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Antarctic Marine Life Adapted to the Cold. Now the Ocean Is Heating Up

These swimmers evolved to brave the icy seas but, as waters warm, they are under threat.

Like other Antarctic animals, the spiny plunderfish may not be able to survive in warming waters. 
Lloyd Peck/British Antarctic Survey

Despite the Southern Ocean's freezing temperatures, thousands of marine species call these Antarctic waters home. Over many millennia, they've become tailored to the extreme environment -- but under the veil of a worsening climate crisis, their life-saving adaptations may steadily turn into survival disadvantages.

On Wednesday, in the journal Royal Society Open Science, researchers warn that as a trade-off for accommodating the icy seas, Antarctic creatures have lost their ability to healthily grow and flourish in warm waters. As a result of global warming, though, warm water is precisely what the future holds for them.

"As ocean temperatures increase with global warming, it is a timely reminder of the differences in species that have evolved to live at widely different temperatures," Keiron Fraser, a marine biologist at the University of Plymouth and lead author, said in a statement. "If Antarctic fish are increasingly exposed to higher temperatures, it will have implications for their survival, as well as effects on many critical physiological processes, including growth."

The researchers arrived at their conclusions by comparing two types of fish, one native to the Antarctic and called the spiny plunderfish and another acclimated to warmer regions and called the shanny. They looked at how the fish react to the same water temperature of 3 degrees Celsius (37.4 degrees Fahrenheit). 

The researchers chose this experimental climate because it's both warmer than Southern Ocean temperatures, which reach lows of minus 2 degrees Celsius (28.4 degrees Fahrenheit), and colder than the shanny subjects' original tank located in the United Kingdom. When the researchers collected the shannies, these animals were in a commercial enclosure heated to about 14 degrees Celsius (57.2 degrees Fahrenheit), the paper says. 

After the fish were exposed, the team found the spiny plunderfish consumed 20% less food than the shanny and grew at only half its rate. 

Per the study, this observed block in growth is because the spiny plunderfish's cold water-specific protein metabolism didn't fare well in warm water. In other words, its body couldn't efficiently break down proteins to facilitate growth at the heated temperatures it isn't used to. On the other hand, the shanny didn't seem to struggle much, which also suggests temperate climate fish have a better chance than Antarctic fish at adapting to new climates.

"Antarctic fish are highly thermally constrained and cannot live long-term at temperatures much above those that they currently inhabit," Fraser said. "In contrast, many temperate species are more tolerant of a wide range of temperatures as they often inhabit extensive latitudinal ranges."

Further, the researchers emphasize that beyond their own study, many cold water species studied so far have been seen undergoing similar problems with protein metabolism. "It seems this is a ubiquitous constraint on life at low temperature," Lloyd Peck, a physiologist with the British Antarctic Survey who studies animal adaptations in extreme environments, said in a statement.

And that's just on the note of protein metabolic constraints. 

The approximately 20,000 species in the Southern Ocean, the researchers say, have developed many other cold-water-specific body modifications that may not bode well in warm climates.

"There are many other unique adaptations in Antarctic marine species, such as 16 species of fish that are the only animals with backbones that do not have red blood cells or hemoglobin to carry oxygen around their bodies, or giant sea spiders thousands of times heavier than the largest in temperate zones," Peck said.

Many of these adaptations make life easier in an environment with constantly low temperatures. But, like the protein metabolism issue, they could be detrimental to the animal in changing, heating environments. Thus, considering the terribly rapid pace of global warming, Peck calls prospects for many Antarctic marine species "bleak."