It's true that zooplankton are drifters; tiny, often microscopic, organisms floating en masse through the seas, oceans and freshwaters of the world. But they don't just drift. Up close, they are really quite active. Just like many animals, zooplankton move to find food and mates, and do so using a wide range of propulsion methods to get them where they need to go. But as they go, microscopic zooplankton create tiny wakes, alerting predators to their location. Worse yet, to feed, zooplankton must filter or examine large volumes of water while they swim. More water, more wakes. Because zooplankton must move, and moving puts them at risk for predation, Thomas Kiørboe, of the Centre for Ocean Life at the Technical University of Denmark, and his team of international collaborators wanted to understand how these tiny creatures balanced the need to move with the need to stay alive. By comparing zooplankton that feed while they swim with those that are able to feed and swim (to relocate) independently, they sought to answer the question, do zooplankton minimize predation risk when given the opportunity? That is, is swimming independent of feeding as risky as swimming while feeding, in terms of the signals that zooplankton are sending to predators?
The authors examined the wakes (fluid disturbances) caused by swimming and feeding zooplankton across a range of size, morphology, ecology, species and swimming modes: from single-celled, rugby-ball-shaped dinoflagellates, which propel themselves with one tail and one side whip, to ~1-mm-sized copepods – crustaceans with four to five pairs of swimming legs on their underside, plus feeding appendages that generate a current. They used high speed cameras to film the zooplankton swimming through liquid filled with tracer particles to visualize the flow of the liquid around the organisms, and measured the peak and variability in swimming speed of the zooplankton, how far the flow field extended out from the zooplankton and how quickly the disturbances dissipated. They also noted how the organisms were swimming and whether zooplankton that swim to relocate and swim to feed use different movements for each task.
Kiørboe and his team discovered that the zooplankton that were only swimming did so in a way that disturbed the fluid around them significantly less than when they were feeding, and less than the zooplankton that always swim and feed. Swimming by itself resulted in fluid disturbances that dissipated more quickly than swimming and feeding, which meant that the wakes did not reach as far out from the zooplankton, potentially alerting fewer predators in the area.
The authors suggest that with zooplankton, similar to many animals, there are tradeoffs associated with the daily activities necessary for survival. Because feeding requires the movement of large quantities of water (relative to zooplankton size, of course) and food is an absolute necessity, the fluid disturbances caused by feeding are worth the risk: if they don't feed, they will die; if they do feed, they might die trying. Either way, zooplankton must feed, so the risks that come with feeding are offset by the benefits of feeding. However, swimming to relocate does not require the processing of large volumes of water, so zooplankton have evolved to minimize predation risks when swimming. And so these tiny drifters swim softly on, with their predators none the wiser.
