Significant modulation of phytoplankton swimming speed and vertical velocity was observed when H. akashiwo was exposed to the actively grazing predator, Favella sp. was observed by Menden-Deuer and her colleagues when H. akashiwo was exposed to the actively grazing predator, Favella sp.
They observed predator-induced defense behaviors previously unknown for phytoplankton. Modulation of individual phytoplankton movements during and after predator exposure resulted in an effective separation of predator and prey species. The strongest avoidance behaviors were observed when H. akashiwo co-occurred with an actively grazing predator. Predator-induced changes in phytoplankton movements resulted in a reduction in encounter rate and a 3-fold increase in net algal population growth rate.
“It has been well observed that phytoplankton can control their movements in the water and move toward light and nutrients,” Menden-Deuer said. “What hasn’t been known is that they respond to predators by swimming away from them. We don’t know of any other plants that do this.”
In a series of laboratory experiments, Menden-Deuer and Harvey found that the phytoplankton not only flee when in the presence of the predatory zooplankton, but they also flee when in water that had previously contained the predators. They found only a minimal effect when the phytoplankton were exposed to predators that do not feed on phytoplankton.
“The phytoplankton can clearly sense the predator is there. They flee even from the chemical scent of the predator but are most agitated when sensing a feeding predator,” said Menden-Deuer.
When the scientists provided the phytoplankton with a refuge to avoid the predator – an area of low salinity water that the predators cannot tolerate – the phytoplankton moved to the refuge.
The important question these observations raise, according to Menden-Deuer, is how these interactions affect the survival of the prey species.
Measuring survival in the same experiments, the researchers found that fleeing helps the alga survive. Given a chance, the predators will eat all of the phytoplankton in one day if the algae have no safe place in which to escape, but they double every 48 hours if they have a refuge available to flee from predators. Fleeing makes the difference between life and death for this species, said Menden-Deuer.
“One of the puzzling things about some phytoplankton blooms is that they suddenly appear,” she said. “Growth and nutrient availability don’t always explain the formation of blooms. Our observation of algal fleeing from predators is another mechanism for how blooms could form.
Amazingly, looking at individual microscopic behaviors can help to explain a macroscopic phenomenon.”
The researchers say there is no way of knowing how common this behavior is or how many other species of phytoplankton also flee from predators, since this is the first observation of such a behavior.
“If it is common among phytoplankton, then it would be a very important process,” Menden-Deuer said. “I wouldn’t be surprised if other species had that capacity. It would be very beneficial to them.”
In future studies, she hopes to observe these behaviors in the ocean and couple it with genetic investigations.
Funding for this research was provided by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the U.S. Department of Agriculture. The study was conducted, in part, at the URI Marine Life Science Facility, which is supported by the Rhode Island Experimental Program to Stimulate Competitive Research.