If you thought jellyfish were simply plastic bags floating aimlessly in the ocean, think again! Although jellyfish are often seen (and felt) near the surface of the water, they actually move through the entire water column. In a study led by professor Graeme Hays of the Swansea University in the UK, the vertical movements of barrel jellyfish (Rhizostoma octopus) were monitored over several weeks using pressure sensitive data loggers attached to the stalk between the bell and the feeding tentacles. As it turns out, these jellyfish move up and down through the water column (to 10 m depth) dozens of times a day regardless of the time of day, ocean conditions, or water temperature.
So, why are barrel jellyfish swimming so much? They’re hunting! These predators cruise for large densities of phytoplankton and zooplankton by “sniffing out” their scent trails through different layers in the water column. Once they have had their fill, the jellyfish return to their original depth. This active hunting behavior has also been seen in the Atlantic sea nettle (Chrysaora quinquecirrha). These findings just go to show that even the simplest of ocean creatures may actually be more complex than we imagined!