Got strong shoulders, will somersault!

On the biomechanics of the somersault movement in Hydra

The stages in the somersault movement of Hydra. In stage 1 (top), the body column is stretched, and the tentacles hold onto the substrate.  In the next stage, the basal end is released. Then the body column contracts and finally the body column is lifted. The second half of somersault is a reversal of this process. (Image Courtesy: Prof. Sanjeev Galande)

In a recent paper published in the Journal of Experimental Biology, the research groups led by Prof. Sanjeev Galande from the Biology department and Dr. Shivprasad Patil and Dr. Apratim Chatterji from the Physics department at IISER Pune, along with Prof. Irit Sagi’s group from Weizmann Institute of Science, reveal the secret of Hydra’s somersault movement on a solid surface.

“Animals move and plants don’t. Both are multicellular organisms. It is not exactly clear when this difference first occurred in evolution. The invention of movement by animals is an important milestone,” says Prof. Sanjeev Galande. It is suspected that the Phylum Cnidaria, comprising of mostly marine life forms, has played an important role in this.

Many Cnidarians such as the jellyfish swim, while others like Hydra can additionally perform movement on surfaces similar to the terrestrial animals. However, many other members such as sea anemones are sessile and prefer to stay in one place, much like plants. In fact, early naturalists thought that these were plants. “The needle of suspicion about the beginnings of animal movement is pointed towards Cnidarians who are actually pioneers of many animal traits, which later evolved,” says Dr. Shivprasad Patil who studies nanomechanics.

Often found in freshwater bodies such as ponds, Hydra exists in a slender cylindrical form called polyp that is typically about 6-10 mm long and 0.5-1 mm thick. With its simple body plan and regeneration capacity, Hydra has been an important model organism for studies in cell and developmental biology.

“When trying to measure certain aspects about the mechanical response of Hydra tissue in the context of its regeneration ability, we serendipitously stumbled upon a rather curious property of Hydra,” says Dr. Patil. Using a sensitive instrument called Atomic Force Microscope for probing the basic physical properties of the body column, the team discovered that the Hydra’s tissue stiffness is not uniform but exhibited a peculiar differential across the body column. The shoulder region, where tentacles spawn forth, was found to be three times stiffer than the rest of its body column. Such a variation was unprecedented and it caught their attention. 

The team from IISER Pune that came together to study the mechanics of movement in Hydra: (Left to Right) Prof. Sanjeev Galande (faculty member), Dr. Shivprasad Patil (faculty member), Shatruhan Singh Rajput (PhD student, Dr. Patil’s group), Suyash Naik (BS-MS Project student, Prof. Galande’s group), Dr. Manu Unni (PhD student, Prof. Galande’s group), Dr. P. Chandramouli Reddy (Postdoctoral Fellow, Prof. Galande’s group), and Dr. Apratim Chatterji (faculty member). Another member of the team from IISER Pune, Devanshu Sinha, is not in the photo. Photograph was taken prior to the ongoing physical distancing and masking measures. (Photo Courtesy: Prof. Sanjeev Galande)

The team proposed a hypothesis that such strong shoulder enables Hydra to pull off the somersaulting stunt. Subsequently, they realised by performing certain biochemical and mechanical alterations, that this differential in tissue stiffness can be eliminated. The Hydra polyps lacking this differential stiffness were unable to perform somersault. Using this biological readout, they provided experimental evidence to their hypothesis and proved that the observed variation in the stiffness across the body column was indeed essential to perform the somersault.

The team also revealed that this differential stiffness is directly controlled by the organisation of the collagen fibers in the extracellular matrix that is an important contributor to the tissue stiffness. Physical or chemical perturbation of such organisation leads to loss in the observed differential stiffness as well as the ability to perform somersault.

They further performed simulations to understand the mechanics behind the stiffness differential enabling the somersault stunt. Their comprehensive simulations, wherein they treated Hydra’s body as an elastic cylinder moving in viscous medium, underscored the role of the stiffness differential played in somersault. If the shoulder region is three times stiffer compared to the rest of the body column, then the energy stored in it in an intermediate step of the somersault, just before the organism raises itself up, is sufficient to overcome the viscous drag and gravitational force.

“Corroborating with our hypothesis, the computer simulated Hydra with uniform stiffness lacked the mechanism of energy transfer, storage and its subsequent utilization to stand upside down,” describes Dr. Apratim Chatterji, who applied his expertise in the area of soft matter physics to understand the somersault movement of Hydra.

The authors indicate that this work opens up possibilities to understand not only animal movement but also the evolution of the musculoskeletal system in highly evolved animals.

“As we see around us, tissue stiffness in these animals varies from being jelly-like to that of metal alloys such as steel. It is likely that Cnidarians are the harbingers in yet another remarkable invention of evolution,” says Dr. Patil.

This work was supported by the Centre of Excellence in Epigenetics program of the Department of Biotechnology (DBT), Government of India, the DBT/Wellcome India Alliance for Intermediate Fellowship and Early Career Fellowship, JC Bose National Fellowship from Science and Engineering Research Board (SERB). The computer cluster employed for the simulations was obtained using a grant from the Department of Biotechnology.

Article citation

Differential tissue stiffness of body column facilitates locomotion of Hydra on solid substrates. Suyash Naik, Manu Unni, Devanshu Sinha, Shatruhan Singh Rajput, P. Chandramouli Reddy, Elena Kartvelishvily, Inna Solomonov, Irit Sagi, Apratim Chatterji, Shivprasad Patil, Sanjeev Galande. Journal of Experimental Biology 2020: jeb.232702 doi: 10.1242/jeb.232702

- With inputs from Dr. Shivprasad Patil, Prof. Sanjeev Galande, Dr. Apratim Chatterjee