Divers feel envy when they see tropical fish floating motionless in coral reefs. Humans can barely swim with oxygen tanks and heavy lead weights attached, while fish rest with no effort at all. It’s the underwater version of a hummingbird hovering in the air.
The truth was the opposite. Scientists at the Scripps Institution of Oceanography at the University of California, San Diego (UCSD) noted on the 8th that "fish that hover in the water actually expend more energy than when they are resting." The research results were published that day in the journal Proceedings of the National Academy of Sciences (PNAS).
◇High energy expenditure to maintain posture
Fish possess a buoyancy bladder, or swim bladder. When air is injected into the bladder, it expands, creating buoyancy that leads the fish upward, while deflating it causes the fish to sink. The research team led by Professor Valentina Di Santo at UCSD recorded the oxygen consumption of 13 species of fish with swim bladders when they floated or moved, as well as when they were motionless at rest. They used high-speed cameras to capture the movement of the fins while the fish hovered.
Professor Di Santo said, "Contrary to previous beliefs, it has been shown that fish use about twice the energy when hovering in the water compared to when they are resting." In other words, the fins were not moving naturally with the currents but were being vigorously flapped.
The research team explained that fish hovering in the water is similar to a person trying to maintain balance on a bicycle. While fish generate buoyancy with their swim bladders, their center of mass does not perfectly align with the center of buoyancy, leading to unstable postures. Fish adjust their position by flapping their fins to avoid tilting or rolling.
In this regard, the hovering of fish is different from that of birds. Birds create lift by rapidly pushing their wings downward. Hummingbirds, the symbol of hovering, are known to flap their wings up to 80 times per second. Fish easily create buoyancy underwater with their swim bladders, but they must constantly move their fins to maintain their posture.
The research team revealed that the larger the distance between the center of mass and the center of buoyancy, the more energy fish expend when hovering. They also found that the shape of the fish and the position of the fins affected the efficiency of hovering. Fish with pectoral fins positioned toward the rear of their bodies used less energy when hovering. Professor Di Santo explained that this is due to the increased leverage effect, moving a large object with a small force.
◇Hovering efficiency varies by habitat
The shape of the body also influenced hovering. Long and slender fish, like the cichlid (species name: Lamprologus ocellatus) that lives in snail shells or the giant danio (Devario aequipinnatus), were less efficient at hovering, whereas shorter and deeper-bodied fish, like the goldfish (Carassius auratus) or the pufferfish (Dichotomyctere ocellatus), performed better.
The research team explained that the differing hovering efficiencies among fish are due to their varying habitats. To navigate complex habitats like coral reefs, hovering ability is essential. Tropical fish like the triangular-shaped featherfin blenny (Plata teira) or the garibaldi (Hypsypops rubicundus), known as the sea goldfish, excel at hovering for this reason. Slender fish may not hover well, but they excel in agility, living in open environments.
The research team stated that understanding fish hovering abilities could aid in the design of underwater robots or submarines. Underwater robots that explore complex environments, such as coral reefs or shipwrecks, need hovering capability. In such cases, a goldfish shape would be more advantageous. Conversely, submarines designed for agility might also adopt an elongated shape.
The researchers explained that to enhance hovering capabilities, one must first induce instability, similar to that of fish. Afterward, a posture control system, like fins, should be reinforced. Professor Di Santo remarked, "If you want an underwater robot to maneuver in tight spaces, you should design it considering fish instability and then add a system to maintain dynamic stability."
References
PNAS (2025), DOI: https://doi.org/10.1073/pnas.2420015122