We show here that increased variability of temperature and pH synergistically negatively affects the energetics of intertidal zone crabs. Under future climate scenarios, coastal ecosystems are projected to have increased extremes of low tide-associated thermal stress and ocean acidification-associated low pH, the individual or interactive effects of which have yet to be determined. To characterize energetic consequences of exposure to increased variability of pH and temperature, we exposed porcelain crabs, Petrolisthes cinctipes, to conditions that simulated current and future intertidal zone thermal and pH environments. During the daily low tide, specimens were exposed to no, moderate or extreme heating, and during the daily high tide experienced no, moderate or extreme acidification. Respiration rate and cardiac thermal limits were assessed following 2.5 weeks of acclimation. Thermal variation had a larger overall effect than pH variation, though there was an interactive effect between the two environmental drivers. Under the most extreme temperature and pH combination, respiration rate decreased while heat tolerance increased, indicating a smaller overall aerobic energy budget (i.e. a reduced O 2 consumption rate) of which a larger portion is devoted to basal maintenance (i.e. greater thermal tolerance indicating induction of the cellular stress response). These results suggest the potential for negative long-term ecological consequences for intertidal ectotherms exposed to increased extremes in pH and temperature due to reduced energy for behavior and reproduction.

The change in oceanic carbonate chemistry due to increased atmospheric P CO2 has caused pH to decline in marine surface waters, a phenomenon known as ocean acidification (OA). The effects of OA on organisms have been shown to be widespread among diverse taxa from a wide range of habitats. The majority of studies of organismal response to OA are in short-term exposures to future levels of P CO2 . From such studies, much information has been gathered on plastic responses organisms may make in the future that are beneficial or harmful to fitness. Relatively few studies have examined whether organisms can adapt to negative-fitness consequences of plastic responses to OA. We outline major approaches that have been used to study the adaptive potential for organisms to OA, which include comparative studies and experimental evolution. Organisms that inhabit a range of pH environments (e.g. pH gradients at volcanic CO 2 seeps or in upwelling zones) have great potential for studies that identify adaptive shifts that have occurred through evolution. Comparative studies have advanced our understanding of adaptation to OA by linking whole-organism responses with cellular mechanisms. Such optimization of function provides a link between genetic variation and adaptive evolution in tuning optimal function of rate-limiting cellular processes in different pH conditions. For example, in experimental evolution studies of organisms with short generation times (e.g. phytoplankton), hundreds of generations of growth under future conditions has resulted in fixed differences in gene expression related to acid-base regulation. However, biochemical mechanisms for adaptive responses to OA have yet to be fully characterized, and are likely to be more complex than simply changes in gene expression or protein modification. Finally, we present a hypothesis regarding an unexplored area for biochemical adaptation to ocean acidification. In this hypothesis, proteins and membranes exposed to the external environment, such as epithelial tissues, may be susceptible to changes in external pH. Such biochemical systems could be adapted to a reduced pH environment by adjustment of weak bonds in an analogous fashion to biochemical adaptation to temperature. Whether such biochemical adaptation to OA exists remains to be discovered.

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Invasive species can have large impacts on food webs if their metabolic demands are higher than those of extant species. The clam Corbula amurensis is believed to have caused a large shift in the pelagic food web in the northern reach of the San Francisco Estuary (USA) since its introduction in the 1980s. This shift has been attributed to the clam's high density, high suspension-feeding rates, and ability to thrive in a wide range of salinities. To understand how environmental salinity alters the potential metabolic impacts of C. amurensis on the pelagic food web, we investigated clam metabolism following acclimation to constant low, constant high, and fluctuating salinities. We measured growth rate, feeding rate, respiration rate, activity of the metabolic enzyme malate dehydrogenase (MDH), and osmoregulatory performance. Clams did not grow during a 3 mo period at either high or low salinity, although they fed more rapidly following acclimation to high salinity than low. C. amurensis had higher metabolic rates in both high and low salinity than in fluctuating salinities. Activity of MDH was positively correlated with salinity in both foot and mantle tissues. MDH activities of C. amurensis were twice those of other clam species. Osmotic pressure of C. amurensis tissues was always lower than that in the acclimation water, but clams hyporegulated to a greater extent in high-salinity conditions. Overall, our results suggest that clams experiencing higher salinities increase metabolic rates to support greater osmoregulation and compensate by increasing their filter-feeding rate.KEY WORDS: Corbula · Clam · Metabolism · Salinity · MDH · Osmoregulation · San Francisco Bay · Invasive speciesResale or republication not permitted without written consent of the publisher Aquat Biol 11: 139-147, 2010 dances of other pelagic taxa including mysid shrimp Neomysis mercedis (Orsi & Mecum 1996), longfin smelt Spirinchus thaleichthys (Kimmerer 2002a), and striped bass Morone saxatilis (Kimmerer et al. 2009), all presumably due to food limitation. Furthermore, food limitation has probably played a role in the continuing decline of these and other estuarine fish species (Feyrer et al. 2007.Salinity patterns in the upper estuary respond strongly to freshwater flow, and climate shifts and water management practices in the Sacramento-San Joaquin Delta alter the seasonal and interannual patterns of flow (Roos 1989, Knowles & Cayan 2002, Kimmerer 2002b. Furthermore, a large area of subsided farmland in the delta, poorly protected by weak levees, may catastrophically flood, shifting the salinity field far to landward (Mount & Twiss 2005). Shifts in the salinity field are likely to affect freshwater supplies and alter the distributions of key estuarine species, and are therefore of key interest for management (Moyle 2008).Rapid and substantial fluctuations in salinity can force physiological responses of estuarine organisms such as Corbula amurensis. Chaparro et al. (2008) showed that the suspension-feeding gastropod Crepipate...