The nautilus is a unique and ancient group of marine animals, representing the sole surviving lineage of cephalopods that possess a hard, external shell. Often called a “living fossil,” this creature has remained morphologically similar to its ancestors for hundreds of millions of years. Understanding the size of a nautilus requires looking beyond a single measurement, as its overall scale is determined by both its coiled shell and the soft body mass within. The dimensions of this animal vary significantly depending on the specific species and the geographic location of its population.
Typical Dimensions of the Chambered Nautilus
The species most commonly known and studied is the Chambered Nautilus, Nautilus pompilius, which serves as a benchmark for the group’s size. An average adult shell diameter for this widely distributed species ranges between 6 to 8 inches (15 to 21 centimeters), measured across the coiled shell. A mature Nautilus pompilius usually weighs between 1 and 2.8 pounds (0.4 to 1.3 kilograms).
While the shell volume is substantial, the weight is moderate due to the shell’s internal structure. Exceptionally large individuals have been recorded, particularly near Indonesia and Northern Australia, with some specimens reaching a shell diameter of up to 10 inches (25.4 cm).
Size Differences Across Nautilus Species
The size of a nautilus is not uniform across the entire genus, which contains several distinct species and subspecies. The overall range for adult nautilus shell diameters spans from about 4 to 10 inches (10 to 25 cm). For example, the smaller species Nautilus macromphalus, found near New Caledonia, typically exhibits an adult shell diameter of about 6.5 inches (16 cm).
Even within the dominant Nautilus pompilius species, isolated populations show considerable size variation based on their location. A dwarf subspecies, Nautilus pompilius suluensis, inhabits the Sulu Sea and is significantly smaller than its relatives, with an average shell diameter of just 4.55 inches (11.56 cm). The genus Allonautilus, which contains the rare Allonautilus scrobiculatus, is generally on the smaller end of the spectrum, with individuals reaching a shell height of up to 7 inches (18 cm).
Shell Size Versus Body Mass
The physical size of a nautilus is dominated by its coiled, calcareous shell, which is not a solid mass but a buoyancy apparatus. The animal’s soft body occupies only the outermost and largest chamber of the shell, known as the living chamber. The remaining internal space is divided into 30 or more smaller chambers that are sealed off and filled with a mixture of gas and fluid.
These internal chambers function like the ballast tanks of a submarine, allowing the nautilus to precisely control its buoyancy in the water column. A tissue tube called the siphuncle runs through the center of all the chambers and regulates the gas and fluid levels. This structure means that a large portion of the nautilus’s overall volume is occupied by air-filled space, resulting in a relatively small soft body mass compared to the total size of the shell.