By Catherine Offord
They might be eyeless, gelatinous creatures consisting of up to 98% water, but jellyfish seem to know where they’re going, according to a study published in Current Biology in February this year.
The first people ever to attach accelerometers to jellyfish in the name of science, researchers at the University of Swansea have now provided evidence that wild Rhizostoma jellyfish (more commonly called ‘barrel’ or ‘dustbin’ jellyfish) orient their swimming in relation to the direction of the tides. The finding has significant implications for predicting the occurrence of jellyfish blooms, which can see thousands or even millions of jellyfish aggregating in coastal areas for months at a time.
To investigate the behaviour of individual jellyfish in the bloom, the team attached accelerometers to 18 jellyfish in the Bay of Biscay, France, and observed hundreds more swimming near the surface of the water from their research boat. They found that when the tide was pulling water back from the shore, jellyfish oriented to swim against the current. When the tide flowed in towards the shore, jellyfish also swam in a consistent direction, although whether they swam with or against the current depended on the depth of the water. However at slack tide, when the waters settled, the direction of swimming was not clearly towards or away from the shore.
According to the researchers, who also simulated this active orientation behaviour in a computer model of swimming jellyfish, the finding helps to explain why Rhizostoma can stay together in these huge aggregates for such a long time without becoming stranded. What’s more, it may improve the accuracy of predictions of future bloom locations, which will no doubt come as good news to beach-goers and fisherman, for whom the blooms are often a potentially dangerous nuisance.
The way the jellyfish detect the current, however, remains a bit of a mystery. “Detecting ocean currents without fixed visual reference points is thought to be close to impossible and is not seen, for example, in lots of migrating vertebrates including birds and turtles,” said Professor Graeme Hays, who worked on the study, in a press release in late January. One possibility, according to the paper, is that jellyfish swimming near the surface of the water may detect shear forces from the horizontal movement of wind and water at the boundary between sea and air.
But however they manage it, the discovery of directed swimming behaviour in jellyfish comes as an interesting revelation about what are generally considered some of the simplest multicellular animals alive today.
As Professor Hays puts it, “Jellyfish are not just bags of jelly drifting passively in the oceans. They are incredibly advanced in their orientation abilities.”
Catherine is a PhD student in collective behaviour at Princeton University with an interest in science communication. She is a certified diver and writes about swarms of animals, marine and otherwise, over at notesfromtheswarm.wordpress.com.
Copyright © 2015 by Marine Science Today, a publication of Marine Science Today LLC.