The Lucky Luke Effect: How Zooplanktons Eat in a Vast Ocean

Stop and think about this for a second: how does a zooplankton eat in such a vast ocean? Turns out, it's not a trivial task: copepods, a type of zooplankton, filter a volume of water approximately 1 million times their own body volume to survive every day ... and at their scale, water has the consistency of syrup.

Scientists discovered a particularly interesting "ambush feeding" technique dubbed the Lucky Luke effect:

“So far, we know of four ways in which zooplankters tackle the engineering feat of finding food in water which appears as thick as syrup. Our contribution has been to describe the mechanism at work for the last of these: How some copepods perform spectacularly precise and rapid surprise attacks on their single-cell prey after first having registered the prey by means of hydrodynamic signals,” explains Professor Thomas Kiørboe, DTU Aqua.

The solution for the ambush-feeding copepods builds on what Thomas Kiørboe calls the Lucky Luke effect:

“Our recordings show that the sub-mm copepods accelerate to a speed of 100 mm per second in a few milliseconds, while at the same time rotating perhaps 180 degrees. Like Lucky Luke who is faster than his shadow, the copepods jump forward so rapidly and with such precision that they, so to speak, shake the viscous boundary layer off, in that way getting close enough to their prey to capture it with their feeding limbs.”

The viscous boundary layer is the layer of water which the copepods pull with them when moving their bodies through the syrupy water. The larger the animals are and the faster they swim, the thinner it seems.


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My favourite thing about this story is the word "copepod". Have you noticed how nice it is to say out loud? I have been going round all day saying "copepod, copepod, all I want is a proper cup of coffee, made in a proper copper copepod..."
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For you non-European-cartoon-aficionados: Lucky Luke is a Belgian cartoon character about a cowboy in the American Old West.
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A natural consequence of having a velocity term in the equation for the Reynolds number. I'm surprised that they didn't work this out beforehand.
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