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How does a swim bladder help a bony fish stay at a chosen depth?

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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.

The swim bladder acts as a hydrostatic or ballast organ, allowing bony fish to maintain a specific depth without sinking or floating upward.12

Fish control their buoyancy by regulating the volume of gas—typically oxygen—contained within the bladder. This process functions through two primary mechanisms:2

Increasing Buoyancy: Special glands secrete gas into the bladder, which increases the fish's overall buoyancy and allows it to rise or maintain its position in the water column.2
Decreasing Buoyancy: When the fish needs to sink, the gas is absorbed from the bladder back into the bloodstream, reducing the volume of the organ and decreasing buoyancy.2

While most bony fish utilize this gas-filled organ, some deep-sea species have evolved to use oil instead of gas, while others—particularly bottom-dwelling varieties—have reduced or entirely lost the swim bladder as it is no longer necessary for their specific environment.12