Animals whose blood isn't red (2021)
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. Unlike every other known type of backboned animal, they don't have any red blood cells - or hemoglobin - at all. 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. 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.