Poikilohydry

Poikilohydry refers to the condition where an organism's water content fluctuates directly with that of its environment. Unlike poikilotherms, which regulate their body temperature externally, poikilohydric organisms do not have mechanisms to maintain a stable internal water balance. Their hydration levels are thus highly dependent on the availability of water in their surroundings.

This physiological state is common among many plants, algae, and lower animals such as lichens and mosses. These organisms can survive periods of extreme desiccation by entering a state of suspended animation, often referred to as anabiosis or cryptobiosis. When water becomes available again, they can rehydrate and resume their normal metabolic activities.

Examples of poikilohydric organisms include desert plants that can dry out significantly during droughts and revive after rainfall, and lichens which are symbiotic associations of fungi and algae, capable of withstanding prolonged dryness. The ability to tolerate such water fluctuations is a crucial adaptation for survival in environments with unpredictable water availability. This strategy allows them to persist in habitats where other organisms would perish.