Coordination of metabolism, acid‐base regulation and haemocyanin function in cephalopods

Abstract

The role of cephalopod haemocyanins in oxygen transport is analysed in the light of the coordination of metabolism, acid‐base regulation and gas exchange processes. Results obtained in squid, the most active among cephalopod species, indicate that the pH dependence of their haemocyanin supports a Po2‐buffer function for the pigment. The release of base equivalents from the tissue during aerobic exercise and the minimal release of protons during anaerobic octopine formation protect arterial pH and, thus, oxygen binding. The extent of respiratory acidification and haemocyanin deoxygenation on the venous side is higher in blood returning from the mantle than from the head. In vivo blood gas measurements reported for squid and for other cephalopod species support the conclusion that CO2 accumulation and respiratory acidification of the blood occur in excess of the effect expected from the consumption of haemocyanin bound O2 and RQ values derived from protein catabolism. This suggests that a considerable fraction of the oxygen consumed by the animal enters via the skin, especially in the mantle. Model calculations demonstrate that skin O2 uptake in the mantle increases during activity in squid. In other cephalopod species like cuttlefish, the special process of arterial CO2 binding to oxygenated haemocyanin and its release during venous deoxygenation may provide the excess CO2 required for venous acidification. All of these processes allow the classical Bohr effect to function supporting oxygen loading at the gills and oxygen unloading in the tissues. The large pH‐dependent cooperativity and the Bohr effect combine to maximize the Po2‐buffer function of the respiratory pigment. These adaptations probably evolved after the ancestors of modern cephalopods lost their shells and locomotor activity assumed a greater role in their lifestyle.