Hydra attached to a surface. Photo: Coveredinsevindust/Wikimedia Commons, CC BY-SA 3.0
What do hydra, tiny freshwater organisms that look like inverted jellyfish, have in common with four-time Olympic gold medallist Simone Biles? Their incredible ability to somersault, it seems. Hydra are smaller than the width of a human finger but can bend their bodies and perfectly flip over their tentacles. Now, with a combination of biophysical experiments and computational modelling, researchers have found that differences in tissue stiffness along hydra’s body length makes this unique motion possible.
Named after the serpent-like monster in Greek mythology that regrows two heads for each one that’s cut off, hydra are known for similar powers of regeneration. Slice open a hydra and each half will grow into two new bodies. In addition, they don’t exhibit ageing, leading some biologists to label them ‘immortal’. This drew Suyash Naik, an undergraduate student at the Indian Institute of Science Education and Research (IISER), Pune, into studying the organism.
“My initial goal was to study what physical changes occur across the hydra body during regeneration,” Naik said. “While studying this, I noticed that hydra tissue is notably stiffer in its head region.”
This serendipitous discovery changed the direction of his research towards locomotion.
Naik measured the stiffness of hydra tissue by calculating a property of materials called the Young’s modulus – a number that determines how much a certain amount of force can stretch something. Stretchy rubber bands have low Young’s moduli while metal rods have high values.
It’s common to see experiments in physics labs where this quantity is measured by suspending heavy weights on rods and observing how much they bend. But measuring this quantity for the soft tissues of a tiny, delicate organism is tricky.
To achieve this, the researchers used an atomic force microscope (AFM) on hydra for the very first time. An AFM can measure material properties down to the scale of individual atoms using a cantilever, which looks like a microscopic diving board, with a needle pointing downwards at one end. The cantilever is moved gently across a sample without damaging it, like a back massager. Forces from individual molecules within the sample make the cantilever vibrate, through which scientists can obtain a ‘force terrain’ of the sample surface. The strength of these forces can be translated to various material properties, including the Young’s modulus.
Using the AFM, Naik found that the Young’s modulus at the hydra head is three times greater than at its base – i.e. a sharp decrease in stiffness from head to toe.
The researchers pondered what the function of this stark gradient might be. “We thought of a system of springs, with two parts of different stiffness connected like a slinky,” Naik said. This could explain hydra’s somersaulting motion – where its stiff ‘shoulder’ stores energy when it bends, before releasing the energy to land.
To verify that the change in stiffness governed hydra’s ability to somersault, the researchers used two techniques. First, they disturbed the shoulder tissue’s stiffness by manipulating the medium that binds cells together, after which the hydra was unable to somersault. In an independent approach, they created a computational model of springs with the right stiffness, and were able to replicate hydra’s motion with it.
Studying locomotion of the simplest organisms is important from an evolutionary standpoint. It sheds light on how animals embraced the need to move around in search for food, water and to evade predators.
“The fact that hydra may have evolved to actively manipulate its own body structure in order to bring about this complex motion is exciting,” says Sudhakaran Prabhakaran, a biologist at Cambridge University who was not involved in this study. “This is a fascinating interdisciplinary collaboration between biology, physics and computation experts coming together to explain a fundamental aspect of a living organism.”
There are few ways more aesthetically pleasing than a somersault to go from point A to point B. A trait picked up millions of years ago by an ostensibly immortal organism is mimicked today by humans seeking sporting immortality. When asked to compare hydra’s somersault to the elegance of an Olympic gymnast, Naik smiled and said the twirl of the tiny, tentacled creature is far superior.
Sumeet Kulkarni is a fourth-year PhD candidate studying physics at the University of Mississippi and an aspiring science writer. He has written for Scientific American, AAS Nova, Astrobites, and I, Science.