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Monday, December 22, 2008

Oysters - Gem of the ocean

A Dozen Oysters and a Pint of Guinness, Please

From The Economist print edition

Bridgeman Art Library

I would love to own this painting.

JUST as household trash tells you a lot about a family, so mankind’s rubbish heaps reveal much about the species. One of the best lies in the waters around Manhattan. There, archaeologists have found mounds of oyster shells, known as middens, dating back to 6950BC.

People have fed on oysters so long that the man whom Jonathan Swift called brave for first eating one is quite out of range of history’s eye. Sergius Orata, a Roman engineer who lived in the first century BC, cultivated oysters in southern Italian lakes by bringing them to spawn on rock piles that he surrounded with twigs. Larval oysters settled on the twigs, which the cultivator could monitor easily; when the oysters grew to marketable size, they were plucked off and sold.

In Manhattan the oyster trade really took off with the arrival of Europeans: as Mark Kurlansky writes in the opening to “The Big Oyster”, his marvellous examination of the dark and salty crossroads where bivalency and humanity meet, “To anyone who is familiar with New Yorkers, it should not be surprising to learn that they were once famous for eating their food live.” Yet had the Europeans examined those shell mounds more closely, they would have found something ominous: the shells grow larger toward the bottom. Left alone, oysters never stop growing. The largest ones were taken first. As more people arrived, the average oyster’s lifespan fell: even in pre-European America, overfishing threatened.

Those ancient New Yorkers and the Dutch and English who followed plundered the area’s oyster supply without a second thought. The waters around New York once teemed with oysters, as did those around London. Both cities were built on estuaries, allowing the constant yet changing mixture of fresh and salty water that oysters love. Both cities progressed rapidly from manufacturing to industrial to financial capitals, and in the process, voraciously abetted by the appetites of their citizenry, both killed their oysters (Paris, the third great oyster metropolis, protected its beds far more successfully, and to this day shuckers presiding over crates of oysters packed in ice remain a common sight on the city’s corners in winter). In so doing, New York and London may have destroyed something far more than a delicious source of protein: oysters are not only among the strangest and tastiest creatures in the sea, but as far as the health of marine ecosystems go, they may also be the most important.

Edible oysters fall into one of five main species: Ostrea edulis, the European oyster, is the most regular, rounded and attractive in appearance; they are most often sold as Galway or Mersey flats in Britain and Ireland and belons in France (like wine, oysters take on characteristics of the terroir, so to speak, in which they are raised; the wildly different tastes result not from biology but from the variant diets, temperatures and salinity offered by the water in which the individual oysters spend their lives). Ostrea lurida, sold most often as the Olympia, is the only species native to America’s west coast; it is small, sweet and tastes of grass and earth rather than the sea.

Crassostrea sikaema, known as Kumamotos, are small and quite deep-shelled; they were brought to America’ s west coast from Japan’s Kumamoto prefecture, and have a crisp texture and a taste that is reminiscent of melons or cucumbers. Crassostrea gigas are native to the Pacific but grown around the world—notably in France as the green-tinged Marennes-Oléron and the fine de claire.

Gigas are closely related to Crassostrea angulata, formerly known as the Portuguese oyster. The story goes that C. angulata were introduced to northern Europe, particularly France and Britain, when a ship carrying a cargo of Portuguese oysters, took shelter from a storm in southwestern France. Believing his oysters ruined, the captain jettisoned them. They flourished. It was either these or O. edulis that M.F.K. Fisher, an American food writer, had in mind when she recounted an old recipe for a single roasted oyster: “You start with an oyster. You put it inside a large olive. Then you put the olive inside an ortolan (a wee bird called ‘the garden bunting’, in case you are among the underprivileged), and the ortolan inside a lark, and so on and so on. In the end you have a roasted oyster. Or perhaps a social revolution.”

The teardrop-shaped Crassostrea virginica thrive on America’s east coast, and can appear in guises as diverse as the small, intensely briny Malpeque, from Prince Edward Island, to the large and sweetly bland Apalachicola, from Florida. Historically, however, most virginicas—a significant portion, if not an outright majority, of oysters eaten in America, from the time of the Civil War until the mid 1980s—came from the Chesapeake Bay, situated mostly in Maryland but with a watershed stretching 64,000 square miles across six states and the District of Columbia.

Although the Chesapeake region might be best known for its blue crabs, in fact oyster harvesting and processing formed the most commercially viable operation in the region as far back as the Civil War. And the waters teemed with oysters long before that: when John Smith first sailed into the Chesapeake in 1608, he wrote that they “lay as thick as stones”—so profuse, in fact, that they made navigation difficult.

The stone-thickness of the oyster beds that Smith saw attest not just to the Chesapeake’s ideal salinity—situated as it is just in from the Atlantic, and fed by dozens of rivers from across the watershed—but also to the beds’ age: left undisturbed, oyster beds would indeed thicken impressively, because oysters like setting their shell nowhere as much as on the back of another oyster shell, because they grow larger the longer they live, and because proximity aids successful spawning. Spawning occurs in the warmer part of the year—hence the historic injunction against eating oysters in months that lack an R. This has nothing to do with illness (though obviously oysters, like other raw meat, spoil faster in warm weather), but because, as Ms Fisher reminds us, “oysters, like all men, are somewhat weaker after they have done their best at reproducing”—the meat tends to be thin and flat-tasting. They spawn by releasing gametes into the water: a female Atlantic oyster tends to release clouds of eggs in a series of wet puffs, while males send sperm forth in a stream. But male oysters can spawn in the style of females, and vice versa; and hermaphroditism, in which eggs and sperm shoot out of the same oyster at the same time, also occurs, albeit rarely.

Fertilisation occurs when opposite gametes meet in the water: hence the advantage offered by proximity. Generally, the male releases his gametes first, which acts as a signal to any females nearby. The spawning process takes about 45 minutes, during which a female will emit anywhere from 10,000 to around 60m eggs, only a small fraction of which will be lucky enough to meet their mates. Once the pair of gametes connect, they become a larva that drifts and swims in the tidal current, propelling itself by means of a little organ ringed with cilia called a velium. This is an oyster’s only taste of free movement. When the larva grows to around 300 microns (roughly one-third of a millimetre), it extends its foot and seeks a suitable surface on which to set. Having found one, it grows into a spat, which when seen beneath a microscope already resembles a tiny oyster, with the shape of a shell already visible. It prefers settling on hard, chalky surfaces. Farms often use tiles as the foundations of their beds, but when given a choice spat seem to prefer oyster shells.

And there’s the rub: most of the Chesapeake Bay’s oyster operations have been public fisheries rather than aquaculture—anyone with a license could take oysters from state-owned bars, and though size and number limits were set, often they were more honoured in the breach than in the observance. Once a tipping point was reached, oysters were too far apart for enough of their gametes to meet, so the population could not sustain itself. And those few larvae that were lucky enough to live long enough to extend a probing foot too often found only silt. The oyster population in the Chesapeake today stands at just 1% of its pre-1980 levels.

It wasn’t just overfishing that depleted the oyster population. Between 1950 and 2000 the human population of the Chesapeake Bay watershed region has more than doubled, from 8m to over 16.7m. The Eastern Shore, long a relatively isolated patch of America’s east coast best known for the odd quasi-Elizabethan English spoken by its inhabitants, became an increasingly popular weekend and second-home destination. Dick Cheney and Donald Rumsfeld have weekend homes in the harbour town of St Michaels. Mr Rumsfeld’s is called Mount Misery: Frederick Douglass, a renowned American abolitionist and statesman, was enslaved there in the early 19th century.

Scraping the sea bed in Chesapeake Bay

The charm is obvious enough: rather like the Norfolk Broads, it contains few breathtaking vistas but, taken as a whole, its quiet, undulating, slithery beauty and ramshackle little towns leave few unmoved, and if your correspondent had to choose a place to see his last sunrise, this might be it. Of course, every golf course, condo development and chain restaurant chips away at the very thing that made people want to move there in the first place. And they inevitably bring environmental problems: sewage, agricultural run-off and increased burning of fossil fuels, all of which produce large quantities of pollutants, which find their way into the bay.

As far as the health of marine ecosystems go, perhaps no single pollutant does more harm than nitrogen. It occurs naturally in human and animal waste. Fossil-fuel combustion produces nitrogen oxides, which rise into the atmosphere and come down in rainfall as nitric acid. And fertilisers often contain large quantities of nitrogen, which seeps into the groundwater and is washed into the bay. In the water, nitrogen serves as a major nutrient for microscopic organisms called phytoplankton. Individually, they are invisible to the naked eye, but when present in large quantities they cause massive blooms, clouding the water reddish, green, yellow or brown and preventing sunlight from filtering through the water. Also, as these phytoplankton die, they, like all organic matter, are eaten by bacteria, which, also like all organic matter, breathe, using up valuable oxygen in the water. Nitrogen thus harms aquatic life in two ways: by allowing phytoplankton to live, it keeps sunlight from reaching underwater plants and grasses, which removes an important source of food and habitat for numerous marine species. And the bacteria that feed on dying phytoplankton use oxygen, leaving less for fish and crabs.

Fortunately, few species filter nitrogen from the water as effectively as oysters—as Bill Goldsboro, a senior scientist with an environmental advocacy group called the Chesapeake Bay Foundation, explains, “The oyster is pretty particular about what it eats, but it’s not particular about what it filters.” A single oyster can filter about 50 gallons of water per day. A few decades ago, the Chesapeake had enough oysters to filter the entire bay every week: that same task would take its existing population a full year. As an oyster eats plankton, it draws in everything else around it, including nitrogen; what it does not eat it expels into the water as solid pellets of waste, which eventually decompose and bubble up into the atmosphere as nitrogen.

In oyster-farming, both economics and the environment winThe efficacy with which an oyster expels everything that displeases it puts paid to a long-standing myth: that pearls are formed when a grain of sand gets into an oyster (or other bivalve), and it protects itself by forming shell material around the intruder. Oysters live in sandy beds; they constantly ingest and expel the stuff. A pearl actually begins from a parasite adhering to an oyster’s mantle, which is a thin organ that surrounds the inside of its shell. The mantle secretes nacre, or mother-of-pearl, by synthesising calcium carbonate from materials in the water. If a parasite tears off a bit of the mantle and carries it to another part of the oyster’s body, that piece of mantle will still secrete nacre, forming a pearl sac around the parasite, which, over years, turns into what people consider a jewel.

This happens very rarely, and so, on a blustery Saturday morning on the Eastern Shore, when the wheezing remnants of Hurricane Gustav turned sky and water alike pearl grey and your correspondent held 6m oysters in the palm of his hand, he was, alas, fairly certain that none of them would facilitate his early retirement. The oysters were being grown in a hatchery run by the University of Maryland just off the Choptank River, on Maryland’s Eastern Shore. Like most rivers in Maryland, the Choptank flows into the Chesapeake. Its mixture of salty water from the bay and ocean and fresh water from streams in the mountainous west of the state make it a perfect habitat for oysters, and thus an ideal testing ground for a theory: it is not so much that oysters live in clean water, as that water with an abundance of oysters in it will be clean. In other words, dirty water doesn’t drive away the bivalves; rather a lack of bivalves invites the filth.

Predators going after the oysters

Don Meritt, a bluff, burly, deeply-tanned PhD waterman who runs the hatchery (and whom everyone—university president and beaker-scrubber alike—calls Mutt) explains that this is a gross oversimplification, but it contains a grain of truth. “Oysters aren’t the magic bullet, but they’re an important bullet,” he says. Dr Meritt has been studying oysters for the university since 1972. His kingdom is a warren of green-roofed institutional buildings hulking alongside a winding two-lane road, near enough to the Choptank to use its water, which flows in through underground pipes. Inside, oysters spawn in black plastic tubs; algae in every shade of drab seethe and multiply in glass jugs; and cheery young students hunch over notebooks. The future of the bay—and more than just the bay, if the experiments work—may depend on what happens here, for oysters are a keystone species: if they thrive, others will too.

Oysters filter nitrogen, and their beds offer the same multispecies home as hard coral in the tropics. Oysters have relatively few natural predators: mainly starfish, which attach themselves to the shell with multitudinous teeth and patiently chew through, and the oyster drill, a species of carnivorous snail that attaches itself to a mollusc shell with a multi-toothed organ and inserts its proboscis, which releases enzymes that digest the creature in its home, making it easy to hoover up. Watermen once tried to defeat starfish by cutting each one they dragged up in half; unfortunately, since they regenerate, this doubled the starfish population. Even a few predators, however, attract predators of their own. And as the oysters remove both plankton and nitrogen from the water, it grows clearer, allowing eelgrass and other species of marine plants to return, which provide comfortable shelter for crabs, scallops and other aquatic life.

In the hatchery, oysters grow from larvae to spat; a group called the Oyster Recovery Partnership (ORP) then carries the spat out to the Chesapeake or one of its tributaries and places them in an oyster bed. In 2008 the ORP planted over 450m hatchery-raised oysters. Not all will live, of course, but many do: over 200m through the ORP’s efforts alone, since 2007, totalling around 1,100 acres of new oyster reefs (historically Maryland held about 200,000 acres of oyster reefs; today it has about 36,000). Half of the oysters have been seeded in sanctuaries and cannot be harvested; the other half are in managed-reserve beds, which watermen tend and can harvest from once they reach marketable size. Only a small portion of available oysters will be harvested, whether publicly or privately; most will be left in situ for the environmental benefits they provide.

And Maryland will likely turn away from public fisheries and toward private ownership of beds—after all, people tend to take better care of what they own. Fortunately, farmed oysters, unlike other seafood, suffer no decline in taste. They grow, breed, eat and filter just as they do wild. Indeed, oyster farming is one of the few situations in which both economics and the environment win: any body of water that can support a vibrant oyster industry will almost certainly be cleaner and more vital than one that cannot. Farmed salmon may turn flabby, bland and, without the addition of dye to its diet, dully grey, but eating an oyster will always be, as Léon-Paul Fargue, a Symbolist poet, said, “like kissing the sea on the lips”.


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