There is a fish (16+ recognized species in one family) which has rejected hemoglobin. They do not produce it at all, despite of course still having the genetics kicking around in deactivated form. They appear to compensate by playing with a lot of anatomy sliders they otherwise wouldn't and all around trying to coax physics into just barely working out in their favor. Their existence is completely at the mercy of their native environment's specific-ass conditions. Someone found 60 million active nests in a single breeding colony in 2021.
This isn't the formal animal facts™ post because I am still squinting at the screen muttering to myself. Fish that live in really really cold water tend to be bullshit somehow, but this is going above and beyond. Err, below and beyond?
