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What we did: CO2 transport in the blood relies in part on haemoglobin-proton binding. Primitive fish species use one mechanism of doing this (intrinsic buffering), while more derived species use a different mechanism (Haldane effect). Here, we explored the haemoglobin-proton binding properties of a group of six intermediate fish species to better understand this transition.
What we found: The four most primitive species showed displayed large intrinsic buffering capacities and low Haldane effects, and the two most derived species displayed low intrinsic buffering capacities and large Haldane effects.
What this means: The evolution of this system appears to have been abrupt, not gradual. In addition, the two derived species in which a large Haldane effect first appears are the same species in which a large and likely usable Root effect first appeared (see accompanying paper), suggesting the two are mechanistically linked.