Andreas Späth

Bad news for mussel lovers

2016-07-11 09:50

Andreas Wilson-Späth


Do you enjoy mussels – steamed in white wine with garlic or in paella? You might want to stop reading right now. Otherwise, I’m afraid I might ruin your appetite.


Those of us who are able to order mussel dishes in fancy restaurants have probably not really felt the impact of climate change and the associated consequences of humanity’s reliance on fossil fuels on a personal level quite yet. We tend to be able to avoid those. Depriving us of mussels, however, may just open some eyes to the realities of the growing global crisis we find ourselves in.


The culprit is something called ocean acidification – you might have seen me rant about it in the past. The carbon dioxide we are releasing into the atmosphere when we burn oil, coal and natural gas doesn’t just act as a greenhouse gas that causes global warming, some of it also dissolves in seawater. As it does so, it increases the acidity (i.e. lowers the pH level) of the water and that is starting to have far-reaching effects on the oceans.


Organisms that rely on skeletons or shells composed of a calcium carbonate as part of their structure are directly impacted. As the seawater becomes gradually more acidic (it’s currently still slightly alkaline on average with a pH of about 8), fewer and fewer of the carbonate ions the creatures need to extract from it for building purposes are available, resulting in thinner and more fragile skeletons and shells. You can guess where this is leading in the case of mussels.


A study published in June shows that the shells of Californian mussels are getting thinner. The largest mussel shells in Native American archaeological sites dating back to more than 2000 years ago are over 90% thicker than their modern counterparts. Shells of mussels collected in the area in the 1970s are, on average, around 32% thicker than those found today.


The simple conclusion: mussels are producing less shell material now than they used to in the past. The most likely explanation: ocean acidification.


So why should we care about the thickness of mussel shells? We eat the mussel flesh, not the shells.


Shells provide a protective armour for mussels that shields them from predators like marine snails specialised to drill through them. Thinner shells will make the sails’ job easier and the mussels more vulnerable.


And then there are the abrasive, highly energetic conditions in the areas of the oceans that mussels inhabit – shallow tidal zones that are constantly battered by strong currents, tides, storm surges and waves. Thinner shells will mean higher fatality rates and fewer mussels reaching maturity.


More acidic seawater isn’t just interfering with mussels’ ability to produce sturdy shells, it’s also likely the affect the strength with which they’re able to attach themselves to hard substrates like the surface of rocks.


New laboratory research conducted at the University of Washington suggests that when ocean water acidity drops below a pH level of 7.6 the adhesive strength of the glue with which mussels secure themselves in place will diminish.


That’s important, because it makes them more likely to be dislodged from their normal habitats and washed to calmer areas where they are more exposed to predatory snails, fish, sea stars and crabs. It also creates a problem for commercial mussel farmers who rely on mussels’ ability to attach themselves to ropes suspended in the sea.


"Due to upwelling and local productivity, our seawater in Washington is already at a baseline of pH 7.8,” says professor Emily Carrington, one of the scientists involved in the research. “Moreover, mussels live in highly dynamic coastal environments that routinely fluctuate up or down 0.5 pH units".


So not fantastic prospects for the future of mussel eating. But that shouldn’t really be our biggest concern. The disappearance of these creatures will have more significant effects on nature than on the menus of seafood joints.


That’s because mussels are a so-called foundation species in many shallow marine environments. The extensive mussel beds they create provide ecosystems in which communities of hundreds of diverse, interdependent species are able to thrive. Damage the mussels and you endanger all of their many associates.


According to professor Cathy Pfister, the lead author of the study comparing mussel shell thickness over time, "the California mussel is a common species along the entire west coast of the United States and their fate will be linked to that of a rich diversity of predators, including sea stars and sea otters, as well as myriad species that are part of the mussel bed habitat”.

- Andreas is a freelance writer with a PhD in geochemistry. Follow him on Twitter: @Andreas_Spath

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