Fish
Icefish
Channichthyidae · also called icefishes, white-blooded fish
Cut an icefish and it barely bleeds red at all — its blood is nearly clear, because it is the only known vertebrate on Earth that survives entirely without haemoglobin, the protein responsible for red blood in essentially every other backboned animal. It gets away with it only because the freezing Antarctic water it lives in holds unusually large amounts of dissolved oxygen, letting an oversized heart and denser blood vessels do the job haemoglobin normally would.
Overview
Icefish, family Channichthyidae, live only in the frigid Southern Ocean around Antarctica, hunting small fish and crustaceans in water few other fish families can tolerate. Like other Antarctic fish, they produce antifreeze glycoproteins that stop ice crystals forming in their blood and tissues.
Blood built for the cold, not for warming
Lacking haemoglobin entirely, icefish rely on oxygen dissolved directly in their plasma, an approach that works only because cold water holds more oxygen than warm water. This makes the whole family unusually vulnerable to ocean warming, since even a modest temperature rise could push conditions beyond what their unique physiology can handle.
Status
Least Concern currently, but decreasing as a long-term concern — climate-driven ocean warming threatens the very oxygen conditions this entire family’s survival depends on.
Icefish: common questions
How can an icefish survive without haemoglobin at all?
Icefish compensate for the complete absence of functional haemoglobin by relying on oxygen dissolved directly in their blood plasma, an approach made viable by the extremely cold Antarctic water they live in, which holds significantly more dissolved oxygen than warmer water would. They further support this strategy with a larger blood volume, an enlarged heart, and a denser network of blood vessels than typical fish, together allowing enough oxygen to be delivered around the body despite lacking the usual haemoglobin-based oxygen transport system nearly every other vertebrate relies on.
Why does icefish blood look almost clear or white?
The red colour of blood in almost all vertebrates comes specifically from haemoglobin, an iron-containing protein that both carries oxygen and gives red blood cells their colour, and because icefish lack functional haemoglobin entirely, their blood has none of this red pigment, leaving it appearing nearly clear or pale, milky white instead. This striking visual difference is one of the clearest and most immediately recognisable signs of the icefish family's highly unusual physiology.
Are icefish related to other fish that live in cold water?
Icefish belong to their own distinct family, Channichthyidae, within a broader group of Antarctic fish called notothenioids, many of which share other cold-water adaptations such as antifreeze glycoproteins in their blood, but icefish are unique even among this already highly specialised group for being the only ones that have lost functional haemoglobin entirely. Other notothenioid fish in the same broader Antarctic fish group typically retain normal haemoglobin-based blood despite sharing some other cold-adaptation traits with icefish.
Why are icefish considered vulnerable to climate change?
Because their entire haemoglobin-free physiology depends on the unusually high levels of dissolved oxygen found specifically in very cold Antarctic water, even a modest rise in ocean temperature could measurably reduce the dissolved oxygen available to icefish, potentially pushing conditions beyond what their specialised, low-oxygen-transport-capacity bodies can successfully tolerate. This makes icefish a particular concern among Antarctic marine species being closely monitored for early signs of climate-driven physiological stress.