Starfish embryos can spontaneously form into giant, waterborne crystals with exotic properties. These crystals can last for many hours before gradually dissolving as the embryos develop.
Tzer Han Tan at Harvard University and Alexander Mietke at MIT, and colleagues, made their discovery while studying how developing starfish embryos interact with each other. “Our experimental observations show how, over the course of their natural development, thousands of swimming embryos come together to form living chiral crystal structures that persist for many hours,” they say.
Starfish begin life as single fertilized cells that divide rapidly and then fold in on themselves in a way that begins to reveal the starfish’s eventual symmetry. At this stage, with embryos just a few hours old, the cilia on the cell surfaces begin to synchronize causing the organisms to rotate slowly, all in the same direction. The embryos also move towards the water surface, where their “top” surfaces naturally become aligned with the air-water interface.
That’s when self-organized patterns can emerge. “Groups of embryos aligned in this manner can spontaneously self-organize into two-dimensional hexagonal clusters,” say Tan, Mietke and colleagues. “Over time, these clusters grow into larger crystals, reaching sizes of hundreds to thousands of embryos and persisting for tens of hours.”
These crystals have well defined properties. For example, they rotate as a result of the embryos' combined turning effect. They also undulate due to a recently discovered property of certain active materials known as odd elasticity. This allows self-sustained waves to propagate through a material, in this case driven by small differences in the rotation between embryo pairs.
Clearly, these crystals are unlike any that form in conventional circumstances. Most metals, rocks, ceramics and ices are crystalline in nature. These crystals are held together by the attractive forces between atoms and molecules that, in certain circumstances, allow them to form highly periodic lattice structures.
Living Chiral Crystals
The formation and dissolution of these living chiral crystals is driven entirely by the embryos' development process. “A striking feature of the living chiral crystals is that they nucleate, grow and dissolve naturally as embryos progress through their development,” say Tan, Mietke and colleagues.
As the embryos grow older, they begin to change shape. This influences the fluid flow around them, which reduces the order within the crystal. “After about 30 hours, disorder begins to dominate and the crystal slowly dissolves over a period of 10 hours,” they say.
The discovery raises interesting questions about the evolution of these crystalline structures and what kind of advantages they offer to starfish. Other species also form crystals. For example, some bacteria form crystalline-like structures and certain kinds of algae, such as volvox, form into spherical colonies made up of tens of thousands of organisms that are well known to “dance” around each other. There is also a potential connection between this kind of crystalline behavior and flocking or shoaling behavior.
Clearly there is much work to be done. But the idea that our oceans and seas are populated with living crystals will surely provoke the keenest interest for further research.
Ref: Development Drives Dynamics of Living Chiral Crystals: arxiv.org/abs/2105.07507