Pontellid copepods live near the ocean's surface where food (phytoplankton) is plentiful. However, living in this open environment exposes them to increased predatory risk. Some of these neustonic ("water surface dwelling") copepods escape predators by jumping out of the water - like flying fish - with a sudden kinetic burst.
While common sense might tell us expending so much energy to break the surface tension of the water is inefficient, the math shows otherwise. This is because air is 850 times less dense than water. The net energy required to travel through air after accounting for the energy required to break the water's surface is still less than the energy required to travel the same distance through water.
If you've observed copepods in your aquarium, you have probably witnessed the jerky bursts they use to dart from place to place. Each burst will only take the copepod so far then it will have to burst again to cover more distance. The researchers found that it takes less energy for a copepod to travel through air with a single burst compared to multiple underwater bursts.
This research by the University of Texas in Austin is published in the Proceedings of the Royal Society B. Here is a video of "flying" copepods escaping larval mulletfish (Mugil cephalus).
[via BBC News]