She and her colleagues found, as the larvae matured, their gills developed the ability to regulate the chemicals in their blood earlier than they began to take up oxygen.

To discover this, the team measured the uptake of ions, which are charged chemical particles, such as sodium.

These ions are necessary for the body's cells to function, but they become toxic if their levels in the blood become too high.

Ms Fu explained that fish take up these ions from the surrounding water, to "maintain this delicate ion balance in their blood".

"In freshwater fish, like rainbow trout, they tend to lose ions from their blood to the water, because the ion concentration in blood is greater than that of freshwater," she said.