It can’t be a coincidence that almost all of them have red blood. Why can’t it be blue? Green? Purple?
In: Biology
It *can* be blue; in fact, blue [horseshoe crab blood](https://en.wikipedia.org/wiki/Horseshoe_crab#Blood_harvesting) is immensely valuable for a number of reasons. But the difference might answer your question. Other animals with blue blood include the octopus (where it was first discovered) and a bunch of other invertebrates.
Red blood relies on [hemoglobin](https://en.wikipedia.org/wiki/Hemoglobin) to transport oxygen. It’s a chemical that can weakly bond with oxygen inside lungs, and then lose that bond inside muscles; partially because muscles have [myoglobin](https://en.wikipedia.org/wiki/Myoglobin), which will bond with oxygen more aggressively. Blue(-ish) blood relies on [hemocyanin](https://en.wikipedia.org/wiki/Hemocyanin) to do a similar thing, but mechanically differently.
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