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Ancient origins of appetite identified in mindless aquatic animals: ScienceAlert

Even the simplest creatures experience hunger. That hunger for food drives decisions and behavior in all living things.

For most of us, the resulting hunger behavior originates in our brains. Then it is up to our outer nervous system to let our brain know when we have eaten enough. But not all animals have brains, so zoologist Christoph Giez and colleagues from the University of Kiel examined jellyfish relatives found in fresh water, called hydra, to see how mindless creatures strike a balance between feeling hungry and full.

To their surprise, they discovered that hydra has more advanced networks of neurons than expected. Despite their mindlessness, hydras also have a nervous system, with one network acting like our central nervous system, which includes our brains, and another network acting like our peripheral nervous system, which includes all the nerves outside our brain and spinal cord, including the brain . nerves in our intestines.

In hydra, the network responsible for digestion (N4) is more internal, while the other network responsible for feeling full (N3) is more external, but the two systems are not separated in completely different parts of the body as our nervous systems are. .

fluorescent microscopy of hydra nerve cells revealing two distinct populations of nerve cells
Hydra’s two different nerve populations, one in blue and the other yellow. (Christoph Giez)

“This proves that a very simple system, such as the diffuse nerve network of the freshwater polyp, is already capable of sensing something as complex as the internal metabolic state and can regulate the associated behavior accordingly,” explains developmental biologist Thomas Bosch from the University of Keel out.

In a series of experiments, Giez and his team show that hydra can indeed detect and change their behavior based on a sense of fullness.

“For example, after feeding the animals, they showed a significantly lower attraction to light stimuli and an equally strong suppression of natural movement patterns,” says Giez.

“One possibility is that Hydra moves towards the light in search of food and performs a somersault-like locomotion. Therefore, the feeling of satiety inhibits these behavioral patterns, because fed animals temporarily do not have to search for food.”

When the researchers removed the hydras’ outer network of neurons (N3), the animals lost their light-orientation ability and were more likely to open their mouths for food. This suggests that N3 neurons play an inhibitory role in mouth opening.

“We could deduce from this that the (outside) population is mainly responsible for locomotion and for the integration of stimuli,” Giez explains. “By demonstrating this subfunctionalization of neurons in a simple system, we were able to show that certain nerve populations in Hydra can already take over central functions similar to those in more complex nervous systems.”

Together, the hydra nervous systems control the translucent animal’s appetite, suggesting that these separate but communicating systems emerged early in the animal’s evolution. Although the researchers could not find any direct physical connections between the two systems, they suspect that their communication takes place chemically.

With incredible regeneration powers and a resistance to aging, hydra has long fascinated researchers. Now it seems that their nervous systems can also teach us more about the evolutionary origins of our hunger.

This research was published in Cell reports.

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