Your question makes perfect sense... here is my stab at answering it:
Octopuses are one of many organisms that use copper-based blood (the molecule containing the copper is called Hemocyanin, analogous to hemoglobin in humans and other vertebrates, which contains iron). Essentially all other mollusks have copper-based blood as well (with a few odd-balls using iron-based blood, mostly hydrothermal vent critters), as well as arthropods (insects, spiders, crustaceans like crabs and lobsters) and some various worms. Really there are probably more species on earth today that have copper-based blood than iron-based.
Why octopuses use hemocyanin (copper-based blood) rather than iron-based blood is a really big question that is very hard to answer simply. The best reason is probably because their ancestors did (other mollusks). But using copper based blood gives them a couple of advantages, one of which being that it can work better when the octopus is in low oxygen environments.
Why is there blood blue? Well that is because copper is blue in the particular oxidation state (fancy phrase meaning how it is bonded to other things) of which octopus blood uses it to carry oxygen. Metals turn different colors depending on how they are bonded to other chemicals, like oxygen. Fort instance pennies appear the familiar "copper" color, because they are bound to a lot of other copper molecules. The Statue of Liberty, however, looks green, because the copper it is made of has "rusted" and bound to oxygen. The copper in octopus blood is bound to both a blood molecule (the hemocyanin) and an oxygen molecule, so it turns blue. You can see the same thing in your blood, with the iron in it. When the iron in your blood is bound to both oxygen and hemoglobin, it is red, but when the iron is not bound to the oxygen anymore (in your veins) it is blue. You may be interested to know that octopus blood when there is no oxygen in it (so when it is in the veins instead of arteries) is actually clear, and not blue.
As for octopuses getting tired out more easily because of using copper-based blood, this is really blown out of proportion by most people. This is very true for a lot of other organisms that use copper blood (like crabs and such...) but octopuses and squid are such high performance animals (especially for invertebrates), that they have overcome more other the limitations of copper-based blood, and really it has performance comparable to some fishes.
Ok, that is a very long response to all of that... if there is anything you don't understand, let me know and I explain it another way, or if you want to know anything else... just holler.
Octopuses are one of many organisms that use copper-based blood (the molecule containing the copper is called Hemocyanin, analogous to hemoglobin in humans and other vertebrates, which contains iron). Essentially all other mollusks have copper-based blood as well (with a few odd-balls using iron-based blood, mostly hydrothermal vent critters), as well as arthropods (insects, spiders, crustaceans like crabs and lobsters) and some various worms. Really there are probably more species on earth today that have copper-based blood than iron-based.
Why octopuses use hemocyanin (copper-based blood) rather than iron-based blood is a really big question that is very hard to answer simply. The best reason is probably because their ancestors did (other mollusks). But using copper based blood gives them a couple of advantages, one of which being that it can work better when the octopus is in low oxygen environments.
Why is there blood blue? Well that is because copper is blue in the particular oxidation state (fancy phrase meaning how it is bonded to other things) of which octopus blood uses it to carry oxygen. Metals turn different colors depending on how they are bonded to other chemicals, like oxygen. Fort instance pennies appear the familiar "copper" color, because they are bound to a lot of other copper molecules. The Statue of Liberty, however, looks green, because the copper it is made of has "rusted" and bound to oxygen. The copper in octopus blood is bound to both a blood molecule (the hemocyanin) and an oxygen molecule, so it turns blue. You can see the same thing in your blood, with the iron in it. When the iron in your blood is bound to both oxygen and hemoglobin, it is red, but when the iron is not bound to the oxygen anymore (in your veins) it is blue. You may be interested to know that octopus blood when there is no oxygen in it (so when it is in the veins instead of arteries) is actually clear, and not blue.
As for octopuses getting tired out more easily because of using copper-based blood, this is really blown out of proportion by most people. This is very true for a lot of other organisms that use copper blood (like crabs and such...) but octopuses and squid are such high performance animals (especially for invertebrates), that they have overcome more other the limitations of copper-based blood, and really it has performance comparable to some fishes.
Ok, that is a very long response to all of that... if there is anything you don't understand, let me know and I explain it another way, or if you want to know anything else... just holler.