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Christophe Dupré and Rafael Yuste have taken neurobiology one step closer to its dream scenario: Reading and understanding, in real time, the activity of an animal’s nervous system.

Dupré, a at the �������� last summer who is working on his Ph.D. in at Columbia University, bred a line of genetically engineered hydras so that every one of their nerve cells would light up when active. Dupré and Yuste, who teaches , recently published their accomplishment – hailed as “the first complete Brain Activity Map” – in .

Hydras are already famous for their apparent immortality, but they are little more than tiny tubes topped with an array of slender, stinging tentacles. The stinging places them in the Cnidaria phylum alongside jellies and anemones, one of the earliest groups of animals to have evolved a recognizable nervous system. Their diffuse “nerve nets” are a good place to begin deciphering the fundamentals of how a complete nervous system functions.

A given hydra will only have a few hundred to thousand neurons spread throughout its entire body — a human brain, by contrast, has 85 billion. In fact, the hydra nervous system is small enough that it’s possible to see the whole thing at once if you put a hydra under a microscope, where it can survive in a tiny drop of water sandwiched between two slides. This is exactly what Dupré did.

When the researchers analyzed the recorded images of hydras – and their neurons – in action, they were shocked to find four distinct, non-overlapping neural circuits in what was supposed to have been a simple, undifferentiated neural net. Each circuit corresponded to a specific behavior. One, for instance, activates when a hydra compresses its tubular body to expel waste (Dupré and Yuste call this “sneezing”); another activates when it nods its “head” side to side.

The surprising complexity of the hydra nervous system highlights the fact that their study is just an “opening salvo,” according to Yuste. The goal is to be able to reverse-engineer an animal's behavior based on its neural activity. Is this hydra hungry? Will it start hunting shrimp soon?

To push forward this research, Dupré and Yuste are bringing six other teams of hydra researchers to �������� this summer for a joint HydraLab, June 8 to August 16. Mysteries abound, but Dupré and Yuste hope their research will help lay bare the fundamental principles that underpin all nervous systems, from hydras to humans.

Citation: Christophe Dupré and Rafael Yuste (2017) Non-overlapping Neural Networks in Hydra vulgaris. Current Biology DOI: Video caption: Tracking neural activity in behaving hydra. The position of the 620 neurons was tracked manually over 20 seconds. Each circle marks one neuron, and the color corresponds to the functional group. All the neurons that are firing at any given frame are bright and the other neurons are completely dark. () ��