What is it about?
Fish often need to hover (stay still in the water) to catch prey, explore, or guard their nests. For a long time, scientists thought that for fish with a swim bladder (which makes them nearly “weightless” in water), hovering would be almost effortless. Our research shows that’s not true. We studied 13 species of these fish with swim bladder and found that hovering actually doubles their energy use compared to resting. Why? Because staying still in water is a balancing act: fish bodies are naturally a bit unstable, so they must constantly move their fins and adjust their posture to avoid rolling or tipping over. Some fish pay a higher energetic price than others, depending on their body shape and where their fins are located. This study shows that hovering is a challenging and costly behavior, not the “free ride” we once imagined.
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Why is it important?
Our study reveals that hovering - previously thought to be nearly effortless for most fishes - can actually double their metabolic rate compared to resting. This means that when fish “stand still” in the water, they are paying a surprisingly high energetic price, often higher than swimming slowly. These results reshape our understanding of what it costs for fish to feed, guard nests, or remain alert in their habitats. By identifying which body shapes and fin positions make hovering more or less costly, our work sheds light on why some fish have evolved deep bodies, heavier masses, or specific fin placements. Species with deep bodies and more rearward fins pay a smaller price to hover, while slender fish with forward fins must work harder just to stay in place. These patterns help explain the incredible diversity of fish shapes and behaviors we see in nature. Beyond biology, these findings can also guide the design of underwater robots: just like fish, robots must balance the trade-off between being stable and being able to maneuver, and now we know that stability comes at a real energetic cost.
Perspectives
My research has always been driven by curiosity about what it truly costs for fish to move, and not move, in water. Years ago, I found that swimming isn’t cheapest at the slowest speeds, but at an intermediate “sweet spot,” forming a U-shaped curve of metabolic cost (https://doi.org/10.1073/pnas.1715141114). But one question nagged at me: what about speed zero? If swimming slowly is costly, is “hovering”-just staying in place-really effortless, as so many assume? This study was my chance to test again the hypothesis of high metabolic costs of going slowly (this time at zero speed!) I discovered that hovering isn’t rest at all. In fact, holding still in open water requires constant, subtle fin movements and postural adjustments. For near-neutrally buoyant fishes, the cost of stability at zero speed is surprisingly high, sometimes even higher than swimming slowly. That’s the hidden price of “doing nothing” in water. For me, this work is a reminder that nature rarely offers free rides. Even stillness takes work, and sometimes the simplest behaviors hold the biggest secrets. I hope these results prompt others to reconsider what it really means to rest, hover, or “stand still”—whether in fish, other animals, or even in robotics.
Valentina Di Santo
University of California San Diego
Read the Original
This page is a summary of: Inherent instability leads to high costs of hovering in near-neutrally buoyant fishes, Proceedings of the National Academy of Sciences, July 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2420015122.
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