You have probably seen fish calmly hanging in the water, seemingly motionless. For a long time, scientists thought this was their way of resting. Surprisingly, a recent study shows that this seemingly effortless action actually costs fish nearly twice the energy compared to real rest.
Image Credit: Noheaphotos via Shutterstock / HDR tune by Universal-Sci
The Hidden Effort Behind Hovering
Scientists from the Scripps Institution of Oceanography at the University of California San Diego studied how fish hover in place. Using specialized tanks and high-speed cameras, the researchers discovered that fish constantly make subtle fin adjustments to prevent tipping, drifting, or rolling.
Valentina Di Santo, the marine biologist who led the study, explains, “Hovering is a bit like trying to balance on a bicycle that’s not moving.” The research, published in the Proceedings of the National Academy of Sciences, challenges decades of assumptions that hovering was effortless because fish have swim bladders—gas-filled organs that allow neutral buoyancy.
Why Is Hovering Hard Work?
Despite swim bladders making fish nearly weightless, there is an inherent instability. According to Di Santo, fish must continuously adjust their fins because their centre of mass (body weight distribution) and centre of buoyancy (swim bladder location) aren't perfectly aligned. The greater the separation between these centres, the more energy fish use.
“What struck me was how superbly all these fishes maintain a stable posture, despite their intrinsic instability,” Di Santo says.
The Body Shapes That Save Energy
Interestingly, fish body shape also plays a significant role. Fish with fins positioned further back use less energy due to better leverage. Compact-bodied fish, like goldfish and pufferfish, hover more efficiently compared to longer, slimmer species like the giant danio.
This finding suggests an evolutionary balance: fish trade hovering efficiency for agility. Di Santo says the high energy cost isn't a disadvantage. Rather, it’s essential for manoeuvring through challenging habitats such as coral reefs, where precise movements matter.
What Fish Teach Robots
Beyond biology, this research holds practical implications. Di Santo suggests that understanding how fish handle instability can guide engineers to build better underwater robots.
“If you want a robot that can manoeuvre through tight spaces, you might have to learn from these fishes to design in some instability and then add systems that can dynamically maintain stability when needed,” Di Santo explains.
Ultimately, this study shows that what seems easy to fish involves hidden effort and remarkable skill—offering insights into the natural world that could benefit technology on our own.
If you are interested in more details about the underlying research, be sure to check out the paper published in the peer-reviewed scientific journal Proceedings of the National Academy of Sciences, listed below.
Sources and further reading:
Inherent instability leads to high costs of hovering in near-neutrally buoyant fishes - (Proceedings of the National Academy of Sciences)
This colour-changing fish can ‘see’ with its skin - (Universal-Sci)
Do fish have feelings? Maybe…- (Universal-Sci)
Can goldfish navigate on land? - (Universal-Sci)
How the zebrafish got its stripes - (Universal-Sci)
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