One of the many times at the beach as a teen, I recall a horseshoe crab and something blue on the sand near it. Years later I learned their blood is blue.
Our blood is red due to hemoglobin which contains iron. Iron binds with oxygen to make rust. Rust is red. So our blood looks red. This hemoglobin on red blood cells carries oxygen from the lungs to the rest of the body.
Hemocyanin which contains copper accomplishes the same task of carrying oxygen. And has a strangely cool color. Some sea creatures are where we typically find hemocyanin.
The other day a news piece talking about the search for alien life brought up the usual life can look different than we think. Carbon-based versus silicon-based life comes as an example. (The prevalence of carbon makes this seem less likely to me.)
But really I wrote this post wondering if the phrase “blue blood” which refers to an aristocrat comes from some animal. According to Etymology Online:
1834, translating Spanish sangre azul, claimed by certain families of Castile as uncontaminated by Moorish or Jewish admixture, probably from the notion of the visible veins of people of fair complexion.