Ancient fish evolved gills to survive acidic oceans
Gill evolution 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.
Hagfish have lurked on the deep ocean floor for millions of years and are actually the closest living ancestor of the first fishes, says Rummer.
The thing is they are not your typical active thick-skinned big fish.
They are eel-like scavengers that generally lie around and the most active they get is when they use their rasping teeth to burrow into a sunken decaying carcass -- playing a key role in recycling ocean nutrients.
Given their body shape, skin type and low metabolism, they can absorb quite enough oxygen through their skin. Studies have shown they get 80 to 90 per cent of the oxygen they need this way.
So, the question, asked Rummer and colleagues, was what did these ancestral hagfish use their gills for? And could this shed light on why gills evolved in the first place?
As part of an ongoing study of the mechanisms used by different fish to cope with ocean acidification, Rummer and colleagues tested what happened when hagfish were put in a highly acid environment.
Their findings show these ancient fish are capable of withstanding higher acidity than any other species of fish studied to date.
In fact, the researchers found the main role of their gills is to regulate their body's acidity so they can cope with an acid environment.
The researchers collected hagfish off the west coast of Vancouver Island in Canada, and exposed them to various levels of acidity, by pumping CO2 into their water.
They then took blood tests and tissue samples at various times after the fish were exposed to the increased acidity, to see how their body chemistry changed.
Previous research has found that many fish become behaviourally and physiologically affected when exposed to the kind of acidity predicted to occur in the world's oceans by 2100.
However, when Rummer and colleagues exposed the hagfish to 50 to 60 times those levels, the fish coped quite well.
The fish's blood pH initially fell by 1.2 units, but after just a few hours their gills were able to correct this by bringing in bicarbonate ions from the surrounding water.
"They're very very good at this -- the best of all the living fish that we know of," says Rummer.
"We think that hagfish are probably going to be the best fish to tolerate high CO2, in the oceans," says Rummer.
"But," she asks. "Do we really want an ocean full of hagfish?"