low pH appeared to elicit a stronger effect than hypoxia (2 mg L−1). Overall, the study showed that hypoxia can aggravate the effects of acidification and that its impacts on physiology and SFG may have ecological and economic ramifications in the short term.
Sui et al. (2017) investigated the combined effects of hypoxia and OA on the defence responses of adult M. coruscus in the East China Sea, one of the largest coastal hypoxic zones observed in the world (Chen et al. 2007). Mussels were exposed to three pH/pCO2 levels (7.3/2800 μatm, 7.7/1020 μatm and 8.1/376 μatm) at two DO concentrations (6 and 2 mg L−1) for 72 hr. Results showed that byssus thread parameters (e.g. number, diameter, attachment strength, plaque area) were reduced by low DO, and shell‐closing strength was significantly weaker under both hypoxia and low‐pH conditions. Expression patterns of genes related to mussel byssus proteins were affected by hypoxia. Generally, hypoxia reduced expressions of some of these and increased expression of others. Overall, both hypoxia and low pH induced negative effects on mussel defense responses, with hypoxia being the main driver of change. In addition, significant interactive effects between pH and DO were observed on shell‐closing strength. Depressed byssus attachment strength and shell‐closing strength may enable predators and water currents to remove mussels from the substrate more easily. In a similar study, the effects of hypoxia and salinity on antipredator responses were examined in juvenile P. viridis from Hong Kong coastal waters (Wang et al. 2012). In hypoxic and low‐salinity conditions, P. viridis produced fewer byssal threads and had a thinner shell and adductor muscle, indicating that hypoxia and low salinity are severe environmental stressors for self‐defence of mussels, and that their interactive effects further increase the predation risk.
Growing human pressures, including climate change, are having profound and diverse consequences for marine ecosystems (Doney et al. 2012). Rising atmospheric carbon dioxide is one of the most critical problems, because its effects are globally pervasive and irreversible on ecological time scales. The primary direct consequences are increasing ocean temperatures and acidity. Also, both warming and altered ocean circulation act to reduce subsurface oxygen concentrations. Furthermore, climbing temperatures create a multitude of additional changes, such as rising sea level, increased ocean stratification, decreased sea‐ice extent and altered patterns of ocean circulation, precipitation, freshwater input and extreme weather events (reviewed in Doney et al. 2012; Howard et al. 2013; USGCRP 2017). In recent decades, the rates of change have been rapid, perhaps exceeding the current and potential future tolerances of many organisms to adapt. In addition, the rates of physical and chemical change in marine ecosystems will almost certainly accelerate over the next several decades in the absence of immediate and dramatic efforts toward climate mitigation.
Notes
1 1 The littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas, which are permanently submerged.
2 2 A Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Peninsula.
3 3 The PDO is detected as warm or cool surface waters in the Pacific Ocean north of 20 °N and, depending on the phase of the oscillation, may regionally ameliorate or counter global climate trends (Stenseth et al. 2003).
4 4The horizontal movement of a mass of fluid (e.g. an ocean current); also, transport (e.g. of larvae or pollutants) by such movement.
5 5 An abnormal weather pattern caused by the warming of the Pacific Ocean near the equator, off the coast of South America.
6 6 Ecological succession is the process of change in the species structure of an ecological community over time.
7 7 Crabs peel bivalve shells by inserting a large dactyl molar, a movable finger of the claw, into the aperture of the shell and progressively chipping away at its lip.
8 8 Mechanistic species distribution models use independently derived information about a species’ physiology to develop a model of the environmental conditions under which the species can exist (Kearney & Porter 2009).
9 9 Aragonite saturation state is commonly used to track ocean acidification because it is a measure of carbonate ion concentration. Aragonite is one of the more soluble forms of calcium carbonate and is widely used by marine calcifiers. When aragonite saturation state falls below 3, these organisms become stressed, and when saturation state is less than 1, shells and other aragonite structures begin to dissolve.
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