3 - Oxygen and Carbon Dioxide Transport in Elasmobranchs

Abstract

1. Introduction 2. Blood-Oxygen Transport 2.1. Hemoglobin 2.2. Red Blood Cell Function and Homeostasis 2.3. Blood-Oxygen Content: Hemoglobin Concentration and Hematocrit 2.4. Whole-Blood-Oxygen Equilibria 3. Transport and Elimination of Carbon Dioxide 3.1. Carbon Dioxide Transport in Blood 3.2. Carbon Dioxide Excretion 4. Conclusions and Perspectives The elasmobranchs are an ecologically diverse subclass of over 1000 species that have evolved to inhabit a wide range of environments and become one of the most speciose groups of vertebrate predators on Earth. This chapter reviews what is known about elasmobranch O 2 uptake, transport, and delivery, as well as CO 2 transport and elimination, and focuses upon two metalloproteins central to these processes, hemoglobin (Hb) and carbonic anhydrase (CA), both of which have undergone distinct functional adaptations. Furthermore, adaptations in relation to life history, which include exercise, hypoxia, salinity, temperature, and, in some species, regional heterothermy, are reviewed. While processes and principles of gas transport and exchange in elasmobranchs are often similar to those of the better described teleosts, there are differences that stand out as clearly worthy of further investigation. Generally, elasmobranch Hbs exhibit a high affinity for O 2 relative to teleosts, which may be associated with a low organic phosphate/Hb ratio and an antagonistic effect of urea on Hb-ATP sensitivity. The Hbs also exhibit a moderate Bohr and Haldane effect, but high buffering by Hb and plasma proteins coupled with the presence of plasma accessible CA greatly reduces the interaction between O 2 and CO 2 exchange relative to the situation in teleosts. Moreover, at least in the dogfish, Squalus suckleyi , current models of CO 2 excretion suggest similar contributions of the plasma and red blood cell (RBC) to CO 2 excretion, a model that contrasts with the pattern of CO 2 excretion typical of other vertebrates in which near exclusive reliance is placed on the RBC. High plasma buffering and plasma-accessible CA in the gill of dogfish favor HCO 3 − dehydration in the plasma, while HCO 3 − dehydration via the RBC is constrained by low RBC CA activity and the absence of a Haldane effect in this species. In the Hb of the whip stingray, Dasyatis akajei , a novel Bohr effect mechanism has been discovered and this same species possesses a novel ATP binding site in Hb. Finally, in the high performing regionally heterothermic sharks, there appears to be a reduction or reversal in Hb temperature sensitivity consistent with regionally heterothermic teleosts, but this remains to be investigated in detail. While gas transport and exchange is a central process associated with the success of elasmobranchs, it has been most thoroughly investigated in just a few species; clearly a great deal remains to be discovered to achieve a more representative understanding of gas transport and exchange in elasmobranch fish.