A nautilus swims through the azure waters off the Great Barrier Reef.
From its lustrous shell to its unblinking eye, the jet-propelled nautilus is unlike any other animal alive today. Paul Chambers meets a mollusc out of time.
For centuries the nautilus was one of the great mysteries of the natural world. Its peculiar coiled shell, with its gorgeous pearly lustre and radiating internal chambers, was known to the ancient Greeks, but the living animal eluded scientific study.
Then, in 1829, a freshly dead specimen was found floating near a Polynesian island. At first it was mistaken for a dead cat. Luckily, a ship’s surgeon saw fit to retrieve the creature and preserve it.
Two years later the specimen reached London where it was dissected by Richard Owen (a leading anatomist, whose achievements included dreaming up the word dinosaur).
He declared that it was a cephalopod, the group to which squid, cuttlefish and octopuses belong. He also noted the similarity between its exquisite, compartmentalised shell and the fossils of extinct animals such as ammonites.
Owen deduced that the nautilus possessed many biological features that were ‘inferior’ to or ‘less progressive’ than those seen in cuttlefish or squid.
It seemed to have more in common with fossilised cephalopods from the Jurassic and Cretaceous periods than with those alive in the modern world. Though our understanding of evolutionary biology has come a long way since then, he did have a point.
The term ‘living fossil’ is contradictory and overused, yet is an apt description of the nautilus, whose fossil history reaches back over 400 million years. For much of this time the world’s shallow seas swarmed with hundreds of different nautilus species, as well as those of their cousins, the ammonites.
Then, 65 million years ago, came the extinction event that wiped out the dinosaurs. This also saw off the ammonites, and marked a steep decline in the diversity and number of nautilus species.
Today just six kinds are left. Truly, they are molluscs out of time.
Design for life
Watch a nautilus in the ocean (or an aquarium – it has become a popular novelty exhibit) and its prehistoric heritage is obvious. Its smooth, tightly coiled shell is unlike that of any other living mollusc.
Most of the owner’s soft parts are hidden, except for the bristling mass of short tentacles protruding at the front. Either side of this is a strange, primeval-looking eye that consists of little more than a round plate with an unblinking pinhole at the centre.
Though unique today, these features are also present in exceptionally well-preserved nautilus fossils. Astonishingly, it would appear that the nautilus body plan has changed very little in about 70 million years.
Even the behaviour of the nautilus is out of step with the modern world. It shows none of the intelligence possessed by octopuses, which can solve puzzles, indulge in play and even have personalities. And it moves like no other creature alive today.
Rather than actively swimming, it hangs in the water as though tethered by an invisible wire. It propels itself by sucking water into its body cavity and then squirting it out through a siphon-like structure called a hyponome.
But the nautilus has a problem. Whereas jet propulsion turns squid and cuttlefish into dashing hunters, it is restricted by its shell. It can’t fit enough water inside to create a high-pressure blast. So it has to make do with swimming backwards in a jerky bobbing style.
Since the nautilus also has poor eyesight, it is limited to stalking crustaceans on the seabed. But it has an ace up its multi-tentacled sleeve: it is ultra-energy-efficient, needing to find just one good meal every couple of weeks.
Location, location, location
To see a wild nautilus in action, you must visit the tropical Indo-Pacific, where the six species live close to the seabed at depths of 100m or more. Perhaps competition for food forced their ancestors out of shallow seas and into deeper water.
This would have placed them out of reach of many large predators, such as dolphins, and away from overcrowded coral reefs, where there is more competition for food from fish and other cephalopods.
But there’s a downside to being an iconic, beautiful, ‘out of this world’ kind of creature. Nautilus shells have become collector’s items and now several nautilus populations are under pressure from overfishing. There’s a danger that some species might end up going the same way as the ammonites.
Did you know?
- The pressurised shell of a nautilus will implode if it swims deeper than 800m. But it cannot survive for long in shallow water since the warmth of the sea will kill it.
Inside a nautilus
The complex structure of the nautilus shell has long puzzled naturalists
- In 1726, it was suggested that the nautilus flooded individual chambers in its shell with water or air to help it sink or float. Later, another theory was that they were gas-filled. The animal (which occupies the final, largest chamber) was said to have little or no control over their contents.
- We now know that the key to understanding the anatomy of this enigmatic mollusc lies in a thin tube of tissue, the siphuncle, that runs through the centre of all of its chambers. Each of these is, in effect, a sealed, pressurised compartment – like an aircraft fuselage – from which the nautilus uses the siphuncle to add or remove air and water.
- By carefully adjusting the air-to-water ratio, the nautilus can achieve neutral buoyancy. This reduces the energy needed to swim and allows it to support the weight of a big shell. Some of its giant fossil ancestors, such as orthocones, could stay afloat while carrying heavy shells up to 9m long.