Final answer:
When there's no food around, a jellyfish's energy use decreases, leading to a reduction in size as they conserve energy by becoming less active in order to survive until more food becomes available.
Step-by-step explanation:
When food is scarce, jellyfish will adjust their energy consumption. Unlike hibernating animals in icy environments, jellyfish, as relatively simple invertebrates, have a different survival strategy. They will generally reduce their physical activity to conserve energy, an action termed as energy conservation. Energy conservation in jellyfish may result in shrinkage in size, rather than an increase in the distance traveled to find more food, which would otherwise expend more energy that they cannot afford to lose.
Jellyfish do not accumulate fat and do not hibernate like many warm-blooded animals. They are ectothermic animals which means they rely on the temperature of their environment to regulate their body temperatures. Their simple body design allows gas and nutrient exchange directly with the environment. In the case of food scarcity, they become less active and decrease their metabolic rate, thus reducing their overall energy requirements. This is a survival mechanism allowing them to endure periods of low food availability by becoming smaller, conserving what little energy they may have stored. In winter, aquatic life, including jellyfish, will adapt by consuming fewer resources and performing fewer movements to survive in the harsher conditions.