Fish make hanging motionless in the water column look effortless, and scientists had long assumed that this meant it was a type of rest. Now, a new study reveals that fish use nearly twice as much energy when hovering in place compared to resting.
The study, led by scientists at the University of California San Diego's Scripps Institution of Oceanography, also details the biomechanics of fish hovering, which includes constant, subtle fin movements to prevent tipping, drifting or rolling. This more robust understanding of how fish actively maintain their position could inform the design of underwater robots or drones facing similar challenges.
The findings, published July 7 in the Proceedings of the National Academy of Sciences, overturn the long-standing assumption in the scientific literature that maintaining a stationary position in water is virtually effortless for fish with swim bladders.
The reason for this assumption was that nearly all bony fishes have gas-filled sacs called swim bladders that allow them to achieve neutral buoyancy — neither sinking nor rising to the surface. The presence of a swim bladder and the stillness of hovering fish caused the research community to assume hovering was a form of rest that was easy for fish to maintain.
Prior research from lead study author and Scripps marine biologist Valentina Di Santo found that the energy required for skates to swim at various speeds followed a distinct U-shaped curve, with slow and fast swimming requiring the most energy and intermediate speeds being the most energy-efficient. Based on these findings, Di Santo suspected there might be more to hovering than meets the eye.
To learn more, Di Santo and her co-authors conducted experiments with 13 species of fishes with swim bladders.
The team placed each fish in a specialized tank and recorded their oxygen consumption during active hovering and motionless resting (when the fish supports its weight with the bottom of the tank). While the fish were hovering, the researchers filmed them with high-speed cameras to capture their fin movements, tracking how each fin moved and how frequently they beat.
The researchers also took a variety of measurements of each fish's body size and shape. In particular, the scientists measured the physical separation between the fish's center of mass, which is determined by weight distribution, and its center of buoyancy, which is related to the shape and location of its swim bladder. All these measurements provided a way to quantify how stable or unstable each fish was.
The study found that, contrary to previous assumptions, hovering burns roughly twice as much energy as resting.
"Hovering is a bit like trying to balance on a bicycle that's not moving," said Di Santo.
Despite having swim bladders that make them nearly weightless, fish are inherently unstable because their center of mass and center of buoyancy don't align perfectly. This separation creates a tendency to tip and roll, forcing fish to make continuous adjustments with their fins to maintain position. The study found that species with greater separation between their centers of mass and buoyancy used more energy when hovering. This suggests that counteracting instability is one of the factors driving the energy expended during hovering.
"What struck me was how superbly all these fishes maintain a stable posture, despite their intrinsic instability," said Di Santo.
A fish's shape and the position of its pectoral fins also influenced its hovering efficiency. Fish with pectoral fins farther back on their body were generally able to burn less energy while hovering, which Di Santo suggested may be due to improved leverage. Long, slender fish, such as the shell dweller cichlid (Lamprologus ocellatus) and the giant danio (Devario aequipinnatus), were less efficient at hovering, and fish with deep, compact bodies, such as the goldfish (Carassius auratus) or the figure-eight pufferfish (Dichotomyctere ocellatus), were more efficient.
"This changes how we see hovering. It's not a form of rest at all," said Di Santo. "It's an energetically costly activity but one that fish engage in anyway because it can be very useful."
Activities like guarding nests, feeding in specific locations or maintaining position in the water column are far more demanding than previously thought. The study's findings also reveal an evolutionary trade-off in fish body shapes, where increased maneuverability comes at the cost of hovering efficiency and vice versa. Rather than being a drawback, Di Santo said, the high energy cost of hovering is a necessary trade-off that gives fishes the exceptional agility required to navigate the challenges of complex habitats such as coral reefs.
These findings could inform the design of underwater robots and vehicles, which must also maintain stability while remaining agile.
"By studying how fish achieve this balance, we can gain powerful design principles for building more efficient, responsive underwater technologies," said Di Santo.
In particular, the findings could help improve the maneuverability of underwater robots, which could allow them to access and explore complex, hard-to-navigate environments like coral reefs or shipwrecks. According to Di Santo, underwater robots have historically been designed with compact shapes that make them stable. As in fish, shapes with more built-in stability are less maneuverable.
"If you want a robot that can maneuver 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," said Di Santo.
In addition to Di Santo, the study was co-authored by Xuewei Qi of Stockholm University, Fidji Berio of Scripps Oceanography, Angela Albi of Stockholm University, the Max Planck Institute of Animal Behavior, and the University of Konstanz, and Otar Akanyeti of Aberystwyth University in Wales. The research was supported by the Swedish Research Council, the European Commission, the Stockholm University Brain Imaging Centre and the Whitman Scientist Program at the Marine Biological Laboratory.
