CARDIFF: The mechanism that allows some plankton to avoid predators by leaping out of the water has been explained by scientists over a century after it was first discovered.
First described in a paper in 1894, the ability of some copepods – a small group of crustaceans, including plankton, that are found in both fresh and salt water environments – to leap out of the water has long eluded science. In a new paper, published in the Proceedings of the Royal Society today, researchers from the University of Texas in the U.S. have shown how some planktonic copepods have mechanisms that allow them to exit the water surface and travel through the air to avoid predators.
“This study, which shows the ability of copepods to break through the surface tension and travel many body lengths out of the perceptive range of the predator; shows that copepods have more specialised survival adaptions than we thought,” said lead author Brian Gemmell.
A flying copepod
Planktonic copepods are especially important to global ecology and the carbon cycle, and are major food organisms for small fish, whales, seabirds and other crustaceans such as krill. Some scientists believe that as a group they form the largest animal biomass on the planet.
The leaping behaviour first appeared in early 19th century scientific literature in a report called “A Flying Copepod” by Russian scientist Aleksiei Aleksandrovitch Ostrooumov. He described how during a spring morning he saw a number of a small, green type of copepods called pontillids rise from the sea, leaping into air before returning again.
Ostrooumov predicted this unusual form of locomotion was part of the moulting behaviour, while a more recent study suggested that the behaviour was part of a predator avoidance strategy, but until now no one could prove how and why it was done.
Plankton escaping predation
Pontellid copepods are a ubiquitous group that reside close to the surface of the water during the day. They are often highly pigmented, which helps to reduce the impact of harmful UV rays. This group is also considerably larger than other copepods, which may leave them more open to predation by fish that rely on their visual sense to detect prey.
Gemmell’s team used high-speed recording techniques to discover that by jumping outside the water, these large copepods are able to escape being targeted by fish looking for a meal.
“Our experiments involved field recordings of copepod behaviour in the presence of natural fish predators (to demonstrate functionality of the behaviour) and high-speed laboratory recordings,” said Gammell. “Laboratory recordings were used to observe the mechanism by which these animals broke through the water surface, and kinematics from these recordings were used to calculate energy requirements.”
An energy efficient escape
After analysing the kinematics (mathematics associated with energy expenditure), the team discovered that by leaving the water, these organisms are able to use less energy to escape a greater distance. Despite using a large portion (more than 30%) of their kinetic energy to simply break the surface tension of the water – the behaviour is still energy efficient. Air mass density is approximately 850 times smaller than the mass density of seawater, which means that after becoming airborne, the copepod will experience less drag resulting in increased distance.
“Despite the large energy loses, this behaviour actually moves the copepod much further away from an attacking predator than the same amount of energy input in an escape that occurs solely underwater,” said Gammell. “As air is so much less viscous than water, the copepods can actually save energy when escaping through air because only a single jump is needed whereas multiple jumps to achieve the same distance is required underwater.”
Plankton escape mechanism
As a group that lives in what’s known as the neustonic environment of the ocean – the first few millimetres below the surface – these copepods have the advantage of being constantly surrounded by a source of food. But this leaves them open to predators, which makes their leaping behaviour a life-saving escape mechanism.
“This is a nice piece of research linking behaviour and ecology, between predators and their prey,” commented Richard Kirby, a research fellow at the University of Plymouth in England. “The research is a great piece of curiosity-driven science in an important area – plankton underpin the food web, without them we wouldn’t have fish in the sea, seagulls in the sky, no seals, or polar bears, no whales or sharks.”
Original paper in Proceedings of the Royal Society
Video of Pontillid leaving the water