Creature of the Month: Chambered Nautilus
By: Haley Chasin

Chambered nautiluses, referred to by scientists as ‘living fossils’ due to the fact that they have remained generally unchanged for the past 500 million years, are defined by their beautifully patterned external shells.

The chambered nautilus is a kind of cephalopod, closely related to cuttlefish, squids, and octopuses. They are found in tropical Western Pacific waters (though not in the Hawaiian islands, capable of living in both shallow waters (300 feet or 90 meters or less) and the very deepest depths (1500 feet or 450 meters). This means that the nautilus can tolerate large ranges in both pressure and temperature.

There are several species in the Nautilidae family, with five in the genus Nautilus (N. belauensis, N. macromphalus, N. pompilius, N. repertus, and N. stenomphelus) and two species in the genus Allonautilus (A. perforatus and A. scrobiculatus). The largest nautilus species, the emperor nautilus is 8-10 inches in diameter, with a body weighing 2.8 pounds. The smallest, meanwhile, is the bellybutton nautilus at about 6-7 inches. Members of genus Allonautilus were actually thought to be extinct but were recently re-discovered in the South Pacific. Allonautilus species have slimy, hairy shells, which, among other body plan changes, make them characteristically different from other nautiluses.

Though nautiluses have poor eyesight, with their simple eyes and no lens, like “hollow bulbs on a stalk” only capable of perceiving light and dark, they are active predators. They feed on shrimp and other crustaceans using 38-90 tentacles which retract into a sheath. These tentacles are lined with alternating grooves and ridges to grip objects. These catch prey and pass it along to the jaws, then to the radula, which shreds the food.

The shell of the nautilus is constructed similarly to snails. It is produced by the mantle tissue and has a genetically hardwired shape called a logarithmic spiral. The shell is separated or compartmentalized into chambers- four in newly hatched nautiluses and thirty in mature individuals. The animal lives in the outermost, or newest, portion of the shell, and as it grows it moves forward. The animal can completely withdraw inside the outer chamber and can even cover itself with a leathery material called the ‘hood’ as a defense mechanism. The shell is counter-shaded, meaning that when you look down, the animal is darker with irregular stripes on top, and when you look up the animal appears lighter due to the pattern.

Nautiluses control buoyancy changes by adjusting the amount of seawater in their shell. The siphuncle is a calcareous tube which connects all interior chambers, and has live tissue running through the shell that serves to pump fluid out of the vacant chamber, thereby adjusting the buoyancy.

The nautilus shell is also designed to maintain neutral buoyancy, as the older sections of the shell are filled with gas to counter the weight of the animal.Nautiluses swim in a see-saw motion by way of jet propulsion, pulling water into the mantle cavity and blowing it out through the siphon.

Experiments at the Waikiki Aquarium have been done to determine what temperature is most optimal in order for nautilus eggs to develop. Oxygen-18 isotope concentrations were used to track changes because it is a tracer and is heavier, so is found farther from the surface. Oxygen-18 values of each septum, or wall between chambers, can be interpreted as the reliability of the temperature in the water where the septa is formed. In the earlier life history stages, the nautilus embryo starts off with a smaller amount of isotopic oxygen-18. Increases in isotopic oxygen-18 between embryonic and post-embryonic septa reflect preparation for colder, deeper water after hatching. Isotopic values of the embryonic septa are thought to reflect the environment inside the eggs,whereas after the embryonic stage it reflects the outside environment, representing changes in habitat after hatching.

Changes in isotopic oxygen-18 in the wild are due to changes in both morphology and environment. Understanding these isotopic concentrations is still a work in progress, but it has allowed aquariums and marine research centers to determine the temperature at which eggs best develop. In the Cretaceous period, nautiluses did not experience the same shift in oxygen-18 compared to extant nautiluses. This indicates that nautiluses during this time period did not relocate to habitats differing in temperature and pressure after hatching.

Nautiluses have been the subject of poetry, artwork, math, and jewelry, and they have also inspired the names of technology from submarines to exercise equipment. In Greek culture, the chamber of the nautilus is a symbol of perfection, and their shell shape has been recognized as a natural mathematical logarithm.

The best time to catch nautiluses is in spring and early summer in deeper waters. Often sought after for their shells, as well as the shell’s nacre, or inner layer, the six living nautilus species are slowly declining in population. This is especially concerning given the difficulty of cultivating nautiluses from eggs in captivity, and scientists are seeking ways to preserve the numbers of these fascinating creatures.