Gills are often thought of as a fish's lungs. They pump oxygen in and carbon dioxide out. But that's not all gills do. They also balance out a fish's electrolytes – the ions that we lose when we sweat. The gills of modern fish evolved from structures that their ancestors – small, worm-like animals – used to pull little specks of food from the ocean. It's not entirely clear when that change occurred, but it's believed to have happened no earlier than when the first fish appeared. Michael Sackville and Colin Brauner from the University of British Columbia, Canada, teamed up with colleagues from the University of Montreal, Canada, and the University of Cambridge, UK, to test this. They looked at the gills of Pacific lamprey (Entosphenus tridentatus) larvae, Florida lancelets (Branchiostoma floridae) and acorn worms (Saccoglossus kowalevskii) to figure out when fish gills took on their modern role.

The team selected these animals because lampreys are an ancient group of fish and, although the gills of adults behave like other fish gills, their larvae use gills for eating. In contrast, lancelets and acorn worms aren't fish. They look more like how we picture the ancestors of fish; even the adults use their gills for eating. But just because they use their gills to eat doesn't mean they can't also use their gills to breathe and restore electrolytes. If this is the case for all three species, Sackville and the rest of the team reasoned, then gills started to take on the characteristics of modern fish gills before a modern fish ever existed. If, on the other hand, only the gills of lamprey larvae breathe and restore electrolytes, then these functions of the modern fish gills would be entirely a modern fish adaptation.

The researchers started by looking at the lamprey larvae, which they placed in divided chambers such that each half of the chamber contained one half of the lamprey. They measured how much oxygen and electrolytes each lamprey half took in and how much carbon dioxide each half released. Because the gills are located near the head, the scientists were able to compare what happened in each chamber and determine whether more movement occurred in the half with the gills. They found that gills always played an important role in balancing electrolytes, but only really started to play a big role in breathing as the larvae got bigger.

Studying the function of gills in lancelets and acorn worms was a bigger challenge, so the researchers had to get creative. The acorn worms were too fragile for the team to put them in the divided chamber, so instead they cut the acorn worms inhalf to determine whether the half with gills played a larger role in breathing. However, it was impossible to divide the lancelet in such a way that they could isolate the role of the gills. It was also impossible to measure the amount of electrolytes that lancelets and acorn worms took in or released, because the animals live in such salty water that measuring small changes in the electrolytes leaving and entering their bodies was impossible. Instead, the scientists looked for cells in the gills with the same features that fish cells use to move electrolytes across the skin and other body surfaces. They found those features in the gills of both lancelets and acorn worms, even though the acorn worms didn't use their gills to breathe.

Based on these observations, the team offer an exciting conclusion. While breathing through the gills seems to be a large fish adaptation, balancing the electrolytes in the body may have been an older trick of the gills.

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Ion regulation at gills precedes gas exchange and the origins of vertebrates