How Does a Cuttlefish Hover? A Rigid Float Full of Gas

The object is an internal shell, lying beneath the skin along the animal's back, made of aragonite and built as many hundreds of thin layers separated by closely spaced supporting pillars, which creates a stack of flat chambers rather than a solid block. Roughly nine tenths of its volume is space. A new chamber is added at the rear as the animal grows, so the number of chambers records age in the way tree rings do, and the whole structure remains rigid, which distinguishes it completely from the flexible gas bladder of a fish.

Control works by moving liquid rather than gas, which is the clever part:

• •Each chamber contains a mixture of gas and liquid
• •Pumping liquid out of a chamber makes the animal more buoyant
• •Letting liquid in makes it less buoyant
• •The gas is mostly nitrogen at less than atmospheric pressure
• •Liquid is moved by osmotic pumping through a membrane, not by muscle
• •Adjustment is slow, taking hours rather than seconds

Holding the gas space at low pressure inside a rigid structure means the full pressure of the surrounding water presses on the walls, and that sets an absolute limit, because beyond a certain depth the structure implodes. Cuttlefish species have limits ranging from roughly two hundred to six hundred metres depending on how strongly their shell is built, and specimens taken below their limit are found crushed. A fish with a flexible bladder has no such limit, since the bladder simply compresses, though it then has to add gas against the pressure to stay buoyant, which is difficult and slow at depth. The two solutions trade rigidity against range.

The regular addition of chambers turns the structure into a record, and researchers read it much as they read a tree. Counting the layers estimates age, since a chamber is added on a roughly regular schedule, though the rate varies with temperature and food supply and is calibrated against animals of known age. Chemical analysis of the successive layers reconstructs the temperature and the water chemistry the animal experienced as it grew, which gives a history of where it lived rather than only where it was caught. Fossil examples are abundant, since the structure preserves well, and they record the same information for species that have been extinct for millions of years.

The object has several unrelated practical uses that between them explain why it is a familiar thing. Cage birds are given it as a source of calcium and as something to wear their beaks against. Jewellers use it as a casting mould, since the soft layered material takes a detailed impression when an object is pressed into a split piece, withstands molten metal poured into it once, and leaves a characteristic striped texture on the casting that is prized rather than hidden. Polishers used it as a fine abrasive. And it washes ashore in quantity because it floats, which is why beachcombers find it long after the animal has gone.

Hundreds of thin layers held apart by pillars make a rigid stack of chambers about nine tenths gas by volume, and the animal adjusts buoyancy by osmotically pumping liquid in and out over hours. Rigidity means water pressure acts on the walls, so each species has a depth beyond which the structure implodes. Jewellers cast metal into it because the layered surface takes a sharp impression.