A nautilus swims through the azure waters off the Great Barrier Reef.
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.