Biofluorescent snailfish brave Arctic waters with built-in antifreeze
These glowing animals have exquisitely evolved to live in subzero temperatures. But a warming climate might threaten their existence.
Some of the most significant scientific inventions—penicillin, gunpowder, the microwave—were discovered by accident. Now a group of researchers investigating how some animals live in the freezing Arctic have another to tack on the list: natural antifreeze. A new study published today in the journal Evolutionary Bioinformatics found that a tiny snailfish species living in Greenland contained sky-high levels of antifreeze proteins that made it possible to survive subzero temperatures.
In 2019, study coauthor David Gruber, a research associate at the American Museum of Natural History in New York and a distinguished biology professor at CUNY’s Baruch College, was out with his team on an expedition to eastern Greenland to look for animals that glowed in the dark under the ice. Located in the Arctic Circle, this region of Greenland gets near-full days of summer sun, but is plunged in darkness during the winter months. The team’s goal was to understand the role light plays in marine species living in these environments with such drastic seasonal periods of never-ending and very limited sunlight. Their search led them to a juvenile biofluorescent snailfish, a small fish with a tadpole-like body typically found in frigid waters dipping well below freezing , at 28.4 degrees Fahrenheit. Biofluorescence is when an animal absorbs blue light and emits either green, red, or yellow light—a rarity among Arctic fishes that live in darkness for most of their lives.
To better understand how snailfish create light, the biology team examined its entire transcriptome—every gene it is making—where they were surprised to find that one of the most actively made proteins in the body was antifreeze proteins. “Similar to how antifreeze in your car keeps the water in your radiator from freezing in cold temperatures, some animals have evolved amazing machinery that prevent them from freezing, such as antifreeze proteins, which prevent ice crystals from forming,” Gruber said in a press release.
Marine biologists had already uncovered the existence of antifreeze proteins 50 years ago. Several species from fish, reptiles, insects, to bacteria are known to have evolved antifreeze proteins to survive in icy habitats. For snailfish, antifreeze protein is developed in the liver where it prevents large ice grains from forming inside cells and body fluids. Without antifreeze protein, the blood of snailfish would turn frozen solid.
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Since the initial discovery, biologists have since found that antifreeze proteins are created through five different gene families. But marine biologists did not know how much energy snailfish spent in creating antifreeze proteins. “In retrospect it makes sense—of course a juvenile fish living on an iceberg is making lots of proteins that prevent it from freezing,” explained Gruber. In their genetic analysis, the team found two gene families in charge of encoding two types of antifreeze proteins, called Type I and LS-12-like proteins. These genes were highly expressed, making up the top 1 percent of expressed genes in snailfish.
The study authors suggest that the high expression levels for these antifreeze proteins are essential for living in extremely cold waters. Some marine biologists, however, have casted some doubts on how big of a conclusion to draw from these results. C.-H. Christina Cheng, an evolutionary biologist at the University of Illinois Urbana-Champaign who was not affiliated with the study, said that LS-12-like proteins also present in the Northwest Atlantic longhorn sculpin did not provide much help in preventing fish from freezing to death. Instead, she says it’s possible the snailfish could be expressing this protein for another developmental reason. What’s more, the expression Type I antifreeze protein found in the snailfish is different from other Type I proteins from the same species.
Cheng said these discrepancies could be resolved by further looking at antifreeze protein activity directly in the blood plasma. “If all these detected transcripts are actually made into functional antifreeze proteins, the plasma antifreeze activity would be high,” she explains. “But if the plasma antifreeze activity is low, then it’s questionable that these transcripts are made into active antifreeze proteins.”
[Related: How polar animals cope with frigid darkness for months at a time]
Still, the new study does highlight the importance of antifreeze proteins in the survival of snailfish living in the Arctic—an environment that is particularly vulnerable to rising global temperatures. Since the past century, the Arctic has been warming four times as fast as the rest of the planet, with predictions projecting an ice-free Arctic ocean in 30 years.
As the region undergoes dramatic changes, ice-dwelling fish will be forced to adapt to warmer climates or face extinction. “For these juvenile snailfish, their superpower of making lots of antifreeze proteins will no longer be a superpower in an Arctic without icebergs,” Gruber said. To make matters worse, warmer waters may introduce more fish species that tend to reside in temperate climates, increasing competition for food and shelter.
In the future, Gruber and his team plan on further investigating the nuances of antifreeze in snailfish and other species living in these frozen environments. “Snailfishes are an interesting family as they have representatives that live at surface to beyond 8,000 meters deep [in the ocean],” he said. “We are curious to investigate if there are any connections between snailfishes ability to survive extreme cold and extreme pressure environments.”