Abstract. 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 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 2 values (0.1 to 0.4 kPa, ca. 1000 to 3900 µatm) than those of unicellular organisms and gametes, for which the ocean (0.04 kPa, ca. 400 µatm) is the extracellular space. A doubling in environmental pCO 2 therefore only represents a 10% change in extracellular pCO 2 in some marine teleosts. High extracellular pCO 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 2 that is directly proportional to the amount of O 2 consumed. In active metazoans, such as teleost fish, cephalopods and Correspondence to: F. Melzner (fmelzner@ifm-geomar.de) many brachyuran crustaceans, exercise induced increases in metabolic rate require an efficient ion-regulatory machinery for CO 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 2 . Compensation of extracellular acid-base status in turn may be important in avoiding metabolic depression. So far, maintained "performance" at higher seawater pCO 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 2 during their regular embryonic development, gametes, zygotes and early embryonic stages, which lack specialized ion-regulatory epithelia, may be the true bottleneck for ecological success -even of the more tolerant taxa.Our current understanding of which marine animal taxa will be affected adversely in their physiological and ecological fitness by projected scenarios of anthropogenic ocean acidification is quite incomplete. While a growing amount of empirical evidence from CO 2 perturbation experiments suggests that several taxa might react quite se...

Through functional analyses, integrative physiology is able to link molecular biology with ecology as well as evolutionary biology and is thereby expected to provide access to the evolution of molecular, cellular, and organismic functions; the genetic basis of adaptability; and the shaping of ecological patterns. This paper compiles several exemplary studies of thermal physiology and ecology, carried out at various levels of biological organization from single genes (proteins) to ecosystems. In each of those examples, trade-offs and constraints in thermal adaptation are addressed; these trade-offs and constraints may limit species' distribution and define their level of fitness. For a more comprehensive understanding, the paper sets out to elaborate the functional and conceptual connections among these independent studies and the various organizational levels addressed. This effort illustrates the need for an overarching concept of thermal adaptation that encompasses molecular, organellar, cellular, and whole-organism information as well as the mechanistic links between fitness, ecological success, and organismal physiology. For this data, the hypothesis of oxygen- and capacity-limited thermal tolerance in animals provides such a conceptual framework and allows interpreting the mechanisms of thermal limitation of animals as relevant at the ecological level. While, ideally, evolutionary studies over multiple generations, illustrated by an example study in bacteria, are necessary to test the validity of such complex concepts and underlying hypotheses, animal physiology frequently is constrained to functional studies within one generation. Comparisons of populations in a latitudinal cline, closely related species from different climates, and ontogenetic stages from riverine clines illustrate how evolutionary information can still be gained. An understanding of temperature-dependent shifts in energy turnover, associated with adjustments in aerobic scope and performance, will result. This understanding builds on a mechanistic analysis of the width and location of thermal windows on the temperature scale and also on study of the functional properties of relevant proteins and associated gene expression mechanisms.

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 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 2 values (0.1 to 0.4 kPa, ca. 1000 to 3900 µatm) than those of unicellular organisms and gametes, for which the ocean (0.04 kPa, ca. 400 µatm) is the extracellular space. A doubling in environmental pCO 2 therefore only represents a 10% change in extracellular pCO 2 in some marine teleosts. High extracellular pCO 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 2 that is directly proportional to the amount of O 2 consumed. In active metazoans, such as teleost fish, cephalopods and

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