5 Animals Whose Blood Isn’t Red

Crocodile Icefish
Unlike every other known type of backboned animal, the crocodile icefish doesn't have any red blood cells — or hemoglobin — at all. Wikimedia

Named for their long, toothy snouts, crocodile icefish (of which 16 species have been recognized) live in the ocean waters around Antarctica. The extremophiles are built to thrive in conditions that would kill most other vertebrates. Crocodile icefish frequent brutally cold portions of the sea where the water temperature can plummet all the way down to 28.5 degrees Fahrenheit (1.9 degrees Celsius). That's below the point at which fresh water freezes.

In extraordinarily cold water, red blood cells turn into a liability. Blood with a high percentage of these cells becomes dangerously thick and hard to circulate when the outside temperature gets too low. That's why fish who live in cold waters have proportionately fewer red blood cells than their warm water counterparts do.

Crocodile icefish take this to the extreme. Unlike every other known type of backboned animal, they don't have any red blood cells — or hemoglobin — at all. It's astonishing!

Now you might be thinking "Wait a second. Without hemoglobin or red blood cells, how do the fish circulate oxygen through their bodies?" To get the job done, they enlist the ocean itself. Cold water is naturally richer in usable oxygen than warm water. Crocodile icefish absorb some of this oxygen directly from the ocean and send it into their blood streams. The blood itself is a colorless liquid, a fact that really surprised the discoverer of these fish, biologist Ditlef Rustad, when he dissected one in 1928. So abundant is the cold water oxygen that, upon absorption, it doesn't need to hitch a ride on red blood cells to get around. Instead, it can travel from point A to point B inside the fish's hemoglobin-free plasma.

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