How many alternative chemical ways to transport oxygen in living creatures are known?
Yes! A variety of metalloproteins are used for oxygen transport. These include proteins that bind to porphyrins with different central metal atoms, as well as proteins that bind the metal atoms directly, without a porphyrin ligand. Fun fact- hemocyanin, which uses two copper atoms as prosthetic groups to bind oxygen, is responsible for the blue color of horseshoe crab blood!
Here are some relevant links:
https://en.wikipedia.org/wiki/Respiratory_pigment
https://en.wikipedia.org/wiki/Hemocyanin
https://en.wikipedia.org/wiki/Hemoglobin (scroll to &quot;other oxygen-binding proteins&quot; near the bottom)