The gas bladder of a marlin consists of a large number (50-500) of translucent bubble-like chambers which lie in two masses on either side of the midline just below the vertebrae.

It extends from the very top end of the abdominal cavity to the top border of the second anal fin. The numbers and sizes of the chambers are variable, but black marlin appear to have more smaller chambers than blue or striped marlin.

The largest chambers in blue marlin are more than twice the size than those of black marlin.

Individual chambers can be punctured without deflating the adjacent ones, but they are not physically separable. The gas is easily sampled and contains mostly oxygen (PO2).

The gas bladder of marlin is closed to the outside world, and gas volume is regulated by secretion and absorption of gasses to and from blood via vascular retia.

The gas bladder has a translucent covering of flat tissue under which are clusters of capillaries forming the retia.

The retia are not combined into one “oval organ” but occur under all the luminal surfaces of the organ.

This means that partitioning of the chamber allows a much greater surface area for gas secretion and absorption and hence a faster buoyancy compensation mechanism.

The function of the gas bladder is to regulate buoyancy. Changes in the volume of gas in the bladder will cause the fish to rise or fall in the water column. The extensions of the marlin gas bladder top and bottom indicate that the mass of the fish is supported over most of its body length.

The separate chambers may be used to adjust the pitch or attitude according to alterations in body shape as it grows or as reproductive tissues develop, especially in gravid females or as it feeds and fills the stomach.

Most fish aim for neutral buoyancy in the water column at the depth where they wish to live. Some fishes, tunas for example, have no gas bladder, and their natural tendency to sink is offset by hydrodynamic lift created by water moving over their body surface and pectoral fins.

For tunas, this is not such an energy disadvantage as might be assumed, since they must swim continuously to maintain water flow over their gills.

It is generally believed that marlin are also obliged to swim continuously to ram ventilate their gills, but the presence of a gas bladder suggest that adaptations of body and pectoral fins for lift are not needed.

That the gas bladder volume of marlin is near the lower end of the range for marine teleosts however suggest some lift may be generated during swimming.