1. Color Appears Only When Oxygen Is Grabbed
Blue is the color of oxygen and copper holding hands. Hemocyanin is a protein that carries oxygen and contains copper. When it is not carrying oxygen, it is colorless. It turns blue when it binds with oxygen [1].
The reason for the color lies in the state of the copper. When there is no oxygen, the copper is in a colorless state known as Cu(I). When oxygen attaches, it changes to a different state, Cu(II), which looks blue. The oxygen molecule binds in a way that fits between two copper atoms [2].
The color of the blood changes depending on the location in the body. This color change is determined by whether the blood has received oxygen or not. This means that even within the same squid, the blood that is carrying plenty of oxygen and the blood after it has released oxygen will have different color intensities.
2. Copper Pigment Binds Oxygen More Loosely Than Hemoglobin
Hemocyanin binds one oxygen molecule to two copper atoms. The bond with oxygen is said to be weaker than that of hemoglobin in human blood [1].
A weaker bond is not necessarily a disadvantage. There is a relationship where gripping tightly makes it hard to let go, while gripping loosely makes it easy to let go. However, this is a general explanation. How squid and octopus actually benefit depends on the species and water temperature.
Dictionaries state that the copper content in hemocyanin is about 0.18% in arthropods and about 0.25% in mollusks [1]. This is the ratio within the protein, not the ratio within the entire blood.
3. Blue Blood Flows in Vessels and Squid Have Three Hearts
Squid and octopus have a closed circulatory system where blood flows inside blood vessels. Hemocyanin exists dissolved in the body fluid, not inside blood cells [7]. Hemocyanin in arthropods also exists in the blood as large aggregates, such as hexamers [4].
Squid have three hearts. There is one heart that sends blood to the whole body, located between two branchial hearts that send blood to the gills [6].
Blood that receives oxygen at the gills turns blue and is delivered to the whole body. If you imagine this flow, you can see that blue is the color of blood "in the middle of carrying oxygen."
4. Shrimp, Crabs, and Horseshoe Crabs Also Have Blue Blood
Hemocyanin is found in the blood of crustaceans like shrimp and crabs, and mollusks like snails and octopus [1]. The blue blood of squid and some crustaceans is due to hemocyanin [5]. The horseshoe crab is an arthropod closer to spiders than to shrimp or crabs. Its blue blood has the property of coagulating when exposed to bacterial toxins, and it is used for testing medical equipment [5].
Although they look similar, they are different inventions. The hemocyanin of mollusks and arthropods have completely different shapes. A student report from the University of Tsukuba states that the two differ significantly from primary to quaternary structure and are thought to have followed separate evolutionary paths [4]. In snails and octopus compared to spiders and scorpions, the oxygen-binding sites may be similar, but the overall shapes are very different [3]. This is seen as an example of similar mechanisms arising separately.
5. Antarctic Octopus Was Reported to Have Higher Hemocyanin Concentration
The octopus living in Antarctica, *Pareledone charcoti*, has been reported to have a hemocyanin concentration in its blood that is at least 40% higher than octopus in southeastern Australia or the Mediterranean [7]. In cold water, it becomes harder to release oxygen. It is explained that increasing the concentration helps maintain oxygen supply [7].
The abstract of a 2015 paper states that adjustments to hemocyanin properties and concentration support oxygen supply at temperatures near freezing [8].
This "40%" figure is a value compared to other species and does not apply to all octopus. Also, the subject of the research was octopus, not squid.
6. If You Want to Look It Up Yourself
You can compare squid, octopus, shrimp, crabs, and horseshoe crabs in encyclopedias or videos to find creatures said to have blue blood. Whether an explanation of hemocyanin is included is a clue.
If you see a horseshoe crab at an aquarium, try reading the display description. You can check with your own eyes how it differs from a crab. Do not touch the tanks.
If you want to know more, the 2003 report by Yukie Kato in the *Tsukuba Biological Journal* is available in Japanese. You can look at how it writes about the differences between mollusk and arthropod hemocyanin.