Final answer:
Gill-breathing animals need water movement to breathe because their gills extract dissolved oxygen from water. They either move through water or use appendages to ensure water flows over their gills, which is essential due to the low concentration of oxygen in water compared to air.
Step-by-step explanation:
Gill-breathing animals such as fish must have water continuously moving over their gills to obtain oxygen. The process of respiration in these animals involves the passage of water, which contains dissolved oxygen, over the gill surfaces where oxygen is absorbed. Because the oxygen concentration in water is much lower than in the atmosphere, gills are highly branched and folded to increase the surface area for oxygen uptake.
To ensure a sufficient flow of oxygenated water over their gills, these animals either move through water or have specialized appendages to draw water across their gills. Movement such as swimming can be facilitated by muscle contractions that create undulations, propelling the animal forward and causing water to flow over the gills. Appendages may also actively pump water over the gills in stationary animals or those with limited mobility.
Evolution has equipped gill-breathing animals with adaptations such as a streamlined body shape to reduce resistance in water, various fins for propulsion and steering, and a swim bladder for buoyancy control, all of which are integral in their ability to move or create water movement for respiration.