Shrimp eye design
Reflective nanotechnology could inspire new optical coating
The humble shrimp lives on the ocean bottom with very low light levels. But it can see very well. Now some Israeli scientists at the Weizmann Institute of Science and Ben-Gurion University of the Negev (named after Israel’s first president and prime minister) have discovered the amazing fine tuning that enables shrimp to see in very low light.1
Decapod (10-legged) crustaceans, such as lobsters, shrimp, and crayfish have eyes quite unlike ours. Our eyes use lenses to focus light by refraction (bending), while decapods use mirrors to reflect light to a focus.2 The shrimp eye mirrors are themselves a marvel of intricate design, involving photonic crystals—nanostructures that can manipulate light at the wavelength level.
Tiny hollow layered spheres
The researchers analyzed the eyes of the whiteleg shrimp (Litopenaeus vannamei) under the electron microscope. They showed that the mirror (tapetum) comprised an array of nanospheres an average diameter of 330 nm—smaller than the wavelength of visible light. These are packed in regular arrays, then wrapped around the bottom half of the light receptor (called rhabdoms).
Furthermore, the spheres are made of 8–10 concentric thin layers, totalling 70 nm shell thickness, surrounding a hollow core filled with a watery substance. The layers are formed of single crystal plates of isoxanthopterin—even this requires special growth, because this substance normally forms prismatic crystals. The spheres have a property called birefringence (‘double refraction’), where the amount of bending, given by the refractive index, varies with the direction. Along the radius of the sphere, the index is 1.4, not much larger than water. But along the circumference, the index is “1.96—one [of] the highest refractive indices of any biological material.”1
Optimal design
It turns out that everything about the nanosphere is ideal for reflecting light, and the shrimp eye needs all it can get. The researchers performed computer simulations to compare reflectivity from different possible arrangements. The size of the spheres meant that the layer size had the optimal thickness. The shell thickness of the spheres was also optimal, as was the fact that they were hollow compared to solid spheres of the same size and composition. The birefringence also made for better reflectors than spheres that were isotropic, i.e. having the same refractive index in all directions.
Furthermore, they were optimized for blue light, the only colour of sunlight that penetrates appreciably to the shrimp’s habitat (up to 70 m deep). The structure meant that the light was reflected repeatedly until directed to the right place: the light receptors.
The Israeli researchers concluded:
The size, core/shell ratio and particle packing are also optimized to maximize the back-scattering of the tapetum reflector, both in terms of the intensity and spectral properties.1
Humans had made nothing like this
The researchers noted that there had been theoretical studies about the superior optics of photonic crystals comprising birefringent building blocks. But they had not actually been made in practice, because there was no way of orienting them properly. However, the researchers referred to “the ‘genius’ of the shrimp solution”, using spherical birefringent building blocks, so there was no need to worry how they were oriented. So:
the shrimp represents a unique example of a natural photonic system that exhibits optical properties not explored before synthetically.1
Evolutionary handwaving vs biomimetic reality
One report presented the usual fact-free homage to evolution:
Through millions of years of evolution, shrimps have developed unique-shaped eyes that enable them to see well in their immediate environment—the seabed.3
As usual, it’s a case of: this structure exists, therefore it evolved (and don’t you dare mention design, because this is unscientific, regardless of the evidence for it!).4 They fail to explain how this could have evolved by a series of small, advantageous steps.
But the researchers concluded:
This system offers inspiration for the design of photonic crystals constructed from spherically symmetric birefringent particles for use in ultrathin reflectors and as non-iridescent pigments.1
They may well believe in evolution, but they must admit that design is the only way that humans will ever achieve this structure. And it’s unlikely that human designers would have thought of it unless they had seen it in the shrimp first. So if it takes immense ingenuity just to make the copy, then how much more to make the original?
References and notes
- Palmer, B.A. and 9 others, A highly reflective biogenic photonic material from core–shell birefringent nanoparticles, Nature Nanotechnology 15:138–144, 13 Jan 2020. Return to text.
- Sarfati, J., Lobster eyes—brilliant geometric design, Creation 23(3):12–13, June 2001; . Ashcraft, C., New design innovations from biomimetics, Creation 32(3):21–23, July 2010. Return to text.
- In the Eye of the Shrimp, Weizmann Wonder Wander; wis-wander.weizmann.ac.il, 20 Jan 2020. Return to text.
- Doyle, S., Does biological advantage imply biological [evolutionary] origin? J. Creation 26(1):10–12, 2012. Return to text.
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