Mollusks, as we all know, are a large part of marine aquaculture and are an important source of food for many people worldwide. Mollusk culture alone represents almost 25% of aquaculture worldwide (Cochrane et al., 2009). They provide important habitat for other organisms to live, perform important ecosystem functions, and have been a popular organism for habitat monitoring (Bussell et al., 2008). However, with the current changing climate and resulting acidification of the Earth’s oceans, if there were any complications related to mollusk growth and physiology, there could be significant impacts on the aquaculture industry.
The rapid progression of climate change due to greenhouse gases can cause several physical changes to occur in the oceans, such as increasing storm frequency, increasing the amount of carbon dioxide dissolved in the oceans, and further decreasing the pH (Figure 1; Allison et al., 2011). These changes, in turn, could negatively affect the physiological processes that mollusks need to perform in order to survive.
In the case of decreasing pH, when pH drops as a result of the ocean absorbing more carbon dioxide released by human activities, the saturation states of minerals such as aragonite and calcite are reduced, which means that the depth at which those minerals will dissolve in the water column will change. Many marine organisms, including mollusks, use those minerals to build their shells and skeletons (Allison et al., 2011).
It’s been hypothesized that juvenile mollusks differ in their tolerance to environmental changes than in their adult life stages. Thus, in an experiment done by Bressan et al. (2014), researchers found that bivalve (clams and mussels) juveniles experienced a substantially higher mortality in acidified seawater conditions compared to their natural pH conditions; shell thickness began to degrade over time, shell length decreased, and live/dry weights of shell and soft tissues decreased as well. As a result of exposure to acidified conditions, there was damage to the outer shell surface of the mollusks and the prismatic layer (the interior rainbow-colored part of the shell) of mussels (Mytilus galloprovincialis) dissolved in as little as a month (Figure 2; Bressan et al., 2014). It was even worse for clams (Chamelea gallina), whose outer shell severely discolored and deteriorated to the point where the concentric ribs completely flattened out. Ocean acidification not only reduces the concentration of minerals available in the oceans for calcifying organisms to use for their shells, but acidified conditions dissolve the shell that these organisms already possess, inducing mortality and stunting growth.
Climate change can also reduce the salinity of the ocean, whether it be through melting ice caps in the polar regions of the world or increasing the frequencies of storms or floods. Mollusk aquaculture can take place in both marine and brackish water environments, and researchers predict that brackish habitats like estuaries are going be affected more severely by climate change than others (Ivanina et al., 2013). A sudden drop in salinity (due to floods) and the poor temperature buffering ability of estuarine habitats can lead to negative impacts on mollusks residing there. In an experiment done by Bussell et al. (2008), a period of reduced salinity negatively affected the immune function of mussels (Mytilus edulis). The concentration of haemocytes, which are a mussel’s blood cells and primarily carry out immune defense, was reduced in lower salinity, in addition to significant changes in their “metabolic fingerprint” or biochemistry.
So what does this mean for mollusk aquaculture? What can they do to mitigate these negative outcomes?
In the case of the main stakeholders and producers, they’re not entirely sure. While there is current research being done on the effects of climate change on mollusks, there is simply not enough information to address every aspect concerning changes in physiology, adaptive capacity, the possible synergistic effect of multiple stressors, amongst other concerns (Rodrigues et al., 2015). Some possible responses are to move production into deeper waters, turning to foreign hatcheries in case of large larval mortality, or simply moving their period of harvest and sales to earlier in the year (Rodrigues et al., 2015). However, there needs to be more research done on the effects of climate change on mollusks, whether it be investigating the effects of multiple stressors, carryover effects of previous generations, or further effects of carbon on mollusk physiology.
Allison EH, Badjeck M, Meinhold K (2011) The Implications of Global Climate Change for Molluscan Aquaculture. In: Shellfish Aquaculture and the Environment, First Edition. John Wiley & Sons, Inc, Hoboken, pp 461-490.
Bresson M, Chinellato A, Munari M, Matozzo V, Manci A, Marceta T, Finos L, Moro I, Pastore P, Badocco D, et al. (2014) Does seawater acidification affect survival, growth and shell integrity in bivalve juveniles? Mar Environ Res 99: 136-148.
Bussell JA, Gidman EA, Causton DR, Gwynn-Jones D, Malham SK, Jones LM, Reynolds B, Seed R (2008) Changes in the immune response and metabolic fingerprint of the mussel, Mytilus edulis (Linnaeus) in response to lowered salinity and physical stress. J Exp Mar Biol Ecol 358: 78-85.
Cochrane K, De Young C, Soto D, Bahri T (2009) Climate change implications for fisheries and aquaculture. FAO Fisheries and Aquaculture Technical Paper 1: 1-212.
Ivanina AV, Dickinson GH, Matoo OB, Bagwe R, Dickinson A, Beniash E, Sokolova IM (2013) Interactive effects of elevated temperature and CO2 levels on energy metabolism and biomineralization of marine bivalves Crassostrea virginica and Mercenaria mercenaria. Comp Biochem Physiol A 166: 101-111.
Rodrigues LC, Van Den Bergh JCJM, Massa F, Theodorou JA, Ziveri P, Gazeau F (2015) Sensitivity of Mediterranean bivalve mollusc aquaculture to climate change, ocean acidification, and other environmental pressures: findings from a producer survey. J Shellfish Res 34: 1161-1176.