How shellfish create a perfectly symmetrical pearl

And why are they better than humans with all their technology? Sharing the squeeze from the new scientific research

During research aimed at creating highly efficient nanomaterials, scientists at the University of Michigan noticed that molluscs build ultra-strong structures with a level of symmetry that surpasses everything else in the natural world (with the exception of individual atoms)

The researchers found that the symmetry of the pearl becomes more and more precise as it is built, thus answering the long-standing question of how the chaos in the center of the pearl translates into a perfect structure on the outside.

“We humans, for all our power, cannot create anything with nanoscale architecture as complex as a pearl,” said Robert Howden, assistant professor of materials science and engineering at the University of Michigan and author of the article. “We can learn a lot by studying how pearls evolve from a disorderly nothingness to this wonderfully symmetrical structure.”

Layers of mother-of-pearl, an iridescent and extremely durable organic-inorganic composite that also makes up the shells of oysters and other molluscs, are built on a shard of aragonite that surrounds the organic center. The layers, which make up more than 90% of the pearl’s volume, become thinner and closer together as they move away from the center.

Perhaps the most surprising finding is that clams maintain the symmetry of their pearls by adjusting the thickness of each layer of mother-of-pearl. That is, each new layer of mother-of-pearl built over the asymmetric center precisely adapts to the previous ones, smoothing out irregularities and forming a round pearl. It is important here that the mollusk regulates the thickness of the mother-of-pearl layers. If one layer is particularly thick, subsequent layers will be thinner. This helps the pearl maintain the same average thickness over thousands of layers so that it looks perfectly round and uniform. Without this constant adjustment, the pearl would look uneven, and small imperfections intensify with each new layer, which would reduce its spherical shape. The pearl below contains 2,615 thin coats of mother of pearl applied over 548 days.

Explanation of the illustration above

a. Kesha pearl cut into pieces.
b. The enlarged cross-section of the pearl shows its transition from a disordered center to thousands of layers of finely matched mother-of-pearl.
c. Increasing the layers of mother-of-pearl shows their self-correction – when one layer is thicker, the next is thinner to compensate, and vice versa.
d, e: Atomic scale images of nacre layers.
f, g, h, i: Microscopic images detail the transitions between the layers of the pearl.

There is an interaction between each layer, and scientists speculate that this interaction allows the system to correct itself as more and more layers of mother-of-pearl build up.

The team also revealed details of this interaction. Mathematical analysis of the pearl layers revealed a phenomenon known as flicker noise (1 / f noise, excess noise). That is, there is a series of events that seem random, but each new event depends on the previous one. Flicker noise has been observed to govern a wide range of natural and anthropogenic processes, including seismic activity, economic markets, electricity, physics, and even classical music.

For example, when you roll the dice, each roll is completely independent and has nothing to do with any other roll. Flicker noise is different in that each event is linked. “We cannot predict this, but we can see the structure in chaos. And within this structure are intricate mechanisms that allow thousands of layers of the pearl’s nacre to combine to create a harmony of order and precision, ”says Robert Howden.

This photo shows how the layers of mother-of-pearl become more even as they move away from the center of the pearl.
This photo shows how the layers of mother-of-pearl become more even as they move away from the center of the pearl.

However, scientists have found that pearls are not enough long-range order, that is, that carefully planned symmetry that ensures the consistency of hundreds of layers in brick buildings. Instead, pearls exhibit medium order, maintaining symmetry for about 20 layers at a time. This is enough to maintain consistency and durability over the thousands of layers that make up a pearl.

The researchers conducted a study looking at keshi pearls harvested from Akoya pearl oysters (Pinctada imbricata fucata) off the east coast of Australia. They found that mother-of-pearl heals itself, and when a defect occurs, it is removed without external intervention – if we are talking about damage within several layers.

A cross-section of a keshi pearl shows how a round gem grows around a deformed core of debris.
A cross-section of a keshi pearl shows how a round gem grows around a deformed core of debris.

These pearls were chosen with a diameter of about 50 millimeters, because they are formed in a natural way, unlike pearls grown from beads, which have an artificial center. Each pearl was cut with a diamond wire saw into sections with a diameter of three to five millimeters, then polished and examined under an electron microscope. The use of “natural” pearls has increased the credibility of the study.

According to representatives of the research group, the results of the study could help create next-generation materials with precise layered nanoscale architecture. That is, oysters will teach people to create stronger and lighter materials.


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