Surprise! Plankton Swim

Zooplanktons have long been thought of as lazy free floaters - "planktos" even means "drift" in Greek.

New evidence, however, shows that they aren't the ocean's freeloaders. They actually swim quite vigorously - but only up and down.

While they are at the whim of the ocean's horizontal currents, giant patches - up to 100 kilometers - of the critters "treadmill" against upwelling and downwelling currents to maintain a constant depth.

"That small zooplankton are capable of remaining at a constant depth with a precision of centimeters, sometimes in the face of strong vertical currents, implies that these organisms have extremely sensitive depth sensors, the nature of which is unknown," said Amatzia Genin of the Hebrew University of Jerusalem.

Genin and his colleagues are likewise scratching their heads over why it is important for zooplankton to maintain such a specific depth.

"That this depth-keeping behavior has evolved in so many different species implies that this energetically demanding behavior provides significant, yet poorly understood benefits," he said.

Genin and his colleagues used a new imaging system borrowed from the Scripps Institution of Oceanography, named Fish TV, to measure the depth at which zooplankton swim. Using Fish TV's multibeam sonar, they analyzed the swimming behavior of more than 375,000 individual zooplankton swimming in the Red Sea.

These findings were published in the May 6 issue of the journal Science.

--Bjorn Carey

Credit: Mark Wunsch