On December 1, 1927, zoologist Ditlef Rustad pulled ashore on Bouvet Island, 1,750 kilometers off the coast of Antarctica, as part of a Norwegian expedition to claim the remote, wind-whipped island as a whaling outpost. Later that month, casting nets into the frigid waters, Rustad hauled up a very strange-looking fish. It had no scales and was very pale, even translucent in parts. Behind its protruding, crocodile-like jaw, he saw gills that were milky instead of the usual crimson. And when Rustad cut open the fish, he saw that its blood was transparent, like ice water. “Blod farvelöst,” he wrote in his notebook—“colorless blood.”

In a 1954 Nature paper, biochemist Johan Ruud confirmed that Chaenocephalus aceratus lacked red blood cells and hemoglobin, the protein that carries oxygen around the body and gives blood its red color. “It was a shocking discovery,” says William Detrich of Northeastern University, who has spent most of his career studying C. aceratus and the other 15 recognized species in the family Channichthyidae, or the Antarctic icefishes. “Among the 50,000 or so species of known vertebrates, these fish are the only examples that lack both hemoglobin and red blood cells.”