Meet Chionodraco hamatus, known to his pals as an icefish. (You can call him Mister Icefish.) He’s one of the Channichthyidae, a whole family of fish living in very particular conditions.
I promised you guys a story about a fish with no haemoglobin at all, and this guy is one of that select few. Originally, it seems, it was thought that these icefish — who live in cold climes, if you hadn’t guessed — had evolved to lack haemoglobin as an adaptive trait. Red blood cells containing haemoglobin make the blood more viscous, making it more difficult to pump in cold temperatures. It’s an easy guess, and it doesn’t seem to be true — in fact, the icefish have other adaptations to compensate for their lack of haemoglobin. If it was adaptive at any point, it’s become costly, like driving 50 miles to get to a shop instead of 5 because you’ll get a free gift.
It seems the likely answer for how this trait got so established among the icefish species is luck. An ecological niche was open, and they fit in just well enough that they could thrive where other species couldn’t, or wouldn’t. At the same time, because they developed without haemoglobin, which also breaks down nitric oxide, an opportunity arose. With a surplus of nitric oxide, the icefish developed ways of using it to supplement their circulatory system, despite its low oxygen-carrying capacity.
Incidentally, this is one piece of the evidence for evolution that directly disproves the idea of an intelligent creator: things don’t have to be designed perfectly. They just have to work. Icefish get by, but it’s a kludge. Nature’s using duct tape, rather than creating something flawless ex nihilo. Which makes all of nature a bit more fascinating, from where I’m sitting.