Grades 3–5 · Organisms
How can a cuttlefish hover without constantly swimming?
A cuttlefish can hover in the sea without beating fins hard all the time. Inside its body is a light, rigid structure called a cuttlebone. Although it looks solid from outside, much of its interior is a network of tiny chambers and supporting walls.
In the diagramA stiff cuttlebone holds gas and liquidCuttlefish: side viewEnlarged chambers8 gas + 2 liquidTen invented spacesLiquid removed:less mass, same shell sizeLiquidGasSupporting walls keep the gas spaces open
The chambers contain gas and liquid. Their stiff mineral framework resists the pressure of the surrounding water. The gas-filled spaces help keep the animal’s average density low, while the framework keeps those spaces from simply squeezing flat like a soft balloon.
The animal can adjust how much liquid occupies the chambers. Salt differences across a specialized boundary help move water into or out of the cuttlebone. Removing liquid lowers mass without greatly changing the rigid structure’s outside volume. The upward push from displaced seawater can then balance the animal’s weight more closely.
The drawing uses ten invented chamber spaces to separate gas from liquid. Real buoyancy adjustment is not an instant depth-control dial, and chamber strength limits the pressures a cuttlebone can withstand.
About this explanation
Starting knowledge: Know that water pushes upward on an immersed object and gas is lighter than the same volume of water.
Sources
- Yang and colleagues, PNAS — Mechanical design of the highly porous cuttlebone: A bioceramic hard buoyancy tank for cuttlefish
- Denton, Gilpin-Brown and Howarth, JMBA — The Osmotic Mechanism of the Cuttlebone