Just when you think you’ve got it figured out: vertebrates use red blood with hemoglobin. The hemoglobin carries oxygen in the bloodstream. Even some invertebrates use hemoglobin, although not octopuses: they have a blue copper-based compound in their blood instead.

And then there’s the icefish. These fish have no hemoglobin, or anything else to bind oxygen in their blood. Instead, they have clear plasma. Because they live in very cold areas, and have very low metabolisms, icefish can get away with having oxygen simply dissolve in their blood.

Scientists have known about the icefish and its clear blood since at least 2006, but it wasn’t until recently that a specimen has been kept in captivity. The Tokyo Sea Life Park has a mating pair of ocellated icefish. Not only do these deep-water Antarctic fish have clear blood, they have no scales.

Just another example of evolution disproving what we think we know, and laughing at our generalizations. There are certain physical and chemical laws involved, of course: this strategy may save the fish the metabolic cost of maintaining hemoglobin and red blood cells, but would only work for a lethargic fish in cold, deep waters.

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