Scallops are cosmopolitan, bottom-dwelling, marine bivalves, found in all the world’s oceans. Scallops can swim. Many can rapidly spurt short distances to escape prey. Some scallops migrate.
Like all bivalves, scallops lack brains. Scallops suffice with a nervous system controlled by 3 paired clusters of nerve cells (ganglia). The most extensive ganglia group connects to all the tentacles and eyes that a scallop has.
A scallop has up to 200 eyes, each about the size of a poppy seed. Rather than a lens, each eye acts as a mirror, similar to the reflector telescope invented by Isaac Newton. Some crustaceans and deep-sea fish also have mirror eyes.
A scallop’s optical mirrors are formed of guanine, a nucleobase. Each mirror has 20 to 30 layers of crystalline squares, separated by thin layers of cytoplasm. Each layer is composed of tightly packed crystal plates, precisely arranged so that each micron-wide square lies directly beneath another, forming a vertical stack.
The most complex optical function of guanine crystals in Nature is in image formation. This function demands an extremely high degree of ultrastructural organization because light must not only be reflected but also focused. The hierarchal organization of the scallop mirror is finely tuned for image formation, from the component guanine crystals at the nanoscale to the overall shape of the mirror at the millimeter level. ~ Israeli biologist Benjamin Palmer et al
Every scallop eye has millions of crystal squares, each just over 1 micron (μm). These tiles themselves don’t reflect light: they are transparent, but their arrangement fantastically turns them collectively into a mirror.
As a ray of light passes through the layers of tiles and fluid, it gets bent further and further from its original direction. The light eventually reverses its direction and heads back toward the front of the eye.
The scallop eye has 2 retinas: one for direct gaze, the other for peripheral vision. The optical axes of the mirror and the retina are not aligned. While scallop eyes are completely different from mammalian eyes, they function alike: an enigmatic functional convergence given the structural disparities.
There is a striking correspondence between the apparent functions of the scallop’s distal and proximal retinas and the cone and rod cells of mammals. ~ Benjamin Palmer et al
A scallop’s mind collates the disparate inputs and creates a panoramic view. A sharp central view allows scallops to quickly recognize oncoming predators. Well-focused peripheral vision helps a scallop find the perfect spot to settle down to feed. A scallop has a range of vision spanning ~250°.
Chameleons manipulate guanine crystals just under their skin to change their color at will. Scallops mentally control their mirror eyes at nanoscale to see.
The scallop controls the crystal morphology and spacing to produce a tiled multilayer mirror with minimal optical diffraction aberrations, which reflects wavelengths of light that penetrate its habitat and are absorbed by its retinas. The mirror forms functional images on both retinas. ~ Benjamin Palmer et al
There is no physiological explanation for how scallops can control their mirror eyes, or how the simple nerve network that a scallop has can create excellent vision from such a strange arrangement.