Starfish Embryo Spins Into Formation Like Living Crystals

Tiny blobs of jelly spin perfect circles in the water – their movement produces a force that attracts their neighbors. When enough of them come together, this synchronized dance aligns them into a precise six-sided, regular, repeating pattern, much like the carbon atoms in graphene’s crystal structure.

But these are not atoms or any kind of inanimate object governed solely by the forces of physics – they are living, self-propelled, embryonic starfish (patiria miniata).

The spinning starfish embryos gather into living crystals. (Tan et al., Natural2022)

“We know we have crystals of many materials, but we have never really linked crystal formation to actually living components,” said MIT physicist Nikta Fakhri. Natural.

“This is a truly extraordinary phenomenon that has never been reported before.”

Fakhri, MIT physicist Tzer Han Tan and colleagues studied active matter — a system in which each individual component (such as a bird in a flock, or a cell in a drop of water) uses energy to move, putting the entire group out of balance with its surroundings.

Like ordinary matter, these systems can also have emergent and surprising properties. Starfish ‘crystals’ are no exception.

Each embryo rotates independently to the left using hair-like cilia. When enough individuals gather on the surface of the water, spontaneous ripples begin to flow through the structures they form.

It was a very large-scale wave, as big as the crystal itself, explained Fakhri.

Ripple model with arrows and color code to indicate the direction of each embryo. (Tan et al., Natural2022)

“We could see these crystals spinning and wobbling for a very long time, which was completely unexpected,” said Alexander Mietke, a theoretical biophysicist at MIT and the study’s author.

“You would expect these ripples to die off quickly, because water is thick and would dampen these oscillations. This tells us that the system has some kind of strange elastic behavior.”

The researchers modeled the movements of the embryos to see how their spins pulled the water toward themselves. This in turn attracts their circling neighbors towards them as well, with a reciprocal force of attraction.

But that water, eventually flowing downwards, also creates another unreciprocated horizontal force in the system. This causes the entire crystal structure to rotate as well.

Both the overall rotation and the waves exhibit odd elasticity physics properties. This is strange because according to the laws of classical physics, simply changing the shape of an isolated solid object, such as a crystal floating in water, should not under certain circumstances move it through the environment.

However, if an object is made up of active components, this change in overall shape can have some interesting mechanical properties.

At least, that’s the theory. In reality, this is still an area of ​​physics that is being explored.

There are several interesting possibilities that arise in nature. A similar phenomenon has also been observed previously in algae.

The rotating crystal-like structure disappears after about 30 hours. (Tan et al., Natural2022)

Once a baby starfish forms a floating, living crystal structure, it holds itself together for days before embryonic development changes it enough to destroy the pattern.

Better understanding these animate physical properties can help us develop new technologies.

“Imagine building a swarm of soft, rotating robots that can interact with each other like these embryos,” said Fakhri.

“They could be designed to self-regulate to ripple and crawl through the ocean to do useful work. This interaction opens up a whole host of exciting new physics to explore.”

As for why baby starfish galaxies – yes, starfish have the most perfect collective noun! – forming a crystal structure when they come together, is currently a mystery.

However, this phenomenon of living crystals is unlikely to occur naturally in the wild because starfish embryos do not normally roam the surface of the water.

This research was published in Natural.

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