Physiological basis for high CO sub(2) tolerance in marine ectothermic animals: pre-adaptation through lifestyle and ontogeny?

Future ocean acidification has the potential to adversely affect many marine organisms. A growing body of evidence suggests that many species could suffer from reduced fertilization success, decreases in larval- and adult growth rates, reduced calcification rates, and even mortality when being exposed to near-future levels (year 2100 scenarios) of ocean acidification. Little research focus is currently placed on those organisms/taxa that might be less vulnerable to the anticipated changes in ocean chemistry; this is unfortunate, as the comparison of more vulnerable to more tolerant physiotypes could provide us with those physiological traits that are crucial for ecological success in a future ocean. Here, we attempt to summarize some ontogenetic and lifestyle traits that lead to an increased tolerance towards high environmental pCO sub(2). In general, marine ectothermic metazoans with an extensive extracellular fluid volume may be less vulnerable to future acidification as their cells are already exposed to much higher pCO sub(2) values (0.1 to 0.4 kPa, ca. 1000 to 3900 katm) than those of unicellular organisms and gametes, for which the ocean (0.04 kPa, ca. 400 katm) is the extracellular space. A doubling in environmental pCO sub(2) therefore only represents a 10% change in extracellular pCO sub(2) in some marine teleosts. High extracellular pCO sub(2) values are to some degree related to high metabolic rates, as diffusion gradients need to be high in order to excrete an amount of CO sub(2) that is directly proportional to the amount of O sub(2) consumed. In active metazoans, such as teleost fish, cephalopods and many brachyuran crustaceans, exercise induced increases in metabolic rate require an efficient ion-regulatory machinery for CO sub(2) excretion and acid-base regulation, especially when anaerobic metabolism is involved and metabolic protons leak into the extracellular space. These ion-transport systems, which are located in highly developed gill epithelia, form the basis for efficient compensation of pH disturbances during exposure to elevated environmental pCO sub(2). Compensation of extracellular acid-base status in turn may be important in avoiding metabolic depression. So far, maintained performance at higher seawater pCO sub(2) (>0.3 to 0.6 kPa) has only been observed in adults/juveniles of active, high metabolic species with a powerful ion regulatory apparatus. However, while some of these taxa are adapted to cope with elevated pCO sub(2) d