Often, physics can be used to make sense of the natural world, whether it's understanding gravitational effects on ocean tides or using powerful physics tools, like microscopes, to examine the inner workings of the cell. But increasingly, scientists are looking at biological systems to spark new insights in physics. By studying squid skin, researchers have identified the first biological instance of a physical phenomenon called 'hyperdisorder', bringing new understanding into how growth can affect physics.
Published in PRX, an interdisciplinary team from the Okinawa Institute of Science and Technology (OIST) studied the effect of growth on pattern development within squid skin cells. By combining experimental imaging methods with theoretical modeling, they found new insights into the unusual arrangement of these cells, and created a general model of hyperdisorder applicable to a wide variety of growing systems.
Studying arrangements: uniformity and disorder
Hyperdisorder occurs in systems where the variance in number of points within a particular measured space grows faster than the volume of that measured space. Essentially, when you're looking at a tiny area, the system may appear quite ordered, but fluctuations are exacerbated when viewing at a larger scale.
"In other growing systems, such as the cells in chicken eyes, studies have previously seen hyperuniformity, whereby there is long range order and patterning, despite randomness at a close scale," said Dr Robert Ross, OIST Interdisciplinary Postdoctoral Scholar, first author on this study. "This is what we expected to see in the squid. But what we actually observed was completely different, and we have not yet seen any other instances of this packing behavior in biology. However, we think such disorder is very likely to be present in similar growing systems, highlighting the importance of growth on physical properties".
Making sense of disorder
In this study, the researchers observed squid over 12 weeks, using an experimental rig to capture 3D images of the squid, to study the appearance of specialized cells, called chromatophores, on its skin surface. "The chromatophores appear at fixed positions in relation to one another, in a specific pattern," explained Professor Sam Reiter, head of the Computational Neuroethology Unit and co-author on this study. "They are essential in camouflage and communication. Therefore, we were interested in studying the spatial arrangement and the pattern development of these cells."
To understand the physics governing the observed hyperdisorder, the team developed a mathematical model, using hard disks on a growing surface to represent the behavior of the squid skin. Despite the apparent complexity of this problem, they were able to devise a very simple, generally-applicable model.
Co-author Professor Simone Pigolotti, head of the Biological Complexity Unit, said, "This study exemplifies the importance of growth on the physical behavior of different systems, and the unique knowledge that can be gained by studying from interdisciplinary perspectives. We look forward to applying our model to other growing systems, both biological and beyond. The general nature of this model means there are endless possible scientific directions to take."
