No, we’re not talking about a Ford pickup truck diesel engine. University of Texas researchers note that Olympic swimmers aren’t the only ones who change their propulsion strokes to escape competitors, and that tiny marine crustaceans called copepods switch from a wave-like swimming stroke to big power strokes in order to escape from the jaws and claws of predators — behavior that has now been revealed thanks to 3-D high-speed digital holography.
UT reports in a release that copepods found in nearly every aquatic environment — and by some estimates, they are the most abundant animals on the planet — are preserved in part by the copepods’ now observable ability to change up their swimming strokes in cold water in aid of escaping a slew of predators, from larval fish to crabs, oysters and jellyfish. “Copepods are key components of marine food webs eaten by just about everything,” comments Ed Buskey, study author and professor of marine science at The University of Texas Marine Science Institute. “The better question is ‘what doesn’t eat copepods?’”
Dr. Buskey maintains that understanding how the microscopic organisms like copepods respond to changes in the environment is a key element in assessing the health of oceans now and in the future, noting that environmental changes that affect copepods include shifts in water temperature and viscosity associated with climate change, and increases in water viscosity related to pollution and coastal algal blooms, pointing out that water viscosity, or “thickness,” naturally increases as the temperature drops. As a layperson, this reporter has observed this phenomenon in boiling water for brewing tea for example, with hot H2O seeming to be “thinner” and consequently more splash and splatter-prone. It is. And for microscopic copepods, lower water temperatures are analogous to swimming through honey, which could be reasonably assumed to make them more vulnerable to predators? So how’s a copepod to cope?
To answer those questions, Buskey and co-author Brad Gemmell — a former graduate student of Buskey’s who is now at the Marine Biological Laboratory in Woods Hole, Mass. — employed high-speed digital 3-D holography techniques developed by mechanical engineer Jian Sheng at Texas Tech University. Sheng’s technique uses a microscope outfitted with a laser and a high-speed digital camera to capture the rapid movements of the microscopic animals moving in and out of focus in a 3-D volume of liquid, enabling the researchers to observe and monitor copepod movement in water with varied temperatures and viscosities.
Buskey, Gemmell and Sheng published their findings in an article entitled “Compensatory escape mechanism at low Reynolds number,” March 4 in Proceeding of the National Academy of Sciences journal Early Edition, the abstract of which can be accessed here,, noting that “Despite high predation pressure, planktonic copepods remain one of the most abundant groups on the planet. Their escape response provides one of most effective mechanisms to maximize evolutionary fitness.” The article contains supporting information that can be found online at: http://goo.gl/7CjfT
The report observes that copepod larvae swim using three pairs of appendages that act like three pairs of oars moving a boat, but unlike a rowboat propelled by three oarspersons for example, the copepods’ “oars” don’t move in complete synchrony. Instead, when in warmer, less viscous water, the three pairs of appendages stroke in an overlapping, wave-like motion, the first pair starting a stroke, then the second pair commencing their the stroke before the first pair completes, and so on.
However, Buskey and Gemmel found that when cold, thick water, the copepods switch to one big power stroke at a time – the first pair of “oar” appendages completing one big downward stroke before the second pair begins, the third pair not starting until the second is complete.
“These little guys are not very efficient swimmers,” comments Ed Buskey. “They slip backward with every recovery stroke. I guess it isn’t easy swimming in ‘honey.’ ” Brad Gemmel observes that the “power stroke adaptation helps the copepods overcome the negative effects of changing water temperature and viscosity to escape predators,” and that without the power stroke, which the researchers discovered is triggered only by colder temperature, not viscosity alone, the copepods would be even easier prey in cold water, because the muscles that control the copepods’ appendages are affected by temperature.
Ergo, “ if you increase viscosity without changing the temperature — the kind of situation you might find during an algal bloom or pollution event — the copepod’s escape ability declines,” says Gemmell in the release. That’s beneficial for predators of course, but could have negative effects on copepod populations and the marine food web, particularly as coastal algal blooms and pollution are expected to increase in future.
Brad J. Gemmell, Houshuo Jiang, J. Rudi Strickler and Edward J. Buskey have also published a paper entitled “Plankton reach new heights in effort to avoid predators” in Proceedings of the Royal Society (Proc. R. Soc. B, 279, 2786-2792, 2012), in which they demonstrate that despite losing up to 88 per cent of their initial kinetic energy, copepods that break the water surface travel significantly further than those escaping underwater and successfully exit the perceptive field of the predator. For more information, visit: http://goo.gl/4TkgA