Though plankton drift with the ocean currents, that doesn't mean they're incapable of any movement. Many of them can move to find food or mates, and they do so in some surprising and sometimes entertaining ways.
Just have a look at this sampler of dinoflagellaes, ciliates, rotifers, cladocerans, and copepod larvae and adults put together by Thomas Kiørboe and his colleagues at the Technical University of Denmark, Woods Hole Oceanographic Institution in Massachusetts, and the Estación de Fotobiolgoía Playa Unión in Argentina.
The first creature -- a dinoflagellate -- wanders about like a wind-up mouse, complete with comic flapping tail. The rotifer at :12 seems to be some sort of larval Cthulhu. The ciliate protist at :20 looks and moves like some sort of alien probe. A copepod looks like a muscle-armed body builder (:43), and others waves their arms like fan dancers (1:08 & 1:17) or like they're gettin' down with their bad selves at Club Plankton (:54).
But did you notice any patterns? Kiørboe and colleagues did, and that is the subject of their new paper "Flow disturbances generated by feeding and swimming zooplankton" in the Aug. 12 issue of The Proceedings of the National Academy of Sciences.
For plankton, moving is necessary but dangerous. Their predators have a nasty habit of finding prey by sensing moving water. So Kiørboe et al. hypothesized that plankton that are able to just swim and not simultaneously feed will have evolved ways to do so as quietly as possible. On the other hand, plankton both moving and swimming should be willing to disturb more water, because feeding is so critical to survival that it is worth taking a greater risk to do it (in this, penguins are no different from plankton).
Indeed, to get enough to eat, zooplankton must daily process water equivalent to
100,000 1,000,000* times their own body volume, so there's no getting around the fluid disturbance that demands. On the other hand, if the only purpose of swimming is to get from Point A to Point B, there's no reason that natural selection shouldn't have favored methods of getting that done that disturb as little water as possible, as briefly as possible.
That is exactly what the scientists found when they observed particle flow around moving plankton. Plankton simply swimming tended to do so quietly by jumping -- unleashing a sudden power stroke powered by a limb or limbs on one side of the body. Or they used the breaststroke, in which paired limbs are pushed forward and rapidly backward in a jet propulsion-like motion.
Either strategy produces only a small volume of moving water that quickly stills, while maximizing the distance the organism travels. Thus swimmers make only small, brief, disturbances interspersed with stillness that makes them invisible to predators for the majority of the time they are "moving".
Feeders and feeding swimmers, on the other hand, were noisy, constantly beating flagella or other appendages. They continually disturbed much more water.
Further, the scientists found that this pattern of swimming strategies held regardless of how distantly related the plankton or how big or small they were, implying they all evolved similar solutions to the same problem, evidence of our old pal convergent evolution. If you go back and rewatch the movie now, you should be able to see and identify all of these popular strategies in action.
*Thank you to Prof. Kiørboe for pointing out this error! Even more amazing!
Kiorboe T., H. Jianhg, R. J. Goncalves, L. T. Nielsen & N. Wadhwa (2014). Flow disturbances generated by feeding and swimming zooplankton, Proceedings of the National Academy of Sciences, 111 (32) 11738-11743. DOI: http://dx.doi.org/10.1073/pnas.1405260111