Cover photo: Icefish larva by Dr. Habil/Uwe Kils, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=207851
Did you know there is a group of marine fish that thrives in below-freezing temperatures? Meet the members of the fish family Channichthyidae, which live around Antarctica. This clade contains around 16 known species of icefish.
Copyrighted free use under Marrabbio2, https://commons.wikimedia.org/w/index.php?curid=492078
In 1928, Ditlef Rustad, a biologist, was working off the coast of Bouvet Island in the Antarctic. He caught a strange fish and named it the "white crocodile fish," a name that remains in use today. It had a long snout, fanned-out and quilled fins, was scaleless, and had an eerie, pale white body color. When he cut it open, he saw that its blood was colorless. He was one of the first biologists to describe this strange-looking creature--one of the most amazingly adapted to polar waters.
Polar waters often reach below freezing--but without completely freezing. This is possible because as water freezes, it expels any salts that were dissolved in the water. Ocean water expels a lot of salt when it freezes, and the salt is pushed into the ocean water below the newly-formed ice. If you or someone you know lives in a cold location, you may know that government services will disperse salt on public roads to keep roads free of ice after snowstorms or freezing rain. Salts lower the freezing point of water, meaning that as more salt is added, increasingly lower temperatures must be reached for the salt-water mix to freeze. As salt is added to water, the water's density also increases--meaning that the now more salty (and more freeze-resistant) water will sink to the bottom. The less dense ice floats on the ocean's surface and also acts as an insulator to further prevent the ocean water below from freezing.
How do these fish survive in the extreme cold of their environment?
For starters, their components at the cellular level are different from those of other animals. Their proteins, such as cellular enzymes that aid metabolism, channels that move nutrients and wastes, and carriers that transport oxygen through the bloodstream, are all adapted to function at near or below-freezing temperatures. They produce an antifreeze that runs through their veins, and their eggs contain ice-preventing proteins to protect the eggs from freezing. One of their most shocking adaptations is that they have clear blood! This is a result of having no hemoglobin, proteins that give usual vertebrate blood its color and carry oxygen in the bloodstream. Icefish are the only vertebrates--the only animals with a backbone--that live without hemoglobin. This is a massive sacrifice biologically speaking, because hemoglobin is the most efficient oxygen transporter animals have developed. It's what makes all other vertebrates such efficient, high-grade users of oxygen that obtain more energy than other animals can. Scientists have even questioned if this unique feature of icefishes was a disadvantageous adaptation rather than a helpful one, and all their other adaptations occurred to make up for the loss!
One critical factor that was never in the icefish's control was the amount of oxygen dissolved in the ocean water itself. However, the colder water is, the more dissolved gases it can store. This feature of their frigid environment ensures that, with all of the icefish's adaptations, it can still get plenty of oxygen for its energy needs. Alongside its specifically cold-adapted cellular adaptations, the fact that warmer water contains less dissolved oxygen means these fish can only live in very cold waters.
On a larger, anatomical level, they have a heart four times the size of their close, red-blooded fish relatives; they also have larger-than-normal gills, an expanded circulatory system, and no scales, allowing them to absorb oxygen across their skin or through their gills and then transport it in large quantities at a fast rate. In fact, they spend a little over 1/5 of their energy just to pump blood through their body!
As Antarctic scientists have researched and learned the adaptations of cold-water organisms, they've realized that many of these creatures cannot survive in warmer waters. The problem this poses for an abundance of Antarctic creatures is that, as global warming increases ocean temperatures, their own environment may become too warm for their adapted functions, causing many fish and invertebrates of the Antarctic to go extinct. Our stewardship of the planet really does affect everything out there!
Resources:
ITIS, the Interagency Taxonomic Information System: https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=171119#null
A scientific article on the icefish's genome and extreme cold adaptations: https://www.nature.com/articles/s41559-019-0812-7
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