Even the topmost layer of the ocean, just millimetres below the air above, is full of life. This zone, where two worlds meet, is home to small creatures like animal larvae, algae, bacteria, and other plankton. Among the most abundant residents of this zone are copepods – tiny relatives of crabs and shrimp. And some of them have the ability to leave this world altogether, and take to the air.
When threatened by fish, some copepods can jump straight out of the water and shoot over many times their own body lengths. From the fish’s point of view, its prey suddenly disappears. Flying fish use the same tactic to escape from predators. Now, we know that one of the most common groups of ocean animals shares their strategy.
One group of copepods, the pontellids, are found throughout the topmost waters. They tend to be more brightly coloured than other transparent members of the group. It’s thought that the colours help to reflect harmful ultraviolet radiation, but they could make the copepods more conspicuous to fish. Copepods escape from fish with powerful bursts of speed that propel them through the water. But those same movements can send them flying too.
The concept of flying copepods first appeared over 100 years ago. In 1894, one Dr Astroumoff said that he saw small green pontellid copepods rise from the water, curve through the air, and fall back to sea again. He thought that they were probably trying to moult by using the jarring re-entry to slough off their old shells. In 1971, another scientist suggested that the jumping copepods were trying to flee from predators, but never presented any evidence for that idea.
That has now changed. Brad Gemmell from the University of Texas, Austin filmed flying copepods (Anomalocera ornata) both in the field (see below) and with a high-speed camera in the lab. As they escaped from approaching mullet, they kicked back with their legs, pulled their antennae back, and left the water.
By analysing the videos, Gemmell calculated that it takes a lot of energy for copepods to get airborne. Their powerful escape reflex provides a lot of energy, but they lose anywhere from 58 to 88 per cent of that as they break the water surface. By contrast, a flying fish loses less than 0.1 per cent with the same manoeuvre. This is because the surface tension of water is much more important to a small animal than a big one. It’s the reason why a pond skater can walk on water, but a human cannot. And it’s the reason why a flying fish can take to the air with relative ease, but a copepod has its work cut out.
Still, their ‘flights’ are substantial. Each copepod is around 3 millimetres long, but their leaps carried them over an average distance of 80 millimetres. That’s well beyond their own body length. It’s also at least 3 times greater than the length of the pursuing mullet, and further than the fish can actually see. And the longest jumpers even managed to cover 170 millimetres.
Gemmell calculated that, despite the energy needed to leave the water, the copepods travel so far in the air that they recoup their initial losses. To travel the same distance underwater, they would need to spend 20 times more energy.
So, taking off is a efficient defence. It’s also an effective one. Gemmell also found that fish are clearly fooled by their disappearing act. In 89 escapes, only one fish managed to target the same copepod when it landed back in the water.
Now, Gemmell wants to see if, as he suspects, the copepods have any special adaptations that allow them to jump more easily. For example, their bodies might be somewhat water-repellent, which would make it easier for them to leave the water behind.
Reference: Gemmell, Jiang, Strickler & Buskey. 2012. Plankton reach new heights in effort to avoid predators. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2012.0163