Jellyfish not just passive drifters

By Natsumi Penberthy 30 January 2015
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Jellyfish sometimes actively swim against ocean currents, and this information can be used to predict their movements

JELLYFISH ARE WIDELY REGARDED as passive oceanic drifters, capable of showing up without warning in large aggregations known as blooms, carried by ocean currents. But it turns out that jellyfish are a lot more active than anyone thought.

Scientists from Deakin University, Melbourne, Swansea University, UK, and Université de la Rochelle, France, tagged and tracked a common jellyfish species regularly gathered in Pertuis Breton in the Bay of Biscay along the western coast of France.

“What we found was that the entire population showed directional swimming,” says Professor Graeme Hays, Chair in Marine Science at Deakin University and lead author of the study. “They would swim against the current at certain states of the tide, and with the current at other states of the tide.”

Griffith University’s jellyfish expert, Associate Professor Kylie Pitt, who wasn’t involved in the study, says it’s a nice demonstration, although not surprising.

“It’s really a confirmation of what some scientists have suspected for a while – that jellyfish are a lot more complex than what we’ve previously given them credit for.”

Jellyfish blooms actively stick together

The researchers attached tags to the barrel jellyfish (Rhizostoma octopus), a large swimmer found in the Atlantic ocean and the Mediterranean. They recorded the swimming pulse, direction, and location of each animal.

The team then used computer modelling to generate two simulations of jellyfish blooms in the bay. For one simulation, thousands of virtual jellyfish passively drifted in the currents. In the other, the swimming behaviour observed from the tracking data was included; the second simulation better fit the model.

“The bloom stayed together far better, they were far more cohesive, and they didn’t wash out to sea or to the shore so much. In essence, the jellyfish could stay together in their preferred location,” says Graeme.

Jellyfish prediction may be on the way

Understanding the movement of jellyfish has important consequences. Massive and unexpected aggregations of these animals can wreak havoc in fish farms, killing hundreds of thousands of fish overnight. A sudden bloom of stinging jellyfish can be bad news for beachgoers as well.

“People aren’t very good at predicting jellyfish blooms – there are certain years when jellyfish are abundant, and other years when they’re not, and that seems to occur in cycles,” says Kylie.

Prof Graeme Hays believes that the results of this study might be the first step towards providing jellyfish forecasts.

“You need to combine a knowledge of the ocean currents with knowledge of the behaviour of jellyfish,” he says. “Then you can get a far more accurate handle on how a bloom will propagate or disperse, and how it will move.”

Future research needs to focus on other jellyfish species to see whether they behave in the same way.

“I don’t think it’s a behaviour that will be demonstrated for all jellyfish,” says Kylie.

The team has already started trialling tagging methods for some jellyfish species found in Queensland.

The study was published last week in Current Biology.