Temperature dependence of oxygen binding to cephalopod haemocyanins: Ecological implications

The present paper discusses the temperature sensitivity and the control of oxygen binding to haemocyanins of several species of cephalopods, using a similar ionic composition to their blood in vivo. Based on the relationship between the oxygen affinity and the cooperativity of oxygen binding of the cephalopods at their habitat temperatures, we may divide them into three groups: 1) the octopuses plus the sluggish giant squid, Architeuthis, and Nautilus, with P-50 values in the range 10-20 mm Hg; 2) the fast swimming squids with P-50 values around 30 mm Hg; 3) and the Sepia species with P-50 values above 30 mm Hg. These data may predict the optimal environmental temperature range for a given species with respect to gas transport. We have also described the interplay of temperature and protons in the modulation of oxygen binding for one species, the squid Todarodes sagitatus. Within the physiological pH range, the concentration of protons affects mainly the high-affinity state of the molecule without significantly affecting the low-affinity state. The shape of the oxygen-binding curve shows a strong temperature-dependence, since the overall heat of the binding of oxygen to the low-affinity state of the molecule is strongly exothermic and that to the high-affinity stale is very close to zero. The results outline the intramolecular compromise that optimizes oxygen loading and unloading through the interplay of temperature and protons under the various environmental conditions experienced by these animals.