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How do fish and cephalopods achieve neutral buoyancy in seawater?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

Fish and cephalopods utilize various biological mechanisms to achieve neutral buoyancy, allowing them to maintain their position in the water column without expending excessive energy.12

Fish

Most bony fish use a swim bladder, an internal, gas-filled organ derived from the digestive tract. By adjusting the volume of gas within this organ, a fish can alter its overall density relative to the surrounding water. When the bladder expands, the fish’s volume increases while its mass remains constant, lowering its specific gravity and causing it to rise; conversely, decreasing the bladder's volume increases its specific gravity, causing it to sink. Notably, many deep-sea fish and all cartilaginous fish, such as sharks, lack this organ.32

Cephalopods

Cephalopods employ diverse strategies to manage buoyancy:
* Gas-filled structures: The Nautilus uses a chambered shell to control buoyancy, while cuttlefish utilize a porous, laminated cuttlebone to regulate gas levels via the bloodstream.14
* Chemical regulation: Some squids concentrate ions that are lighter than seawater within their body chambers, while others store buoyant oils in chambers associated with their digestive glands to achieve neutral buoyancy.4