The Secret Disorder of Squids
Many densely packed systems in nature have a hidden structure. Their constituent components appear to be randomly arranged on small scales, but upon zooming out, these patterns become more uniform. This phenomenon is known as hyperuniformity. For example, the Universe becomes uniform on larger scales (see Research News: The Cosmos as a Colloid). Count the number of galaxies in increasingly large swathes of the sky, and the variation grows more slowly than if the galaxies were randomly distributed. Conversely, systems where variation increases at larger scales are hyperdisordered. This phenomenon shows up in soft matter and confined fluids. Now Robert Ross and his colleagues at the Okinawa Institute of Science and Technology, Japan, have reported the first example of hyperdisorder in a biological system—the skin of a baby squid [1].
Scientists looking at densely packed systems have mostly focused on inanimate matter such as colloids, crystals, and glasses. But biological systems differ significantly: They can grow. Although researchers have observed hyperuniformity in some biological examples, such as cones in bird retinas and veins in leaves, growth doesn’t appear to play a role in the emergence of hyperuniformity in these systems. To probe how growth shapes the behavior of dense disordered systems, the researchers needed a crowded living system in which they could track individual components.
Juvenile oval squids, whose skins are dotted with pigment-producing cells called chromatophores that look like a smattering of freckles, provided that glimpse. “Oval squid is a perfect candidate because the chromatophores are very visible and they are on one single layer of the skin,” explains Simone Pigolotti, senior researcher on the study. “And they do not move, so you know they also work as tracers to monitor the surface expansion.”
The researchers photographed squids, all aged eight weeks after hatching, and analyzed the chromatophore patterns on their skins. Bigger cells were surrounded by many smaller ones in a lattice-like pattern, with a roughly consistent separation between all cells. Different patches of skin had varying numbers of cells, and these differences became more pronounced for larger skin areas. This trend is typical of a hyperdisordered system.
To understand the origins of this hyperdisorder, the researchers developed a model that represented squid skin as a linearly growing surface where new chromatophores randomly popped up between older ones—but only if there was sufficient space. Over time, the density of chromatophores on its skin oscillated. Growth expanded the skin, so the cell density dropped; then, new cells appeared in the gaps and brought it back up.
In agreement with the experiment, the cells in the model were arranged in pockets of irregularities spread through a lattice-like, evenly spaced structure. The random insertion of cells created short-scale disorder, which growth exported to large scales. The researchers concluded that the interaction between growth and random cell packing produces hyperdisorder in this system.
According to David Brückner, a theoretical biophysicist at the University of Basel, Switzerland, the study “proposes a new mechanism for how hyperdisordered structures can emerge.” It reveals new biology, too. “While animal patterns are often described in the framework of reaction–diffusion systems, here, local couplings orchestrate the pattern as new cells appear in a tissue during growth,” adds Brückner, who was not involved in the work.
In another experiment, Pigolotti and his colleagues tracked the cells’ growth for several weeks after the squids hatched. As the system grew, the increase in cell size and distances between them was offset by new, smaller cells populating the gaps. Consequently, even as the surface area increased, the cell density remained stable—as did their size distribution. The model showed that, for this to happen, younger cells must grow more slowly than older cells, indicating that individual cells must have some notion of the animal’s age. Brückner says that “it raises really interesting questions as to how this could be achieved mechanistically.”
Further research should probe the mechanisms by which chromatophores track the squid’s age and whether their hyperdisordered scaling serves a functional purpose. The simplicity of the mechanism described in this work, the researchers suggest, means that hyperdisorder could be a feature of other growing disordered systems.
It also underlies the kind of surprises biology can offer. “Often the expectation is to explain biology using physics, but this is a neat example where, by trying to explain a biological phenomenon, we discover some new physics,” said Pigolotti.
–Sachin Rawat
Sachin Rawat is a freelance science writer based in Bangalore, India.
References
- R. J. H. Ross et al., “Hyperdisordered cell packing on a growing surface,” Phys. Rev. X 15, 021064 (2025).