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Fish swim in a tank.
Photo Credit: Peter Rejcek
Antarctic toothfish, Dissostichus mawsoni, swim in a tank in the Crary Lab aquarium at McMurdo Station. Scientists are using them and other fish in the Southern Ocean to learn about the broad adaptations that allow them to function in freezing water.

Growing cold

Scientists study broad adaptations of Antarctic fish to freezing waters  

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The aquarium tanks in McMurdo Station’s External U.S. government site research laboratory, the Albert P. Crary Science and Engineering Center External U.S. government site, contained a species of Antarctic fish during the 2012-13 austral summer that has been a rare sight in recent years — Dissostichus mawsoni.

The Antarctic toothfish, one of the largest of the 120-plus species that make up the group of fish known collectively as notothenioids External Non-U.S. government site, has been a favored specimen for study for nearly a half-century. Like many of the notothenioids that inhabit the Southern Ocean around Antarctica, mawsoni have special adaptations that allow them to survive in the frigid waters.

Perhaps the most well-known and intensely studied trait is the ability of these fish to produce a sort of antifreeze protein that binds to ice crystals in the body to prevent them from growing. Otherwise, the fish would quickly freeze and die in seawater as cold as minus 1.8 degrees Celsius.

Peope weigh a fish.
Photo Courtesy: Paul Cziko
Scientists Paul Cziko, left, and Konrad Meister weigh an Antarctic toothfish in the field.

The sheer size of Antarctic toothfish, which can tip the scale at nearly 150 kilograms, makes it an ideal candidate for researchers who need tissue and blood samples for various analyses, some studies delving deep into the organism’s cell biology, as they attempt to learn more about how mawsoni and its cousins function in the world's coldest waters.

“You can actually get fluids from tiny little ducts and canals from the mawsoni that you can’t get from the smaller fish,” explained Paul Cziko External Non-U.S. government site, a graduate student at the University of Oregon External Non-U.S. government site who led the field research out of McMurdo Station this past season.

“We use the mawsoni a lot because it gives you a lot of material,” noted Christina Cheng External Non-U.S. government site, a professor at the University of Illinois at Urbana-Champaign External Non-U.S. government site and principal investigator on a project to understand the genetic underpinnings that allow the mawsoni and its cold-loving cousins to exist in such an extreme environment.

“It’s a precarious kind of existence,” Cheng said.

The first insights into how notothenioids cope with cold conditions came back in the 1960s, largely through research by Art DeVries External Non-U.S. government site, also a professor at University of Illinois at Urbana-Champaign and a co-principal investigator on the current project with Cheng. Then a graduate student at Stanford University, DeVries discovered that antifreeze proteins circulated through the bodies of the fish.

Since then — and thanks to the genomic External U.S. government site revolution that now allows scientists to reproduce the blueprint of an organism’s complete set of DNA — Cheng, DeVries and their team have produced a wealth of data about the evolutionary history of the antifreeze protein trait.

The adaptation didn’t emerge until relatively recently, somewhere around 20 million years ago, after Antarctica turned into an icehouse. Cheng’s recent research revealed that the antifreeze protein evolved from a small bit of DNA that was attached to another gene, which was a digestive enzyme.

People stand near wall of ice.
Photo Credit: Paul Cziko
Field team members fish at the Cape Evans wall.

“That piece got duplicated and duplicated and duplicated — maybe changed a little bit — and made this really long repetitive gene that codes for protein and made this antifreeze protein,” Cziko explained. “It was a completely new invention.”

“Here we actually see how the evolution of a single protein contributes to survival of a species or a whole clade of these organisms,” he added.

Thanks to the antifreeze protein and other adaptations — such as the ability to maintain neutral buoyancy despite the absence of a swim bladder — the notothenioids filled many of the ecological niches left vacant by those less suited for frigid Southern Ocean. 

Deoxyribonucleic acid External U.S. government site, or DNA, is the chemical compound equivalent of both a construction and instruction manual for nearly all living things. DNA molecules are made of two twisting, paired strands, each of which is composed of four chemical units called nucleotide bases that comprise the genetic “alphabet.”

The order of the bases determines the meaning of the information encoded in that part of the DNA molecule, just as the order of letters determines the meaning of a word. An organism’s complete set of DNA is called its genome.

Vials sit in a tray of ice.
Photo Credit: Peter Rejcek
Tissue samples from an Antarctic toothfish sit in a tray of ice.

Cheng began her molecular and genomic research after working with a student who sequenced the first antifreeze protein gene. Sequencing External U.S. government site involves determining the exact order of the bases in a strand of DNA.

“I found that sort of thing very interesting. I sort of taught myself molecular biology after the student left,” she said.

The team has since sequenced the entire genome of the Antarctic toothfish. Now Cheng wants to look more broadly at the adaptations that mawsoni and other fish of its ilk in the Southern Ocean possess that allow them to function at nearly minus 2 degrees Celsius.

To figure that out, she and her group need more tissue samples from the Antarctic toothfish, as well as from a non-Antarctic species of Notothenioidei, Eleginops maclovinus, which is as closely related to the mawsoni as possible but without the antifreeze protein or evolutionary cold adaptation.

“This whole genome study is to try to figure out what are these changes in all of the proteins that are needed for everything — from growth, development, reproduction, metabolic functions, energy production. Everything,” Cheng said. “There has to be something that is different so they can work at this low temperature.”

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