Octopuses have three hearts and blue blood

An octopus has three hearts: two smaller ones pump blood through each pair of gills, while one larger heart circulates it through the rest of the body.

That larger "systemic" heart actually stops beating while the octopus is actively swimming — one reason octopuses would rather crawl along the seafloor than swim long distances: swimming quite literally exhausts them more than it should.

The blood is blue because it carries oxygen using a copper-based protein called hemocyanin instead of the iron-based hemoglobin humans rely on — hemocyanin works better in cold, oxygen-poor water.

The bigger picture: octopuses evolved down a completely different branch than vertebrates, and yet they developed one of the most complex forms of intelligence in the animal kingdom — proof that nature has more than one route to a "smart" brain.