siphuncle

The siphuncle is a tubular cord of living tissue that runs longitudinally through the shell of many shelled cephalopods, linking the animal’s soft body with the gas-bearing chambers of the shell. It threads through each septum, connecting the chambers (camerae) along the shell’s axis. In many species the siphuncle is reinforced by a calcareous or organic tube and can vary in diameter and position along the shell.

The primary function of the siphuncle is buoyancy regulation. It actively pumps fluids into and out of each chamber, enabling the animal to adjust the gas content within the chambers. By removing liquid from a chamber and/or altering gas volume, the overall density of the shell changes, allowing the animal to ascend, descend, or maintain neutral buoyancy with minimal active swimming. The tissues of the siphuncle interface with the chamber contents and the animal’s vascular system, facilitating gas exchange and fluid movement.

In fossil and living cephalopods, siphuncle morphology and position are important diagnostic features. The siphuncle may be central, marginal, or ventral depending on the lineage, and its structure—such as the presence of connecting rings or septal necks—varies among groups such as orthocerids, nautiloids, and ammonoids. Because of its central role in buoyancy, the siphuncle is a key subject in studies of cephalopod physiology, evolution, and paleoecology.