Octopuses have three hearts and blue blood, which helps them carry oxygen in cold water.

The longer version

Two small hearts push blood through the gills, and a larger one drives it onward to the body, so pressure does not collapse after oxygen pickup. The pigment carrying oxygen is copper-based rather than iron-based, which is why the fluid looks blue when loaded.

It grabs oxygen readily in cold water where supplies run thin, an advantage low in oxygen-poor habitats, though it carries less per unit of volume, so circulation must work hard. Vessels also pressurise the arms, letting eight flexible limbs extend and grip without any bones inside them. Their vessels branch widely, feeding muscles that work hard during rapid escape.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.