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Why do some deep-sea fishes have reduced swim bladders or no swim bladder?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Deep-sea fishes often have reduced or absent swim bladders primarily as an evolutionary adaptation to the extreme hydrostatic pressure found at great ocean depths.

Because the swim bladder is a gas-filled organ used for buoyancy, it poses a significant physiological challenge in deep-sea environments. At depths of several thousand meters, the immense pressure would compress any air-filled compartment, making it difficult for the fish to maintain its structural integrity and buoyancy control. To survive these conditions, many deep-sea and bottom-dwelling species have either lost the organ entirely or reduced it significantly.123

While some bony fish in these environments may utilize oils instead of gas within the bladder to assist with buoyancy, the complete absence of air-filled cavities is a common strategy to avoid the physical stresses of high-pressure habitats. This adaptation is part of a suite of specialized traits—including modified eyes and metabolic adjustments—that allow these animals to thrive in the extreme cold, darkness, and pressure of the deep sea.234