Starfish embryos swimming in formation like a "living crystal" could influence the design of self-assembling robot swarms

Starfish embryos swimming in formation like a “living crystal” could influence the design of self-assembling robot swarms

MIT scientists have observed that as multiple starfish embryos swirl to the surface, they attract each other and spontaneously assemble into an organized, crystal-like structure. Image credit: Courtesy of the researchers, colorized from MIT News

In its earliest stages, long before it sprout its distinctive appendages, a starfish embryo resembles a tiny bead that rotates through the water like a miniature ball bearing.

Now, MIT scientists have observed that as multiple starfish embryos swirl to the water’s surface, they attract each other and spontaneously assemble into a surprisingly organized, crystal-like structure.

Stranger still, this collective “living crystal” can exhibit a strange elasticity, an exotic property where twisting of individual units — in this case, embryos — triggers much larger ripples across the entire structure.

The researchers found that this rippling crystal configuration can persist for relatively long periods of time before dissolving into individual mature embryos.

“It’s absolutely remarkable — these embryos look like beautiful glass beads and come to the surface to form this perfect crystal structure,” says Nikta Fakhri, Thomas D. and Virginia W. Cabot Career Development Associate Professor of Physics at MIT. “Like a flock of birds that can avoid predators or fly more smoothly because they can organize themselves into these large structures, maybe this crystal structure could have some advantages that we don’t yet know about.”

MIT scientists have found that starfish embryos spontaneously swim together at the surface, forming large crystal-like structures that collectively ripple and twist for relatively long periods of time before dissolving as mature embryos. Photo credit: Massachusetts Institute of Technology

Beyond starfish, she says, this self-assembling, rippling array of crystals could be applied as a design principle, such as in building robots that move and function together.

“Imagine building a swarm of soft, spinning robots that can interact with each other like these embryos,” says Fakhri. “They could be engineered to organize themselves to ripple and crawl through the sea to do useful work. These interactions open up a new set of interesting physics to explore.”

Fachri and her colleagues have published their findings in a study published today in Nature.

Spin together

Fakhri says the team’s observations of starfish crystals were an “accidental discovery.” Her group has studied how starfish embryos develop and in particular how embryonic cells divide in the very early stages.

“Starfish are one of the oldest model systems for studying developmental biology because they have large cells and are optically transparent,” says Fakhri.

The researchers observed how embryos swim as they mature. After fertilization, the embryos grow and divide, forming a shell from which tiny hairs or cilia sprout, propelling an embryo through the water. At a certain point, the cilia coordinate to rotate an embryo in a specific direction of rotation, or “chirality”. Tzer Han Tan, one of the group members, noticed that as the embryos floated to the surface, they kept rotating towards each other.

“Every now and then a small group would get together and dance around,” says Fakhri. “And it turns out there are other marine organisms that do the same thing, like some algae. So we thought this is fascinating. What happens when you bring a lot of them together?”

In their new study, she and her colleagues fertilized thousands of starfish embryos and then watched them swim to the surface of shallow bowls.

“There are thousands of embryos in a dish, and they start to form this crystal structure, which can get very large,” says Fakhri. “We call it a crystal because each embryo is surrounded by six adjacent embryos in a hexagon that is repeated throughout the structure, very similar to the crystal structure in graphene.”

wobbly crystals

To understand what might cause embryos to assemble like crystals, the team first looked at a single embryo’s flow field, or the way water flows around the embryo. To do this, they placed a single starfish embryo in water, then added much smaller beads to the mixture and took pictures of the beads as they floated around the embryo on the water’s surface.

Based on the direction and flow of the beads, the researchers were able to map the flow field around the embryo. They found that the cilia on the embryo’s surface beat in such a way that they turned the embryo in a certain direction and created vortices on either side of the embryo, which then attracted the smaller globules.

Mietke, a postdoc in Dunkel’s applied mathematics group at MIT, processed this flow field from a single embryo to a simulation of many embryos and ran the simulation forward to see how they would behave. The model produced the same crystal structures that the team observed in their experiments, confirming that the crystallization behavior of the embryos was most likely a result of their hydrodynamic interactions and chirality.

In their experiments, the team also observed that a crystal structure, once formed, persisted for days, during which time spontaneous waves began to propagate across the crystal.

“We could see this crystal spinning and jiggling for a very long time, which was totally unexpected,” she says. “One would expect these waves to decay quickly because water is viscous and would dampen these oscillations. That told us that the system had strange elastic behavior.”

The spontaneous, long-lasting waves could be the result of interactions between the individual embryos, turning in opposite directions like interlocking gears. With thousands of gears turning in the crystal formation, the many individual spins could trigger a larger, collective movement across the entire structure.

The researchers are now investigating whether other organisms, such as sea urchins, show similar crystalline behavior. They are also investigating how this self-assembling structure could be replicated in robotic systems.

“You can play with this design principle of interactions and build something like a swarm of robots that can actually work on the environment,” she says.

Researchers build embryo-like structures from human stem cells

More information:
Nikta Fakhri, Strange Dynamics of Living Chiral Crystals, Nature (2022). DOI: 10.1038/s41586-022-04889-6.

Provided by the Massachusetts Institute of Technology

This story is republished with permission from MIT News (, a popular website covering news about MIT research, innovation, and teaching.

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