Mussels repair shell damage despite limitations imposed by ocean acidification

Abstract

Bivalves frequently withstand shell boring attempts by predatory gastropods that result in 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 impaired by elevated pCO2. To better understand the stereotypical shell repair process, we monitored mussels (Mytilus edulis) with sublethal shell damage within both field and laboratory conditions to characterize the deposition rate, mineral composition, and structural integrity of repaired shell. These 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 four distinct stages; shell damage was first covered with an organic film, then mineralized over the course of weeks, acquiring the appearance of nacre after 8 weeks. OA did not impact the ability of mussels to close drill holes, nor the strength or density of the repaired shell after 10-weeks, as measured through mechanical testing and µCT analysis. However, as mussels progressed through each repair stage, significant interactions between pCO2, the length of exposure to treatment conditions, and the strength, inorganic content, and physiological condition of mussels within OA treatments were observed. These results suggest that, while OA may not prevent mussels from repairing shell damage, sustained exposure to elevated pCO2 may induce physiological stress responses that impose energetic constraints on the shell repair process.