Fish may use less energy to support and move their bodies against gravity because buoyancy counteracts their weight. However, water’s greater viscosity creates more resistance than air, so swimming is not automatically less energetically costly than movement on land.
This is part of IB Biology A1.1 Water and applies at both SL and HL. Water is denser than air, so it exerts a substantial upward buoyant force on an immersed organism. According to the principle of buoyancy, this force equals the weight of the water displaced.
As a result, fish do not need heavy supporting limbs or as much continuous muscular effort to hold their bodies above the ground. Their skeletons can be relatively lightweight, while structures such as swim bladders help many bony fish achieve neutral buoyancy, meaning they neither sink nor rise without additional effort.
Water also has greater viscosity than air. Viscosity is a fluid’s resistance to flow, so a moving fish experiences considerable drag as water moves around its body. Fish reduce this resistance through streamlined bodies, smooth surfaces, and fins that generate controlled thrust.
| Property of water | Effect on fish movement | Comparison with land animals |
|---|---|---|
| Buoyancy | Supports body weight and reduces the energy needed to resist gravity | Land animals require strong skeletons and muscles for support |
| High viscosity and density | Creates drag that opposes movement | Air produces much less drag at similar speeds |
| Fluid medium | Allows fins and body movements to generate thrust | Land animals push against a solid surface using limbs |
A common misconception is that fish always use less energy because water “holds them up.” Buoyancy reduces support costs, but viscosity increases locomotion costs; the total energy requirement depends on speed, body shape, size, and swimming method.
Exam technique: For an “explain” question, link each property to a consequence: buoyancy provides upward support, reducing work against gravity, whereas viscosity increases drag. A balanced answer must include both effects rather than claiming that water makes all movement easier.
