Why do giant squid have blue blood?

 A few weeks ago I attended at cinema showing of Inside Nature's Giants: The Giant Squid. In the series a team of experts dissect and examine some of the World's largest animals.

The giant squid (ref)

The Giant Squid is a little understood member of the class Cephalopoda, the cephalopods, known as Architeuthis dex (among other names). They are truly amazing animals, growing up to 20 meters long without a bone in their body (only a beak formed of protein and chitin).

Among the mysteries which were discussed in the programme and the Q&A session with some of the cast afterwards was the question;

"Why do giant squid have blue blood?"

Answered simply, it is because the chemical makeup of their blood is different to that of humans and other animals which use haemoglobin to transport oxygen. Squid have haemocyanin as their oxygen carrier. Haemocyanin looks blue when bound to oxygen.

Haemoglobin uses iron atoms and haemocyanin employs copper. These are both transition metals, meaning they can change their oxygenation state (electrical state) to allow the reversible binding of molecules such as oxygen. While held, or complexed, by the protein around them these metals change colour dependent upon their oxygenation state.

This still hasn't answered the question; "Why?"

Squids and Humans had a common ancestor ~590 million years ago, though Richard Dawkins admits in The Ancestor's Tale that this dating is "difficult and controversial". That the two both have oxygen carrying proteins in their blood is probably an example of convergent evolution. So it's just a quirk of evolution...?

Haemocyanin, however, is much less efficient than haemoglobin at oxygen transport.

According to the Map of Life some insects use haemoglobin while other "more primitive" (or deeper branching) insects use haemocyanin. At some point insects must have recruited or independently evolved haemoglobin. But giant squid didn't.

Haemocyanin seems to have become intertwined with other functions in arthropods (a phylum of which squid are members), including immune response and cuticle (skin) colour.

There is another consideration when looking at biological systems, how available are the elements in the environment. Iron accounts for between 5 and 6% of the Earth's Crust, copper in contrast accounts for less than 0.01%. In the oceans iron has an average concentration of 540 pmol/kg (540 x 10^-12 mol/kg) and copper 2.4 nmol/kg (2.4 x 10^-9 mol/kg). Also copper concentration increases continually with depth, while iron reaches a maximum midway down.

Giant squid live at between 300 and 600 meters down, compared to an average ocean depth of 3790 meters. This implies that the squid do not occur at depth beyond the iron maximum, however they may still experience a high copper abundance not found by land animals.

Abundance may, but is highly unlikely to cause the observed use of haemocyanin not haemoglobin, other ocean dwelling animals, such as fish, use haemoglobin.

It appears that giant squid have blue blood due to evolution and the subsequent co-option of haemocyanin for a range of functions other than oxygen transport.

And of course the question still remains; are giant squid really blue-blooded?