Thermal stimulation temperature is encoded as a firing rate in a Hydra nerve ring

Many animals that lose neural tissue due to injury or disease have the ability to maintain their behavioral abilities by regenerating new neurons or reorganizing existing neural circuits. However, most small model organisms used for neuroscience like nematodes and flies lack this high degree of neural plasticity. These animals often show significant behavioral deficits if they lose even a single neuron. Here we show that the small freshwater cnidarian Hydra vulgaris can maintain stable sensory motor behaviors even after losing half of the neurons in its body. Specifically, we find that both the behavioral and neural response to a rapid change in temperature is maintained if we make their nervous system roughly 50% smaller by caloric restriction or surgery. These observations suggest that Hydra provides a rich model for studying how animals maintain stable sensory-motor responses within dynamic neural circuit architectures, and may lead to general principles for neural circuit plasticity and stability. Significance StatementThe ability of the nervous system to restore its function following injury is key to survival for many animals. Understanding this neural plasticity in animals across the phylogenetic tree would help reveal fundamental principles of this important ability. To our knowledge, the discovery of a set of neurons in the jellyfish polyp Hydra vulgaris that stably support a response to thermal stimulation is the first demonstration of large-scale neural plasticity in a genetically tractable invertebrate model organism. The small size and transparency of Hydra suggests that it will be possible to study large-scale neural circuit plasticity in an animal where one can simultaneously image the activity of every neuron.