At the Zoo: The Nautilus Shell Game
by Katie Venit
As the fading Fijian sun retreats from the tropics,
promising a bit of relief from the oppressive heat,
a strange animal stirs in the depths of the ocean. This
small sailor anxiously waits in its sandy bed as the
ominous yellow orb dies away.
Quickly, the animal jets closer to the surface, traversing
almost a thousand feet in a couple of hours. By the
time it reaches its feeding grounds in the coral, the
water in these shallower depths has cooled. The migrant
feeds ravenously on the discarded exoskeletons of spiny
lobsters and other crustaceans. Eventually, it senses
the reemergence of the sunlight, bringing another day
of heat. The lone nautilus abandons its feast and sinks
back down to the depths to sleep in the sand through
the day.
At the peak of their Paleozoic reign 650 million years ago, 3,000 nautilus species shared the sea with the prodigies of that era, the trilobites, as well as the ammonoids, other shelled invertebrates that usurped the ocean from the nautiluses and nearly pushed them into extinction. The nautiluses had the last laugh, however, as the ammonoids and trilobites died out by the end of the Cretaceous period. Today, six species of nautilus—including one with a layer of fur lining the bottom of its shell—survive in pockets of tropical waters in the western Pacific and Indian oceans.
The nightly journey of nautiluses has puzzled scientists for centuries. How can these creatures survive at depths of up to 1,800 feet and then rise to waters as shallow as 300 feet every night? This rapid pressure change would cause other aquatic life either to implode from the hydrostatic pressure in the deep water or to explode from internal pressure in shallow water.
The reason for this journey seems easy enough to identify. Nautiluses are extremely sensitive to warm temperatures: If they spend extended periods of time in an environment warmer than 80 degrees Fahrenheit, they become sick and die. Kept in sufficiently cool water, such as in zoos and aquariums, a nautilus can survive for up to 17 years; the weak hydrostatic pressure found in surface tanks does not itself imperil the creature. To cope with this vulnerability to warmth, nautiluses, which live only in tropical climates, retreat to the cool ocean depths during the day while hiding from predators. However, because there are few appropriate food sources living that deep, these scavengers need to make their way toward the surface once the sun sets to search for sustenance.
How a nautilus manages this feat is a more complicated question, whose solution centers on the creature’s shell. Nautilus, cuttlefish, squid, and octopus are cephalopods, considered the most intelligent members of the Mollusk family and of all invertebrates. Unlike their cousins, the nautiluses are the only cephalopods with an external shell—basically a tube that has evolved into a spiral. This spiral is divided into a few dozen chambers, and each chamber leading out from the middle of the spiral is younger and larger than the previous one. A hard membrane called a septum separates the chambers, which are connected by the siphuncle, a vein through which liquid is removed or added to the chambers.
The soft parts of a nautilus—the head, tentacles, and internal organs—rest in the last and youngest chamber, and they can be retracted into the septum and covered by two large and tough hoods if a nautilus feels threatened. The tiny creatures, which grow four to ten inches long, have between 70 and 90 tentacles, depending on the species and the sex. The tentacles all serve different functions, ranging from catching and holding food to clinging to oceanic objects. Nautiluses also have propulsion organs called hydroponones, protruding forward from between the mass of tentacles. Early scientists dismissed this mechanism as too weak to be responsible for the depth-change mystery and turned instead to the animals’ chambered shell for an explanation.
Think back to Archimedes’ Principle: An object will float in the water if its mass is less than the mass of the water it displaces. It is easiest to understand how this concept works by thinking of a submarine. When the submarine floats stationary under the water, it has neutral buoyancy. It is essentially weightless, having the same density as the water around it. When the vessel dives, it takes on liquid, increasing its mass but not its volume, thus making it heavier than the surrounding water. It now has negative buoyancy and will maintain this negative buoyancy until it sinks to the denser water below. When the sub surfaces, the process is reversed.
Although it seems feasible that nautiluses—after which Jules Verne’s famous fictional submarine was named—would use this same passive mode of locomotion, they cannot. It would take weeks for a nautilus to adjust its density enough to travel the distance that it does in only two hours. Contrary to earlier theory, the hydroponone itself is responsible for the daily migration, using a process of jet propulsion. Water is sucked in and then shot out, and the nautilus goes in whatever direction it wants.
Steady at one atmosphere, the gas pressure in a nautilus’s chambers has been found insufficient to bolster shell integrity at great depths. What then keeps its body from yielding to the vast change in pressure? Although early scientists dismissed this possibility, it turns out that the shell’s structure itself is strong enough to withstand depths of up to 2,000 feet without the aid of additional internal gas pressure.
Nautiluses use their internal chambers as fish use their swim bladders: to maintain neutral buoyancy. As a nautilus grows, it gradually drains liquid out of previously formed chambers to compensate for the added mass of the expanding shell and retain neutral buoyancy. For a full-grown nautilus, occasional minor adjustments to the amount of liquid in its chambers are made to compensate for large meals or chips in its shell.
A nautilus tends to establish neutral buoyancy for its average depth, which is usually around 900 feet in the ocean. Nautiluses rarely spend much time at this depth, but maintaining this average level allows greater flexibility in depth change. If a nautilus adjusted its buoyancy so it could rest at the bottom without having to cling to the sand, it would have an extremely difficult time propelling itself all the way to its feeding grounds. The opposite is true if it maintained neutral buoyancy at the surface. Its hydroponone and clinging tentacles are enough to move a nautilus around when it is not neutrally buoyant, as long as it does not wander too far out of range. When a nautilus’s depth range changes suddenly, as when it is placed in an aquarium tank, it senses the pressure difference and over time adjusts accordingly to float comfortably in this new environment.
The Smithsonian’s National Zoo currently houses seven chambered nautiluses (Nautilus pompilius) in the Invertebrate House inside two tanks kept at 63 to 64 degrees Fahrenheit. Although these nocturnal creatures tend to be fairly inactive during Zoo hours, they usually perk up when served a gourmet dinner of frozen shrimp, squid, crab claws, or fish smelt. Yet even when they are slumbering, merely catching a glimpse of these beautiful and otherwise elusive marvels of adaptive engineering is a treat not many are able to experience.
ZooGoer 29(6) 2000. Copyright 2000 Friends of the National Zoo. All rights reserved.
