Why jellyfish can't rise to the surface

Using box jellyfish as an example, researchers from Kiel University show how the physics of density, not behavior or physiology, can prevent animals from reaching the surface even as they actively swim upward.

The journey to Everglades National Park began as a routine field trip: scientists from the Nanoelectronics research group at Kiel University (CAU) traveled to the vast wetland region in Florida, U.S., to collect box jellyfish—animals whose nervous systems they study to better understand how biological systems process information. But after a tropical rain shower, the team noticed something unexpected. "We normally find the jellyfish close to the surface. After the rain, they had suddenly disappeared from there," recalls first author Jan-Frederik Freiberg, a doctoral researcher in the group.

This observation led to a new study now published in the Journal of Experimental Biology. The researchers show that not only biological limitations, but also physical resistance at the boundary layer itself can prevent some aquatic organisms from crossing certain water layers.

From the Everglades to the laboratory

The Kiel research group led by Professor Hermann Kohlstedt primarily investigates how nervous systems process information and which principles can be derived from them for technological applications. In the current study, however, a different aspect takes center stage: the physical conditions of the environment in which these model organisms move.

The tiny box jellyfish Tripedalia cystophora plays a special role in this work. Despite their comparatively simple nervous system, the animals possess sophisticated eyes and display complex behavior. In mangrove habitats, they orient themselves using light and preferentially swim toward the water surface, where they search for food.

This made it all the more striking that the jellyfish in the Everglades were suddenly swimming much deeper below the surface than before. Following heavy rainfall, coastal waters can become stratified, with lighter freshwater forming a layer above denser saltwater. Between them lies a so-called halocline—a transition zone between water layers with different salinity.

Back in Kiel, the team tested their observations under lab conditions. In a darkened tank, they created an artificial halocline and filmed the jellyfish as they moved upward toward a light source. Although the animals repeatedly attempted to cross the boundary layer, they were unable to make the transition.

"Using AI-assisted trajectory reconstruction, we were able to track the vertical movement of the jellyfish and quantify how effectively the halocline prevented them from crossing it," says Niels Röhrdanz, co-author of the article.

A new physical explanation for haloclines

Until now, two explanations have mainly been discussed for how aquatic organisms respond to haloclines: either they actively avoid certain regions of the water, or altered salinity conditions temporarily impair their swimming ability or cause them to sink.

The findings show that these two explanations are not sufficient. In stratified water columns, an additional effect acts alongside hydrodynamic drag: as the jellyfish swim, they displace denser saltwater into lighter layers. The resulting stratification drag increases the animals' energy loss and reduces buoyancy. Water stratification, therefore, does not merely slow the animals down; their own swimming movements generate additional resistance that can prevent them from crossing the boundary layer. "It is not the animals' behavior or physiology that holds them back, but the physics of the boundary layer," summarizes Dr. Jan Bielecki, senior author of the study.

For Kohlstedt, the findings resonate beyond biology. "In electronics, interfaces between materials are where the most interesting things happen—they control what passes through and what doesn't. A halocline is nature's version of exactly that: an invisible boundary that determines where animals can and cannot go. What surprises me is that the same physical logic governing a transistor can dictate the vertical distribution of an entire animal population in the wild. This is precisely the kind of interface phenomenon that drives our work in KiNSIS—Kiel Nano, Surface and Interface Science," says Kohlstedt.