Scientific Mystery: The Antarctic Icefish: Clear-Blooded

What These Fish Actually Are

There are about 16 species in the family. They live in the Southern Ocean around Antarctica, typically on the continental shelf at depths of hundreds of meters, in water that sits near or slightly below the freezing point of freshwater (about -1.8°C to -2°C). Most grow to 25–50 cm. They are predators that eat smaller fish and krill. Some species look vaguely crocodilian, with large heads, prominent teeth, and big eyes suited to dim light under ice.

Their standout feature is the blood. Icefish are the only known adult vertebrates that lack functional hemoglobin and mature red blood cells. Their blood is colorless to milky-white. Oxygen is carried only by dissolving directly into the plasma—the watery part of blood. That gives their blood roughly one-tenth the oxygen-carrying capacity of a typical red-blooded fish.

A Norwegian zoologist, Ditlef Rustad, first pulled one from Antarctic waters in December 1927 and noted the pale, almost translucent appearance. Johan Ruud confirmed the lack of hemoglobin in 1954. The trait is shared across the family, meaning it arose in a common ancestor millions of years ago after the Southern Ocean cooled.

X users have been circulating clips and photos of the fish’s translucent skin, visible organs, and in some cases a see-through skull. Those images are accurate. Many icefish lack scales, have reduced bone density, and some species also lack myoglobin in the heart, leaving that organ pale as well.

Icefish have translucent or transparent bodies and almost no scales: in some specimens, the only organs that can be seen are their eyes pic.twitter.com/aJ0KcwUrf8

— Massimo (@Rainmaker1973) August 25, 2026

How They Survive Without Red Blood

Cold water holds more dissolved oxygen than warm water. Icefish have low metabolic rates. Together, those two facts make plasma-only oxygen transport viable in their habitat. They compensate with extreme cardiovascular remodeling:

• Hearts that can be four to five times larger relative to body size.
• Blood volume two to four times higher than in red-blooded relatives.
• Wider blood vessels and denser capillary networks.
• Large gills and scaleless skin that allow some oxygen uptake directly through the body surface.

They also produce antifreeze glycoproteins (AFGPs) that bind ice crystals and stop them from growing inside tissues. Without those proteins, their body fluids would freeze.

Genetically, the hemoglobin genes are gone or reduced to remnants. In 15 of 16 species the beta-globin gene is deleted and only a fragment of an alpha-globin gene remains. Transposable elements appear to have helped delete the gene clusters. One species retains a more complete but still non-functional version. The loss is a derived trait unique to this family among vertebrates.

Scientists still argue whether losing hemoglobin was an accident that happened to be survivable in oxygen-rich, low-competition Antarctic waters, or whether it conferred an advantage—thinner blood in the cold, or reduced demand for scarce iron. Both ideas have support. The trait is metabolically expensive (that oversized heart works hard), so it is not an obvious win. It persisted anyway.

An Antarctic fish survives without the oxygen-carrying system most vertebrates rely on.

Icefish lack hemoglobin—and mature red blood cells.

In ordinary vertebrate blood, hemoglobin inside red blood cells carries oxygen through the body. But these fish evolved in the cold,… pic.twitter.com/IJWjZO0Drs

— Ro (@Rotshaq) August 28, 2026

Is It the Mystery the Video Claims?

No. The video treats the absence of hemoglobin as an ongoing puzzle that “scientists still can’t explain.” That oversells the gap. Researchers have mapped the genomes, reconstructed the gene-loss events, measured the physiology, and identified the compensatory changes. Papers from the last two decades treat icefish as a natural experiment in anemia, oxygen transport, and cold adaptation—not an unsolved riddle.

What is still open is the precise evolutionary “why” and some molecular details of how the deletions happened. Those are normal scientific questions, not evidence that the fish defies biology. Popular X posts get the core facts right: no hemoglobin, clear blood, antifreeze proteins, oversized heart. They correctly note that the cold, oxygen-rich environment made the mutation viable. They do not claim scientists are baffled.

A 2021–2022 discovery added scale to the story. Researchers found a breeding colony of Jonah’s icefish (Neopagetopsis ionah) on the Weddell Sea floor with an estimated 60 million active nests across roughly 92 square miles—the largest known fish nesting ground on Earth. Each nest held about 1,700 eggs, often guarded by an adult. That colony shows these animals are not rare oddities; they can dominate local ecosystems.