Imperiled Mussels

In ancient Greece, electoral votes were cast by scratching the names of candidates inside mussel shells.

Centuries later, in one of the earliest (14th century) French cookbook transcripts, Le Viander de Taillevent, a mussel recipe appears—with mint of all things. The reknowned Taillevent rose from meager beginnings as a young kitchen hand by the common name of Guillaume Tirel to become the heralded master chef for the king of France, Charles V (“the Wise”). More than a quincentennial later, Taillevent’s name graced a famous Parisian restaurant which opened shortly after the close of World War II.

Mussels are bivalves of the marine family Mytilidae, most of which live on exposed shores in intertidal zones. The external shell is composed of two hinged halves (valves) joined together by a ligament, and closed by robust internal muscles. They have tough, elastic byssal threads—their notorius “beards.”

Mussels are another sad example of how humankind has altered the ocean environment in recent times – exhausting the limits of an ecosystem’s endurance.  The iconic coastal California mussel may be the casualty this time.

A study published in the journal <em>Science</em> predicts that by mid-century, western coastal waters will become sufficiently acidic to hinder shell formation by mussels, oysters and corals.  These waters are particularly fecund  because winds that blow surface water out to sea allow water laden with nutrients to swell near the shore.  This upwelling renders those waters especially vulnerable to ocean acidification. Increased acidity levels develop in the waters as they absorb carbon dioxide which accelerate as trends of anthropogenic greenhouse gases continue to soar.  <strong>Ocean acidification</strong> has been dubbed the <em>osteoporosis of the seas</em>.

What does this have to do with our cherished shellfish?  As carbon dioxide dissolves in sea water, saturation levels of the mineral calcium carbonate, a critical building block for shells and skeletons, decreases. Undersaturation can reach perilous levels depriving these sea creatures of the basic component needed to develop and maintain their shells.  According to these researchers (who were using optimistic models), by 2050 west coast seawater will no longer have sufficient saturation states to maintain adequate calcium carbonate levels.  This places mussel populations at serious risk.  This is indeed a dire finding given that mussels provide habitat, refuge, and food for some 300 other species.

A correlative finding was reached in a later study conducted at UC Davis Bodega Marine Laboratory.  Researchers there noted that increased concentrations of carbon dioxide in the atmosphere that climate scientists attribute to human activity have resulted in increased ocean acidification.   This team focused on mussel larvae, which swim in the open ocean before settling down on the shoreline and attaching to reefs as adults.  As with many other marine creatures, mussel larvae are more vulnerable to environmental stresses.

Larvae were grown in the lab at present acid levels, levels projected for the end of the century if carbon dioxide emissions continue, and at levels which might be reached if emissions are reduced.  The shells were measurably thinner and the mussels’ bodies smaller at projected acid levels.

Other researchers have sung the same refrain:  if human actions continue unabated, oceans will continue to absorb rising levels of atmospheric carbon dioxide which causes ocean acidification whose corrosive effect ultimately threatens to decimate certain shellfish species. 

Do we welcome such a sea change?

 

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