A new clue to how multicellular life may have evolved
Exploring the fluid dynamics of cooperative feeding
Emory University
image:
Tracer particle tracks from a still image of time-lapse video of a Stentor coeruleus individual.
view moreCredit: Shashank Shekhar
Life emerged on Earth some 3.8 billion years ago. The “primordial soup theory” proposes that chemicals floating in pools of water, in the presence of sunlight and electrical discharge, spontaneously formed organic molecules. These building blocks of life underwent chemical reactions, likely driven by RNA, eventually leading to the formation of single cells.
But what sparked single cells to assemble into more complex, multicellular life forms?
Nature Physics published a new insight about a possible driver of this key step in evolution — the fluid dynamics of cooperative feeding.
“So much work on the origins of multicellular life focuses on chemistry,” says Shashank Shekhar, lead author of the study and assistant professor of physics at Emory University. “We wanted to investigate the role of physical forces in the process.”
Shekhar got the idea while watching the filter feeding of stentors — trumpet-shaped, single-celled giants that float near the surface of ponds.
Through microscope video, he captured the fluid dynamics of a stentor in a liquid-filled lab dish as the organism sucked in particles suspended in the liquid. He also recorded the fluid dynamics of pairs and groups of stentors clumped together and feeding.
“The project started with beautiful images of the fluid flows,” Shekhar says. “Only later did we realize the evolutionary significance of this behavior.”
Shekhar and his colleagues discovered that grouping together benefits a stentor colony as a whole by generating more powerful flows to sweep in more food from a greater distance away.
The stentor’s multicellular-like behavior could be used as a model system to help understand how life evolved from single-cell organisms to complex organisms like humans — made up of trillions of cells with specialized tasks.
Co-corresponding authors of the paper are John Costello, a marine biologist at Providence College in Rhode Island and Eva Kenso, a mathematician in the Department of Aeorospace and Mechanical Engineering at the University of South California, Los Angeles.
The project began in 2014 when Shekhar participated in the physiology program at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, an international center for research and education in biological sciences. Shekhar has since held a visiting position at MBL
“Renowned scientists come there every summer from around the world for organic collaborations," Shekhar says of the MBL. "You have the time and resources to explore extreme questions that capture your interest.”
Shekhar drew particular inspiration from three scientists who became his co-authors on the Nature Physics paper. Costello and Sean Colin (a marine biologist from Roger Williams University) study the biomechanics of marine creatures like jellyfish and zooplankton. Wallace Marshall (a cell biologist from the University of California, San Francisco) uses the stentor as a model organism to explore phenomenon like regeneration — for example, the ability of an octopus to regrow a leg.
“You can chop up a stentor and each tiny piece will become a complete organism within 12 hours,” Shekhar says. “They are fascinating in many ways.”
These single-celled eukaryotes, common in freshwater ponds and streams, are named after the mythological Greek herald Stentor, due to their horn shape.
At the narrow end of the stentor is a gripping mechanism known as a “holdfast” which allows the organism to anchor to a twig, leaf or other organic matter floating in water. The wide end of the stentor is essentially a giant mouth rimmed with hair-like cilia. The cilia beat in the water, generating currents that drive food particles, such as bacteria or algae, into its mouth.
Stentors can secrete a kind of goo from their holdfast end. This goo enables them to stick to organic surfaces and temporarily form into colonies that take on a half-hemisphere shape.
Perhaps the most remarkable thing about stentors is their size. Most human cells are at least 10 times smaller than the width of a human hair. A single-celled stentor, however, is visible to the human eye at about 1-to-2 millimeters long — the widths of the tip of a sharpened pencil or crayon.
The size of stentors makes it easy to record detailed imagery of their behaviors under a microscope.
Shekhar decided to investigate the fluid dynamics involved in the filter feeding of stentors. He first focused on a single stentor, from the species Stentor coeruleus, attached to the surface of a fluid-filled lab dish.
“I added micron-sized plastic spheres to the liquid to see what would happen,” he says.
The tiny, plastic particles served as tracers, making the flows generated by the stentor’s cilia visible. Shekhar captured striking time-lapsed video of twin vortices forming around the mouth of the stentor.
Shekhar wondered if the behavior of stentors to occasionally form pairs or colonies was related to their quest for food.
To test the idea, he videoed the fluid dynamics of pairs of stentors. Their heads swayed towards and away from one another. “I call that movement ‘I love you, I love you not,’” Shekhar says.
As their heads drew together, the flows generated by the two stentors combined into a single vortex that created a stronger current, able to draw in more particles from a greater distance.
Shekhar wondered why the stentors would move their heads apart since having them together seemed to provide a clear benefit.
A similar behavior was observed in colonies of stentors joined into half-hemisphere-shapes. In this configuration, their heads swayed between an array of adjacent partners and generated flows more powerful than the those of pairs.
Forming colonies seemed to further enhance their ability to suck in particles. So why did individual stentors occasionally break away from a group to swim off on their own?
The researchers theorized that weaker stentors benefitted more from joining forces than the stronger ones.
“The colonies are dynamic as the stentors keep changing partners,” he explains. “The stronger ones are being taken advantage of, in a sense. They change partners often so that everyone benefits similarly.”
The researchers developed mathematical models to test this theory in experimental setups through the expertise of Kanso and co-author Haniliang Guo, a mathematician at Ohio Wesleyan University, Delaware.
The results showed that one stentor always gained more advantage than the other in a paired system. And that forming a large colony, including the dynamic relocation of individuals, enhances the feeding flow rate for individual stentors on average.
The findings provide new insight into the selective forces that may have favored the early evolution of multiceullar organization.
“It’s amazing that a single-celled organism, with no brain or neurons, developed behaviors for opportunism and cooperation,” Shekhar says. “Perhaps these kinds of behaviors were hard-wired into organisms much earlier in evolution than we previously realized.”
The stentor project is a new research direction for Shekhar. His lab is known for uncovering insights into actin — a protein that assembles into filaments in living cells and is essential to their mobility.
“The stentor work was a passion project,” Shekhar says. “It’s wonderful to work at your own pace, over many years, on a question that fascinates you and wind up with such beautiful and significant results.”
Stentor colonies in a half-hemisphere shape.
Stentor colonies in a half-hemisphere shape.
Credit
Shashank Shekhar
Shashank Shekhar
Stentor colony [VIDEO] |
Journal
Nature Physics
Method of Research
Computational simulation/modeling
Subject of Research
Animals
Article Title
Cooperative hydrodynamics accompany multicellular-like colonial organization in the unicellular ciliate Stentor
Article Publication Date
31-Mar-2025
“She loves me, she loves me not”: physical forces encouraged evolution of multicellular life, scientists propose
Marine Biological Laboratory
image:
Stentor coeruleus is a giant unicellular, filter-feeding protist that uses the coordinated motion of its oral ciliary structure to generate feeding currents. These currents allow the organism to capture and direct prey toward its oral opening. This image displays tracer particle tracks from a time-lapse recording, revealing the flow patterns generated by an individual S. coeruleus in its immediate vicinity. Credit: Shekhar et al., Nature Physics, 2025
view moreCredit: Credit: Shashank Shekhar, Emory University
WOODS HOLE, Mass. -- Humans like to think that being multicellular (and bigger) is a definite advantage, even though 80 percent of life on Earth consists of single-celled organisms – some thriving in conditions lethal to any beast.
In fact, why and how multicellular life evolved has long puzzled biologists. The first known instance of multicellularity was about 2.5 billion years ago, when marine cells (cyanobacteria) hooked up to form filamentous colonies. How this transition occurred and the benefits it accrued to the cells, though, is less than clear.
This week, a study originating from the Marine Biological Laboratory (MBL) presents a striking example of cooperative organization among cells as a potential force in the evolution of multicellular life. Based on the fluid dynamics of cooperative feeding by Stentor, a relatively giant unicellular organism, the report is published in Nature Physics.
“We took a step back in evolution, to when organisms were independent. Why did they even come together in a colony before they ever became fixed in position relative to each other?” says John Costello of Providence College, senior author on the study and an MBL Whitman Center scientist along with co-author Sean Colin of Roger Williams University.
“So much work on the origin of multicellular life focuses on chemistry. We wanted to investigate the role of physical forces in the process,” says lead author Shashank Shekhar, Assistant Professor of Physics at Emory University and a former Whitman Center Early Career Awardee at MBL.
Many mouths are better than one
Stentor is a trumpet-shaped, single-celled organism that can grow up to 2 mm long. In its native habitat of ponds or lakes, Stentor attaches its slender end (called the holdfast) to leaves or twigs while the trumpet end sways freely, creating a vortex of water to suck in food, such as bacteria, with its cilia-lined mouth.
In the lab, the scientists noted, when Stentors are dropped into a dish of pond water, they quickly form a dynamic colony where the cells don’t actually attach to one other, but their holdfasts touch together on the glass. By quantifying fluid flows, the team showed that two neighboring Stentors in a colony can double the flow rate of water into their mouths, as compared to their individual capacity. This allows them to suck in more prey and faster-swimming prey, by creating stronger vortexes that sweep in water from a greater distance.
“She loves me, she loves me not”
However, the feeding benefits accrued by two neighboring Stentor aren’t equal, the team found. The weaker Stentor gains more from teaming up than the stronger one does. And, curiously, they display what Shekhar calls “she loves me, she loves me not” behavior. When paired Stentors sway their trumpet ends together, their fluid flows increase, but then they invariably oscillate, pulling their mouths apart again. Why?
To answer this, they turned to mathematical modeling of fluid dynamics across the colony led by co-authors Hanliang Guo of Ohio Wesleyan University and Eva Kanso of University of Southern California.
Guo and Kanso confirmed a “promiscuity” in the colony, where individuals keep switching between neighboring partners. And the result is all the cells in a Stentor colony, on average, gain stronger feeding flows.
“In a colony, even though an individual might appear to be moving away from one neighbor, it is actually moving closer to another neighbor,” the team writes. This makes sense from an evolutionary standpoint, “as individuals are expected to seek the most favorable energetic payoff by associating with a neighboring individual that benefits them most.”
“You might look at them as always attempting to optimize their income,” Costello says. And the colony as a whole reaps more food.
Just a phase
But wait. The Stentor we know and love today is not multicellular. The colonies it forms are ephemeral; they disperse just by bumping the lab table. If the individuals collectively benefit from working together, why do they separate again?
The scientists don’t know for sure. But they’ve noted that when they give Stentors plenty of food, they are happy to remain attached to the glass and feed in colonies. But when the food is taken away and becomes scarce, the Stentors detach and go into individual foraging mode.
“Humans do that, too,” Shekhar says. “When there are plenty of resources and prey, we collaborate and cooperate. But when the resources reduce, it’s each one to its own.”
We are not clones
In other models of early multicellular life, such as the green algae Volvox cateri, cells that failed to divide properly eventually evolved a matrix between them, forming a colony of genetically identical cells which later differentiated. But the ephemeral Stentor colonies are formed not of clones, but of genetically distinct individuals.
That’s why the scientists think their Stentor model precedes other models of early multicellularity (which is believed to have evolved at least 25 times in different lineages).
“This is earlier, much earlier in evolution where happy single cells said, OK, let’s hang out together and benefit, but then let’s go back to being single again. Multicellularity wasn’t done permanently yet,” says Shekhar.
Shekhar began this study as a student in the 2014 MBL Physiology course with then course co-director Wallace Marshall of University of California, San Francisco, an expert on Stentor.
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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.
Tracer particle tracks in the [VIDEO] |
Flowtrace movie showing the particle tracks representing the resulting flowfield of a pair of Stentor individuals. Movie consists of a maximum intensity Z-projection images over a moving window of 0.5s.
Tracer particle tracks showing [VIDEO] |
A Flowtrace movie showing the particle tracks representing the flowfield of the individual Stentor. The movie consists of maximum intensity Z-projection images over a moving window of 1.5 s. Credit: Shekhar et al., Nature Physics, 2025
Dynamic colony of Stentor indi [VIDEO] |
A Flowtrace movie showing the particle tracks representing the flowfield of the individual Stentor. The movie consists of maximum intensity Z-projection images ovemoving window of 1.5 s. Credit: Shashank Shekhar, Emory University
Tracer particle tracks showing [VIDEO] |
A Flowtrace movie showing the particle tracks representing the flowfield of the individual Stentor. The movie consists of maximum intensity Z-projection images over a moving window of 1.5 s. Credit: Shekhar et al., Nature Physics, 2025
Dynamic colony of Stentor indi [VIDEO] |
A Flowtrace movie showing the particle tracks representing the flowfield of the individual Stentor. The movie consists of maximum intensity Z-projection images ovemoving window of 1.5 s. Credit: Shashank Shekhar, Emory University
Credit
Shashank Shekhar, Emory University
Shashank Shekhar, Emory University
Journal
Nature Physics
Method of Research
Experimental study
Subject of Research
Cells
Article Title
Cooperative Hydrodynamics Accompany Multicellular-like Colonial Organization in the Unicellular Ciliate Stentor
Article Publication Date
31-Mar-2025
