As winter buckles down for most of us in the Northern Hemisphere, we’re reminded of how difficult it can be to endure extreme weather. While numerous terrestrial and aquatic adaptations exist to survive the cold, the Notothenioids (pronounced: No-tow-then-ee-oids) are likely the aquatic winners of the the competition for adaptation to extreme cold.
The Notothenioids are a group of about 120 species that are found almost exclusively in the southern ocean, or the waters surrounding Antartica. These waters are not only often below freezing (the salt in the water depresses the temperature at which water freezes), but their polar location means they are often without sunlight for days.
The first interesting adaptation that many Notothenioids have is an ‘antifreeze protein’ in their blood. This protein basically binds to any ice crystals that may form in the bloodstream and prevents them from turning into ice. Ice formation in the bloodstream would likely be fatal, or minimally cause damage to the fish, and thus, the presence of this antifreeze system in antarctic fishes enables them to survive. These proteins are not made by the liver, and it is currently unknown where exactly they are made.
The icefishes (a subgroup of Notothenioids) interestingly have clear blood. Hemoglobin is a protein that carries oxygen from the source (such as lungs or gills) and throughout the body where it is needed, and hemoglobin is what also makes blood red. Because cold waters are supersaturated with oxygen, less hemoglobin should be needed; however, the icefishes have evolved to produce no hemoglobin (but do have larger hearts and a few other adaptations).
And as if those two adaptations weren’t interesting enough, many Notothenioids lack swim bladders—the air-filled pouch that most fish have to help them regulate their position in the water. While this swimm-bladder-less condition has shown up in many other groups of fish, it’s still uncommon, and is typically attributed to the fact that most Notothenioids not only live their lives on the bottom, but that if they were to ascend in the water, they might hit surface ice (or anchor ice), and so there is little incentive overall to want to be off the bottom.
Notothenioids are relatively understudied, which may come as no surprise given their habitats. But some excellent researchers done pioneering work on these fish, and we encourage you to check out this video and some background studies.