Fish gills evolved to balance pH, not breathe, suggests a new study. This goes against the traditional assumption that gills first evolved so fish could get more oxygen as they became bigger and more active, say researchers in a recent issue of Scientific Reports.
“When we think of the gill we automatically associate it with a human lung,” says co-author Dr Jodie Rummer, a fish physiologist at James Cook University in Townsville. “So the common thought has always been that perhaps the first reason a water breather needed to evolve a gill is to get oxygen.”
According to this so-called ‘oxygen hypothesis’, as organisms got fatter and more active, they needed more oxygen to sustain a higher metabolism.
A lot less oxygen dissolves in water than in air. While slow-moving, slim-lined fish could get away with absorbing oxygen through their thin skin, more active fish tended to be thicker-skinned (for protection) so they had to find more effective ways to get their oxygen.
The intricate folds of the gill provided the perfect solution. They had an immense surface area to allow oxygen to be absorbed into the fish’s bloodstream.
But Rummer and colleagues’ study of hagfish challenges the assumption that getting oxygen was the driver for fish developing gills.
Continue reading ‘Ancient fish evolved gills to survive acidic oceans’