Researchers at the University of California have uncovered in detail the dynamic process that allows the hydra — a simple freshwater animal famous for its regenerative capabilities — to open and close its mouth.
The white hydra (Hydra vulgaris). Image credit: Corvana / CC BY-SA 3.0.
Hydra looks like a column with a ring of tentacles at one end. The other end adheres to a rock or other surface, sticking the animal in place while it waits for unsuspecting prey to swim by.
When a live animal brushes against its tentacles, the hydra shoots out poisoned barbs to sting and paralyze its prey.
Then the hydra contracts its tentacles, a special group of cells splits apart to display a black maw, and it sucks the prey in.
Once the meal is digested, the hydra rips open its mouth to spit out any leftover materials, seals it back up into a continuous sheet of tissue, and waits for the next prey.
“The feeding response of hydra has been well-characterized physiologically and is regarded as a classical model system for environmental chemical biology,” said team leader Dr. Eva-Maria Collins and her colleagues.
“However, due to a lack of in vivo labeling and imaging tools, the biomechanics of mouth opening have remained completely unexplored.”
The scientists were inspired to investigate the mechanism while studying the regeneration abilities of a species called Hydra vulgaris (white hydra).
They showed that the cells of Hydra vulgaris change their shape, rather than move around, when the mouth opens.
“Because mouth opening involves major morphological changes, other biologists had suggested that in order to open its mouth, hydra had to rearrange the positions of the cells between its tentacles to create and then expand the opening,” the scientists said.
“But through live imaging, we discovered that the process by which hydra opened its mouth fully occurred on fairly fast time scales, on the order of 60 seconds.”
Dr. Collins added: “it’s fascinating that hydra has to tear a hole every time it opens its mouth. And that this process happens so quick; this was the first indication to us that mouth opening did not involve cellular rearrangements.”
By tracking the position of the tagged cells and analyzing the changes in position, Dr. Collins and co-authors confirmed that the process did not involve rearrangement of cells.
Instead, using shape analysis, they discovered that mouth opening was achieved through dramatic elastic deformations of the cells surrounding the mouth.
“Through cell position and shape tracking, we show that mouth opening is accompanied by changes in cell shape, but not cellular rearrangements as previously suggested,” the researchers said in a paper in the Biophysical Journal. “Treatment with a muscle relaxant impairs mouth opening, supporting the hypothesis that mouth opening is an active process driven by radial contractile processes (myonemes) in the ectoderm.”
“Furthermore, we find that all events exhibit the same relative rate of opening. Because one individual can open consecutively to different amounts, this suggests that the degree of mouth opening is controlled through neuronal signaling.”
“Finally, from the opening dynamics and independent measurements of the elastic properties of the tissues, we estimate the forces exerted by the myonemes to be on the order of a few nanoNewtons.”
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Jason A. Carter et al. 2016. Dynamics of Mouth Opening in Hydra. Biophysical Journal, vol. 110, no. 5, p.1191-1201; doi: 10.1016/j.bpj.2016.01.008
