The chiton mollusc may have provided a unique way to create multifunctional materials. The armour of this mollusc has been examined and results suggest this may also function as discernible eyes. A possibility may exist to create comparable manmade multifunctional materials.
Chiton molluscs are unique among highly developed molluscs as these creatures have an absence of a calcareous shell, instead the back of the chiton is protected by strong cuticula of the mantle, its “armour”. This mantle extends only over the perimeter of its backside, known as the girdle, eight overlapping calcareous shell plates protect the chiton from predators and allow it to roll into a ball. The plates may be akin to a gauntlet glove. In the dorsal shell plates are sense organs which may be connected to several micro-aesthetes and where eyes are embedded in the animals’ protective shell. The animal is unique in other respects as it may employ its teeth crystals for magneto-reception, the ability to sense the earth’s magnetic field and utilise this information for navigation.
Most eyes existing in nature seem to be made of organic materials, the chiton’s eyes however are inorganic and composed of a crystalline mineral named aragonite. This allows the animal to sense changes in light and react to any predators in the environment. Joanna Aizenberg and colleagues first suspected these crystals held the key to how sea dwelling brittle stars responded to predators in response to light changes, when compared to other organisms without these structures. The hypothesis was; the crystals may focus light onto photoreceptive cells and the team demonstrated how this may be by examining photosensitive material after light exposure. The remarkable element is how the brittle star achieves sight without a brain although still possesses a complex visual system. This previous research prompted new research into chitons.
The new study by Li and colleagues from the Wyss institute for biologically inspired engineering at Harvard university and MIT, aimed to demonstrate how the chitons armour also acted as a source of vision for the animal. The team achieved this by first employing optical experiments to show the hundreds of aragonite lenses may actually form images. How the light scattering is reduced by the polycrystalline lenses may be determined by the crystal structure and how this affects light reception. The anatomy of the eye was examined finding an outer cornea, lens and photoreceptive cells needed for transmitting images to the chitons nervous system. The most important finding was aragonite crystals are larger in the lens than in the protective shell and are highly organised for consistent and distinct light perception.
“By studying isolated eyes, we identified how exactly the lens material generates a defined focal point within the chamber which, like a retina, can render images of objects such as predatory fish,” Said Ling Li, co-first author of the study. “The team also learned how optical performance was developed as a second function to the otherwise protective shell with mutual trade-offs in both functionalities. The material properties are favoured for mechanical robustness so the evolving chiton had to balance out its mechanical vulnerabilities by limiting the size of the eyes and placing them in regions protected by strong protrusions,” commented Li.
Multifunctional materials may have an abundance of practical applications. The researchers speculated how the findings extracted from the functions of the chiton may be used to develop sensory materials with the aim of incorporating these into buildings. This may allow the material to sense light, wear and tear and environmental conditions. Additionally, the findings might support improved drug transport and bioengineered organs.
“This study shows how amazing nature is at solving complex challenges in simple and elegant ways. By uncovering the design rules this simple organism uses to self-assemble a multi-functional shell that simultaneously provides physical protection from the environment and an eye capable of sensing oncoming invaders, the team is now in a position to leverage these insights to engineer synthetic materials which may lead to entirely new solutions for both industrial and medical applications,” said Wyss institute founding director Don Ingber.
How may life provide solutions for more dynamic and efficient materials?