science
Octopuses have blue blood and three hearts, and two of those hearts actually stop working during one common activity.
An octopus has one main heart pumping blood to its body and two side hearts pushing blood through its gills to pick up oxygen. When it swims by jetting water through its body, the muscle contractions involved squeeze and temporarily pause those two gill hearts.
This is why swimming tires an octopus out quickly, and it prefers crawling along the seafloor, which doesn't interrupt circulation the same way. Sleeping and eating don't have this heart-pausing effect at all.
Octopus blood is blue rather than red because it uses a copper-based molecule instead of iron-based hemoglobin, which works better at carrying oxygen through cold, low-oxygen water.
science
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