Hydra is a useful animal model to study basic behavior, as it is among the first instances in evolution to have a nervous system. A freshwater polyp mostly found in ponds and lakes, it is radially symmetric with a body essentially made of a cylinder with a head and tentacles at one extremity and a foot at the other extremity. It can move its body column in various directions to explore its surroundings and contract it to hide and escape threats.
Using these simple motion primitives Hydra spends most of its time foraging, defined as an exploration of the local environment for food items. The number of components that make foraging behavior is not known, and the temporal structure of this behavior still needs to be elucidated. Hydra is most of the time sessile, i.e. it stays attached with its foot to a substrate. However, it can sometimes move to a different location by somersaulting, which is done by attaching its head to the substrate and detaching its foot to reattach it somewhere else.
To decipher the structure of movement in Hydra, we created a paradigm where we can track it in 3D over multiple days and reconstruct it with a model. We found that Hydra somersault in bursts separated by intervals of a few hours, which creates sequences of nomadic states interspersed with sequences of sessile states.
We suggest that somersaulting and sessile states alternate as a strategy to explore the environment. Further, within somersaulting and sessile states there are other substates that exist and can help the animal tune its exploration. Together, this provides a description of an algorithm that can be used to implement foraging behavior in a radially symmetric animal.