Cold Acclimation Provides a Robust Overwintering Strategy in Hydra vulgaris

AbstractCold acclimation is a biological process that allows animals to survive at low temperatures. The freshwater invertebrate Hydra is subject to broad changes in environmental temperature and does not have the required motility in order to move to warmer environments during the winter. For this reason, Hydra had to develop robust mechanisms to achieve cold acclimation at the onset of winter. How Hydra detects the onset of winter and activates its acclimation mechanism is unknown. Here, we used thermocyclers to induce cold acclimation in Hydra and study its properties. We found that Hydra cultured at room temperature does not survive an abrupt transition from 22 to 4 °C. However, it can be treated to become cold acclimated and survive at 4 °C by exposure to intermediate temperatures such as 12 °C if the treatment duration exceeds more than a week. Once cold acclimated, Hydra is considerably more robust to thermal changes. It survives repeated abrupt transitions from 4 to 22 °C and from 22 to 4 °C. However, acclimation is reversible, and if a cold-acclimated Hydra stays at room temperature for more than a week, it will gradually lose its cold acclimation. We developed a mathematical model representing the dynamics of this process and used it to predict survival according to temperature data recorded in one of their natural habitats. The results of these simulations provide an explanation for how Hydra survives winter under natural conditions. Accordingly, daily fluctuations are too short to cause injury, and seasonal fluctuations, which are long enough to be lethal, allow acclimation to incrementally build up and protect the animal. Cold acclimation in Hydra is therefore an example of a strategy that has adapted during evolution to match the animal's needs for survival.