Oysters are off the Menu

Posted by Science Explored on April 09, 2013

You may have heard of acid rain, but what about ocean acidification?  Marine biologists, chemical engineers, and oceanographers all have something to say about the effects of carbon dioxide emissions on our oceans.  Since the Industrial Revolution, humanity has created vast amounts of carbon dioxide as a byproduct, or secondary (sometimes unintentional) product, of making various chemicals such as plastics, driving our cars, and burning coal to make electricity.  Carbon dioxide is not harmful in low quantities; in fact, plants rely on carbon dioxide during photosynthesis to make gluccose and oxygen for us as a byproduct.  Nonetheless, CO2 emissions have been steadily on the rise as world populations grow.  One of the main places much of the carbon dioxide gets deposited is in Earth’s oceans because it reacts with seawater.  As CO2 levels accumulate in the ocean, the effects are starting to be noticed by fishermen and seafood lovers across the country.

Crustacean Populations

Crustaceans are a “subphylum” of the phylum arthropoda.  Like arthropods, crustaceans are invertebrates that posses a segmented body and hard exoskeleton made from calcium carbonate (CaCO3), a natural occurring substance found in rock and dissolved throughout seawater.  Over 67,000 species of crustaceans have been identified that range from shrimp and crayfish to barnacles and crab.  Crustaceans grow by molting, or casting off, their exoskeletons and forming new, larger exoskeletons.  As carbon dioxide levels increase in Earth’s oceans, crustaceans seem to be thriving and growing larger.  Conversely, other animals such as corals, scallops, and oysters with calcium carbonate exoskeletons that do not molt are beginning to suffer.  “Higher levels of carbon in the ocean are causing oysters to grow slower, and their predators – such as blue crabs – to grow faster,” said Justin Baker Ries, a marine biologist of the University of North Carolina in an interview with The Washington Post.  Why then are we seeing carbon dioxide harm one group but help another?  To understand this we need to look out exactly how carbon dioxide changes our oceans.

Driving down the street hardly seems detrimental to oyster populations, but it has a measurable effect.  As our cars speed down a street, they emit CO2, a byproduct of burning gasoline needed to drive the cylinders that make the wheels turn.  This CO2 gets added to Earth’s atmosphere and can eventually travel to the surface of the ocean.  When carbon dioxide meets seawater, a chemical reaction occurs – the CO2 dissolves in seawater causing a drop in pH, a measurement of the concentration of hydrogen ions (H+).  The more hydrogen ions are present, the lower the pH is, the more acidic a substance is.  Carbon dioxide leads to an increase in acidity because H2O + CO2 form CO32- + 2H+.  Essentially, every molecule of carbon dioxide that dissolves in water forms two hydrogen ions, which contributes to increased ocean acidity by lowering pH.  It’s estimated that since the start of the Industrial Revolution and the present, pH has dropped (lower pH indicates higher acidity) by 11%.  How then does this connect back to ocean life?

Calcium carbonate, the substance oysters, crabs, and similar animals use to make their exoskeletons, is easily weathered by acids.  As a result, animals that cannot molt their exoskeletons have weaker “shells” than they previously had as they age.  Crabs, who benefit from being able to molt, can cast of their shell as the acidic seawater weathers it and form a new exoskeleton while enjoying “softer” prey.  Thus, as ocean acidity increases, crustaceans are becoming more prolific, or plentiful and widespread, and larger because they will molt more frequently in addition to having an easier time hunting their prey.  These consequences effect the seafood industry of the United States and the world.  You would expect that as oyster quality decreases and their numbers decline, the prices for a dozen oysters on the half shell will dramatically increase.  Unsurprisingly, a few states, such as Maryland that relies heavily on the fishing industry, have poured millions of dollars into helping and preserving current oyster populations.  Don’t be surprised then if next time you order oysters, your order is disappointing – you should have gone with the crab cakes instead.  

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