Phototaxis is a satiety-dependent behavioral sequence in Hydra vulgaris.

Understanding how internal states like satiety are connected to animal behavior is a fundamental question in neuroscience. Hydra vulgaris, a freshwater cnidarian with only twelve neuronal cell types, serves as a tractable model system for studying state-dependent behaviors. We find that starved Hydra consistently move towards light, while fed Hydra do not. By modeling this behavior as a set of three sequences of head orientation, jump distance, and jump rate, we demonstrate that the satiety state only affects the rate of the animal jumping to a new position, while the orientation and jump distance are unaffected. These findings yield insights into how internal states in a simple organism, Hydra, affect specific elements of a behavior, and offer general principles for studying the relationship between state-dependent behaviors and their underlying molecular mechanisms.