How did fish evolve gills?

Chuck Bednar for – @BednarChuck

Contrary to popular belief, fish did not evolve gills to obtain more oxygen as they grew larger and became more active – the adaptation arose to allow them to survive acidic ocean conditions, according to a new study published recently in the journal Scientific Reports.

“When we think of the gill we automatically associate it with a human lung,” Dr. Jodie Rummer, a fish physiologist at James Cook University in Australia and co-author of the new study, said to ABC Science Online on Monday. “So the common thought has always been that perhaps the first reason a water breather needed to evolve a gill is to get oxygen.”

The longstanding proposal, known as the oxygen hypothesis, claims that as fish and other marine organisms grew larger and more active, they required more oxygen in order to maintain a higher metabolism. The structure of the gill appeared to provide the ideal solution, giving the organisms a larger surface to absorb oxygen into their bloodstreams.

However, Dr. Rummer and her colleagues report that their recent analysis of the hagfish appears to cast doubts on this hypothesis. Hagfish, they explained, are the ocean-floor dwellers and the closes living relative of the earliest fishes. They are also quite different than most types of big fish, as they absorb most of the oxygen they need through their skin.

Hagfish found to use gills to maintain blood pH balance

Hagfish resemble eels and are usually only active when they use their teeth to dig into a sunken and decaying carcass. Based on their body shape and low metabolism, the creatures absorb 80 to 90 percent of the oxygen they require through their skin, even though they have gills. So if the gills weren’t needed for respiration, why did they evolve?

As part of an ongoing study involving an analysis of the mechanism used by different fish to deal with ocean acidification, Dr. Rummer’s team placed hagfish in a highly acidic environment. The researchers found that the creatures are able to withstand higher acidity than any other species of fish they studied thus far. Furthermore, they discovered the primary role of the hagfish gill is to regulate the body’s pH balance so they can survive these harsh conditions.

The fish were collected off the west coast of Canada’s Vancouver Island and exposed to various levels of acidity. The researchers pumped carbon dioxide into their water, then took blood tests and tissue samples from the hagfish at different points after the creatures had been exposed to the higher acidity levels to monitor body chemistry changes, according to Discovery News.

Previous studies revealed that many types of fish experience behavior and physiological changes when exposed to highly acidic water, but when Dr. Rummer and her co-authors subjected the hagfish to levels more than 50 times those predicted to be present in the world’s ocean by 2100, they found that the fish were able to deal with the conditions. After their blood pH initially fell by 1.2 units, they were able to use their gills to correct this in just a few hours.

“They’re very, very good at this – the best of all the living fish that we know of,” Dr. Rummer said, adding that they “are probably going to be the best fish to tolerate high CO2.”


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