Photo credit: Prof. Dan-E. Nilsson, Lund Vision Group, Dept. of Biology, University of Lund
In Sandro Botticelli’s iconic painting, “The Birth of Venus,” the goddess of beauty is depicted standing inside a giant scallop shell. In nature, inside its radiating ridged shell, the scallop resembles an undifferentiated round blob. But the beloved bivalve mollusk is not as simple as meets the eye. The scallop possesses an unusual form of biological optics. A single scallop can have up to 200 eyes!
A scallop’s eyes grow out of its fleshy mantle, in the undulant folds of its upper and lower shells. Each eye perches on top of a thin tentacle that can extend it forward and retract it back. The eyes often possess a brilliant blue hue, and they not only permit the mollusk to perceive light and motion, but to form actual images.
Scallop eyes use a concave mirror to focus light rather than a lens. The mirror is made from twenty to thirty layers of guanine crystals. Guanine happens to be one of the four* chemical building blocks of DNA and RNA — it is the “G” in G,C,A,T — guanine, cytosine, adenine and thymine. Crystals of guanine are rhombic, each having four sides of equal length like a semi-squashed square. The rhombic guanine crystals grow in thin transparent layers with a high index of refraction, allowing some light to pass through and reflect. In the scallop eye, the crystal layers are themselves arrayed in a square mosaic. The remarkable arrangement of the guanine crystals reduces optical aberrations. The scallop can even adjust the layered mirror structure to best reflect wavelengths of light.
The scallop’s visual system is even more complex because it has a double retina. One retina images what’s in front of the scallop, and the other retina images objects and motion in its peripheral field of view.
It has been noted that the concave focusing mirror in the scallop eye bears a striking resemblance to the segmented mirrors of reflecting telescopes. This similarity provides biomimetic inspiration for the further development of compact, wide-field imaging devices.