Why do octopuses have blue blood? 0 10

Ancient multicellular organisms were small and did not require an oxygen distribution system.

As their sizes increased, animals developed gills and a fluid began to circulate throughout their bodies.

With further growth, there arose a need for respiratory pigments that enhance the oxygen-carrying capacity of the blood.

The "working tools" of these pigments became metal atoms, as they easily bind and release oxygen. The choice of metal was likely the result of a random mutation.

In mollusks and crustaceans, these turned out to be proteins that "work" with copper (hemocyanin, which is blue), while in vertebrates, they work with iron (red hemoglobin).