Abstract
Bivalves frequently withstand shell damage that must be quickly repaired to ensure survival. While the processes that underlie larval shell development have been extensively studied within the context of ocean acidification (OA), it remains unclear whether shell repair is impacted by elevated pCO2. To better understand the stereotypical shell repair process, we monitored mussels (Mytilus edulis) with sublethal shell damage that breached the mantle cavity within both field and laboratory conditions to characterize the deposition rate, composition, and integrity of repaired shell. Results were then compared with a laboratory experiment wherein mussels (Mytilus trossulus) repaired shell damage in one of seven pCO2 treatments (400–2500 µatm). Shell repair proceeded through distinct stages; an organic membrane first covered the damaged area (days 1–15), followed by the deposition of calcite crystals (days 22–43) and aragonite tablets (days 51–69). OA did not impact the ability of mussels to close drill holes, nor the microstructure, composition, or integrity of end-point repaired shell after 10 weeks, as measured by µCT and SEM imaging, energy-dispersive X-ray (EDX) analysis, and mechanical testing. However, significant interactions between pCO2, the length of exposure to treatment conditions, the strength and inorganic content of shell, and the physiological condition of mussels within OA treatments were observed. These results suggest that while OA does not prevent adult mussels from repairing or mineralizing shell, both OA and shell damage may elicit stress responses that impose energetic constraints on mussel physiology.
Highlights
Mytilid mussels are bivalve mollusks that perform ecologically important roles within marine ecosystems [1,2] and support an intercontinental fishery that accounts for 6.9% (1.2 million tonnes) of the USD 250 billion global aquaculture industry [3]
Shell damage was induced in the right valve of remaining mussels as previously described, and individuals were haphazardly placed in one of seven experimental mesocosms that ranged in target pCO2 levels (400, 700, 1000, 1600, 1900, 2200, 2500 μatm) at 16 ◦C in the Ocean Acidification Environmental Laboratory (OAEL) located at Friday Harbor Laboratories, San Juan Island, WA, USA
Evidence of gastropod predation within mussel beds varied significantly during monthly field sampling of intertidal sites, with as many as 8% of mussels (M. edulis, n = 50, 1998–2001) within bed populations carrying shell damage in a given month (Figure 1B)
Summary
Mytilid mussels are bivalve mollusks that perform ecologically important roles within marine ecosystems [1,2] and support an intercontinental fishery that accounts for 6.9% (1.2 million tonnes) of the USD 250 billion global aquaculture industry [3]. Shell breaches can increase desiccation risk during low tide, stimulate an immune response [28], and hinder acid–base regulation [29] To mitigate these risks, mussels and other mollusk species are capable of quickly repairing a shell injury by patching holes [30–32]. Our field observations and experiments serve to determine the frequency with which mussels sustain shell damage and provide a timeline of the shell repair process These results were used to inform laboratory mesocosm experiments wherein mussels repaired sublethal shell damage that breached the mantle cavity away from the shell margin under seven OA treatments (pCO2 targets: 400–2500 μatm) for up to 10 weeks. The impact of environmental pCO2 on the progression of the shell repair process, as well as the composition, microstructure, and strength of repaired shell, was assessed through mechanical testing, SEM imaging, and μCT analysis
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