Thanks so much for the deep dive. I love learning from such concise facts.

To add, also @Semjaza@lemmynsfw.com : There is only one known species of vertebrates without hemoglobin. The Crocodile Icefish, it once had it in its blood and lost some genes to synthesize it. The debate about why is still ongoing, with the currently favored theory that they adapted to a high oxygen and low iron environment.