We are used to thinking of them as soft jelly creatures that let themselves be carried away by currents. Instead - surprise - even jellyfish can perceive the direction of the water push, and actively swim against it to go where they want to go: it is the conclusion of an international study published in Current Biology.

Mass shifts. The research, conducted by the marine biologists of the Universities of Swansea (Wales) and of the Deakin University of Warnambool (Australia) is proving useful for understanding the dynamics and the formation of jellyfish invasions: banks of hundreds of specimens with stinging tentacles that persist in a given area even for weeks, regardless of the currents.

taken for the tentacles. The team used gps locators - carefully linked between the umbrella, ie the "bell-shaped" part, and the beginning of the tentacles - to monitor the movements, acceleration and direction of the body of 18 "sea barrel" jellyfish (Rhizostoma octopus) in the Bay of Biscay, off the coast of France. Other floating sensors were used to measure the actual direction and intensity of sea currents.

we remain united. Researchers have thus discovered that sometimes jellyfish actively swim against the current, controlling their destination, rather than passively being carried away by it. Subsequent simulations showed that this ability would help the jellyfish shoals to remain compact, avoiding that the single specimens are swept away by the current and dispersed.

Open questions. The mechanism that allows jellyfish to perceive currents and move accordingly remains a mystery: a hypothesis is that jellyfish can sense the direction of water through their body. Or, they could move along the lines of the Earth's magnetic field, an ability already observed in other aquatic creatures, such as sea turtles.

MeteoMeduse. The study will be used to refine the forecasts of jellyfish movements: in recent years, insane fishing, pollution and climate change have intensified jellyfish invasions, creating imbalances in marine ecosystems.