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.
- 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.
- 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.