Nautilus: Blast from the past

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.

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Nautilus opening spread

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.
 
Living fossil?
 
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.
 
Awkward mover
 
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.
 
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