, mirrors reflect light.

Using various microscopic imaging approaches, the scientists found that the unique spatial vision in the scallop is achieved through the mirror's layered structure located at the back of each eye, which is tiled with a mosaic of square-shaped crystals arranged and fine-tuned to reflect wavelengths of light that penetrate its habitat.

Palmer and his colleagues note their work demonstrates the remarkable control the scallop exerts over the growth and arrangement of crystals to make a highly reflective mirror capable of forming functional images.

"There are a couple of key features of the scallop's concave mirror system that make them interesting from a bio-inspired materials [development] point of view," said Palmer.

"The scallop has a very compact visual system — each eye is only approximately one millimeter in diameter. It would be extremely difficult to make a lens of this sort of size with a high enough refractive power to bend light onto the retina, especially in sea water, which makes this process even harder. The scallop uses a mirror to focus light instead of a lens, to circumvent this problem," he said.

The mirror forms images on a double-layered retina, to separately image both peripheral and central fields of view. Previously, it was thought that only the upper retina of the scallop was used for image formation. "We believe the two retinas may be used for different functions," Palmer said. "The lower retina seems to be used for imaging the peripheral field of view, whereas the upper retina is used for imaging moving objects in bright light conditions and is the primary sensor for the scallop escape response mechanism."

The mirrored eyes of scallops and other marine animals are often extremely efficient light-collectors and are very light sensitive compared to similarly-sized lens-based systems. Concave mirrored eyes collect light from a very large field of view.

The scallop Pecten maximus on the sea floor. | Ceri Jones, Haven Diving Services]

What's more, scallops also have many eyes with overlapping fields of view. These eyes have an optical nerve which connects to a central nerve hub in the scallop. Previous research has hinted that scallops can process the information from multiple eyes to "compute" a better resolved composite image, making up for the limitations in the images produced from a single eye.

Based on these characteristics, Palmer suspects the scallop eye may someday provide inspiration for small scale imaging devices that need to operate in dim-light conditions.

"We are interested in studying the 'color' of the scallop eyes. Our paper revealed that the thicknesses and spacings of the scallop eye's multilayer mirror are optimized for reflecting blue light. This makes sense, because the scallops we found live in water where almost no red-orange light penetrates," Palmer said. "We would like to investigate scallops living in other habits with different light penetration properties, for example, in clearer tropical waters where the irradiance spectrum is more broadband."

Palmer also plans to investigate whether the eyes of scallops from different oceans and at different depths are optimized for the light properties of their environment, and if the mirrors of these organisms can be fine-tuned like the mirrors in other animals like copepods, neon tetra fish and chameleons.