Blue Blood

HELEN
Here is a question I would appreciate an evolutionist answer for in answer to an email I received:

All these organisms have blue blood

lobster
crab
crayfish
scorpion
octopus
squid
clam
mussel

Question: this being the case, how are all of these cladistically related in order that hemocyanin rather than hemoglobin is the oxygen carrier?

I can understand the lumping of crabs, lobsters, and crayfish -- no problem. Same with octopus and squid as well as clam and mussel.

However, among these groups -- is this considered a case of convergent evolution? Has it ever been considered?
I have not run into this question before from either side.


HRG/ALTER EGO
I'm not a biologist by profession, but I recognize them all as protostomes (in contrast to chordates, which are deuterostomes).


THE BARBARIAN
We can draw it a little tighter than that. All of these are either molluscs or arthropods.
Most arthropods with well-developed trachae, lack hemocyanins, apparently because they don't need them.
Hemocyanin in the arthropods is very much different than that in molluscs. It is thought that they have evolved separately.


HELEN
Barbarian, you could have just said "convergent evolution." That is what I thought the answer would be.

For those who don't know what 'convergent evolution' is, it means that the same thing just fortuitously happened to evolve in separate organisms the same way. The eye, for instance, is said to have evolved independently up to about forty times.

It's an interesting way out, especially since nothing like that is ever seen today...


THE BARBARIAN
No, that's wrong. Antibiotic resistance evolved many times, convergently. Like the hemocyanin case, different paths to the same result were taken. The example of a new metabolic pathway I showed you earlier, is another case like that. The old pathway was lost, but a new one evolved. It wasn't the same one, though. Convergent evolution. How about that? It's not "just fortuitously", though. Nobody who knows anything about biology would suggest that such a complex pathway would evolve by chance. Natural selection takes care of that.

Similarly, we don't think that bats and birds got wings from a common ancestor, because they are convergent.