Biologists now believe fish gills are primarily for regulating chemicals

The traditional view of fish gills as a breathing device may have breathed its last. University biologists in Canada now believe that fish gills are more likely to have evolved for the primary purpose of regulating chemicals in the fish’s body.

Clarice Fu, a zoologist from the University of British Columbia, led the study, whose findings were reported in the Royal Society journal, Proceedings B. The study focused on gill development in rainbow trout larvae. Ms Fu and her colleagues found that as the larvae matured, their gills developed the ability to regulate the chemicals in their blood before they began to take up oxygen.

The scientists looked at the uptake of ions (ie charged chemical particles, such as sodium) which are necessary for the body's cells to function, but which become toxic if too concentrated. Ms Fu said fish take on these ions from the water, in a bid to maintain this delicate balance.

"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 added.

"When the gills are still immature, a significant portion of ion uptake occurs at the skin. As the fish get older and the gills mature, [this] can gradually shift to... the gills," said Ms Fu. "We found that ion uptake shifted from the skin to the gills earlier than oxygen uptake. This led us to propose that the gills are needed for ion regulation earlier than they are needed for oxygen uptake."

A cautionary note, however, was struck by Professor Rick Gonzalez, who studies the physiology of aquatic animals at the University of San Diego in the USA.

Calling the study a "very interesting first step", he said it wasn't clear if it answered the question of why fish evolved gills.

"Gills combine some of the functions of the lungs and kidneys in mammals, which leads to interesting interactions of function," he told BBC News. "The physical and chemical nature of the water can play an important role in their function. So how these all work together to get the various jobs done is very interesting and offers insight into how natural selection works."