Scallop’s amazing mirror eyes
The scallop is an edible saltwater clam with a distinctively shaped shell. But unusually for shellfish, it has a complex visual system. Despite not having a real brain, it has up to 200 eyes, each only about 1 mm across.
These eyes are quite unusual: instead of a lens to focus light into an image, the scallop eye has a mirror for focusing. The leading creationist scientist Sir Isaac Newton invented the first man-made reflecting telescope, but the scallop had it first, on a tiny scale. And recent research from the Weizmann Institute of Science in Israel, using cryo-electron microscopy, shows how sophisticated these eyes are. As they say, the image formation
“demands an extremely high degree of ultrastructural organization because light must not only be reflected but also focused. The hierarchical 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 millimetre level.”1
That is, guanine, one of the DNA ‘letters’, is also the main reflecting surface. But the scallop manages to grow the crystals in a square shape, although guanine normally forms prisms. This means that they are like tiles fitting closely together, and resemble “the segmented mirrors of reflecting telescopes”.1
Also, guanine is mirror-like only when it’s in thin multilayers, otherwise it’s transparent.2 The scallop has 20–30 layers of the tiled guanine alternating with cytoplasm, the material inside cells. The tiles are very thin, only 74 nanometres (nm) thick, and separated by cytoplasm 86 nm thick. This means that it is almost perfectly efficient at reflecting blue-green light with a wavelength of 500 nm. This ‘happens’ to be around the peak colour of the light that penetrates the water to reach the scallop, as well as the colour that the eyes are most sensitive to.
Furthermore, the eye is not symmetrical around the axis. This is not a fault, but a designed feature. It produces two separate images focused onto two retinas. Objects in the centre of view produce images on the distal retina (further from the body), while peripheral images form on the proximal mirror (nearer to the body). The distal retina is good at detecting movement, so the scallop can escape a predator. The proximal retina is more sensitive, and provides information about the scallop’s surroundings.
The report concludes,
“The crystal morphology, multilayer structure, and 3D shape of the scallop’s eye mirror are finely tuned to produce functional images on its two retinas.”1
It continues by explaining that this design might
“pave the way for the construction of novel bio-inspired optical devices. In particular, … the development of compact, wide-field imaging devices derived from this unusual form of biological optics.”1
Animal eye expert Daniel Speiser, from the University of South Carolina, is amazed that such a simple shellfish needs such advanced visual technology, “It’s still a puzzle why they see so well.”2 Of course, the newspaper report contains the required nod to evolution:
“His study shows that scallops have evolved a mastery over forming crystals, guiding them into shapes that researchers didn’t think possible.”2
But how could ‘mastery’ have evolved by slow and gradual changes generated by accidental changes to existing genetic information (mutations), each an advantage over the previous stage? Natural selection selects only for survival advantage, while the scallop seems to have much more vision than it needs. Like thousands of other features in living things, the scallop eye testifies to the engineering skill of its Creator.
References and notes
- Palmer, B.A. and nine others, The image-forming mirror in the eye of the scallop, Science 358(6367):1172–1175, 1 December 2017 | doi:10.1126/science.aam9506. Return to text.
- Zimmer, C., The scallop sees with space-age eyes—hundreds of them, New York Times, 30 November 2017; nytimes.com. Return to text.