A recently published study into how Heterosigma rapidly forms blooms discovered a remarkable behavior: they flee. This fish-killing species of microscopic plant swims away when it senses single-celled predators are feeding on others nearby. In response, they take "shelter" in low salinity water layers which the predators find intolerable. The investigators said they had never seen a plant swim away from a predator before.
In the experiment, predatory plankton would eat all of the algae in a day if there was nowhere for the plants to hide, but they doubled their numbers every two days if there was a refuge available to them.
This behavior explains how blooms can suddenly appear at the surface and kill fish. While wild fish are susceptible, the sudden occurrence of blooms is particularly devastating for farmed fish held in net pens that cannot escape. Losses of wild and net-penned fish in Puget Sound have been estimated between $2 and $6 million per episode and are believed to be increasing in scope and magnitude in this region and worldwide over the past two decades.
Ultimately, when models are developed to predict the occurrence of Heterosigma blooms, algal behavior will need to be included. Early warning of blooms will allow fish farmers to take remedial action, such as moving net pens or pumping deep water without Heterosigma into the pens. Improved knowledge of this species' toxins could open up more effective methods.
The researchers from the University of Rhode Island are funded by the National Centers for Coastal Ocean Science.
Read the research paper: Harvey, E. L., and S. Menden-Deuer. 2012.Predator-induced fleeing behaviors in phytoplankton: a new mechanism for Harmful Algal Bloom formation ? PLOS ONE 7:1-8.