It looks like a honeycomb and there’s no classic star shape in sight, but these bead-like blobs are actually hundreds of starfish embryos that have congregated together and formed an intricate “living crystal”.
The discovery of this behaviour, published in Nature, is something researchers from Massachusetts Institute of Technology (MIT), US, believe could be applied to the development of new robotic systems, where “swarms” of tiny machines work together to perform tasks.
You spin me round: Starfish cells attract each other to form crystals
These “crystals”, which are tiny to the human eye but consist of hundreds or thousands of individual cells, come about thanks to the movement of tiny hairs on the outside of a starfish’s embryonic shell.
These hairs – or cilia – move in a way that causes the embryo to rotate in water, creating a whirlpool effect that pulls others towards it.
In doing so, one starfish embryo can surround itself with six others in a hexagonal formation, all rotating alongside one another.
Over time, more and more get sucked into formation, accumulating and forming intricate living crystals.
The physical interactions of biological entities in this way is known as ”active matter”. This refers to the motion of organisms when they expend energy – in this case the embryos’ spinning movement results in large, crystalline structures.
“It’s absolutely remarkable,” says Nikta Fakhri, an associate professor in physics from MIT who worked on the study.
“These embryos look like beautiful glass beads and they come to the surface to form this perfect crystal structure.”
Embryo masses could lead to robotic swarms
Starfish embryos are useful in studies of developmental biology due to their large, transparent cells.
But this study was conducted by a team of MIT physicists which set out to study the movement of embryos as they continue to undergo cell division.
Exploring further, they found the lattice formations created by neighbouring cells – not dissimilar to the crystalline structures formed between atoms in substances like graphene – endured for hours or days.
“We could see this ‘crystal’ rotating and jiggling over a very long time, which was absolutely unexpected,” Fakhri says.
“You would expect these ripples to die out quickly, because water is viscous and would dampen these oscillations; this told us the system has some sort of odd elastic behaviour.”
Crystals only deteriorate when individual embryos grow larger and change their shape as they work towards becoming our familiar, five-armed marine animals.
It’s when embryos accumulate in this manner that they begin displaying what Fakhri and her colleagues described as “odd” properties, like the strange tendency to spontaneously ripple – a kind of wave cascading across the entire ”honeycomb”.
These movements are the result of hundreds of starfish embryos moving independently in the same direction.
This collective movement, Fakhri says, could present a new opportunity to introduce nature-inspired design to the development of robotic systems.
Just like birds flocking and bees swarming, so too could tiny robots work together in formation to perform tasks.
“Like a flock of birds that can avoid predators or fly more smoothly because they can organise in these large structures, perhaps this crystal structure could have some advantages we’re not aware of yet,” says Fakhri.
“You can play with this design principle of interactions and build something like a robotic swarm that can actually do work on the environment.
“I can encode that into small robots and have a collection of these robots … and end up actually building a robotic ‘swarm’.”
Matthew Agius is a science writer for Cosmos Magazine.
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