Antarctic Icefish
The only vertebrate on Earth with no red blood cells — it runs on transparent blood and has a visible heart
No photo available for Antarctic Icefish
Superpower
Antarctic icefish have no hemoglobin and no red blood cells — the only vertebrates on Earth to have lost both. Their blood is completely transparent. To compensate, their heart is up to four times larger relative to body size than comparable fish, their blood volume is four times normal, and their vessels are significantly wider — pushing oxygen-dissolved blood through the body by volume rather than by hemoglobin chemistry. In the near-freezing, oxygen-rich waters of the Southern Ocean, this unconventional strategy works.
Overview
There are 16 species of icefish, all found in the Southern Ocean surrounding Antarctica. Their transparent bodies reveal ghostly outlines of organs, and their hearts are visible through the body wall — beating slowly, pumping colorless blood. The loss of hemoglobin would be fatal for virtually any other vertebrate; in extremely cold, oxygen-saturated Antarctic waters, it's survivable. Icefish are sluggish ambush predators spending much of their time motionless on the seafloor waiting for krill or small fish.
Found in
The Southern Ocean surrounding Antarctica at depths from the surface to over 2,000m. Most species live on or near the seafloor in continental shelf waters at temperatures between -1.9°C and 2°C.
Things worth knowing
- 1
Icefish bones are partially cartilaginous — their skeletons are less mineralized than most fish, reducing the metabolic cost of building and maintaining bone in an environment where every calorie is critical.
- 2
Icefish have antifreeze glycoproteins in their blood — special molecules that bind to ice crystals and prevent them from growing, allowing survival in water technically below the freezing point of normal seawater.
- 3
The transparent blood creates a visible window into the cardiovascular system — researchers have used living icefish to study heart valve function and blood flow in real time without surgery or imaging equipment, simply by observing the body wall.
- 4
Despite having no hemoglobin, icefish absorb oxygen directly through their skin — their unusually smooth, scaleless skin allows cutaneous respiration that supplements their blood's limited oxygen-carrying capacity.
- 5
Icefish eggs are among the largest of any teleost fish — the yolk provides enough energy to sustain development through the long Antarctic winter with no external food, an adaptation to an environment of extreme seasonal scarcity.