Spirals in Time: The Secret Life and Curious Afterlife of Seashells (18 page)

Adventures of an oyster

Oysters live complicated lives. Watching an oyster grow up, it almost seems as if it can’t quite make up its mind about what it wants to be. It all begins, for the Native Oyster at least, when adult males cast clumps of sperm into the sea around them. All being well, some of the sperm will drift past a receptive female oyster who will draw them in through her siphon and use them to fertilise her eggs inside (in some other oyster species the females pump their eggs out into the water, and fertilisation is external). Problems in the lives of oysters can start right here. If there isn’t a female nearby, those sperm will go to waste.

Sex for bivalves is never as intimate as the slimy clinch of snails, but even though they don’t come into physical contact, mating oysters can’t be too far apart. An oyster here and an oyster there is no use, which is why Andy went to the effort of bringing in thousands of breeding adults. Placing them in clusters on the seabed – around 10 per square metre – gives the breeding oysters the best chance of successful fertilisation. If this happens, the next step is for each female to brood embryonic oysters for up to 10 days among her gills and in her mantle cavity. During this time the youngsters are visible
in a shucked oyster as a milky slick, referred to by some, revoltingly but accurately, as white sick. This gives at least one good reason why it is best to eat oysters only when there is an R in the month (a general rule that was first introduced by the Victorians in Britain); in the northern hemisphere these are the cooler times of year, September to April, when spawning isn’t in full swing. Female oysters are quite harmless to eat during the breeding season but their soupy broods of larvae are perhaps not to everybody’s taste. What’s more it’s a good idea, ecologically and economically, to leave oysters undisturbed during this time so they all have a chance to cast their offpsring into the next generation. As the days pass, the young oysters develop into grey and then black sick, by which point they are ready to leave their mother and fend for themselves.

Depending on her size, a single female Native Oyster will puff out as many as 1.5 million young into the water in one go. It’s one of those facts that makes me think there really should be nothing in the world
but
oysters, but the ocean is a dangerous, difficult place, of course, and only a tiny fraction of those larvae will make it to adulthood.

Now autonomous, each baby oyster secretes a little shell that folds in two halves. It sprouts a brushy cluster of tiny hairs that waggle around and propel it through the water, and in this form it roams around for a few weeks until the time comes to add another piece of anatomy. The larva grows a foot that pokes out between the twinned shells like a tongue, and it sinks down to the seabed before tramping off to begin the most important hunt of its life.

Creeping along, the oyster tests out the substrate, looking for an ideal spot to settle down. If it doesn’t like what it finds, the larva can launch itself back into the water column to catch a brief ride on a passing current and continue its search elsewhere.

What the adolescents are so desperately looking for is a certain smell or a taste that points the way to the ultimate
prize: an empty shell to land on. Volatile compounds waft from living oysters as well as the vacated shells they leave behind, thanks to thin coatings of bacteria and other microbes. These invisible messages tell oysters to flock together. If there is no better choice they will make do and settle onto a stone or piece of wood, but they much prefer the scent of their own kind.

When the minute larva detects the right aromatic trail it crawls towards it and prepares itself for one final transformation. It comes to a halt, squeezes out a drop of chalky glue and cements its left shell in place on the outside of another oyster or onto an empty shell (the glue takes just a few minutes to set). From this point the larva is known as spat. Now, with no further need for mobility, the oyster reabsorbs its foot and grows huge gills which for the rest of its life – perhaps the next 20 years if it’s lucky – will keep it flush with oxygen and particles of food.

Over the next 12 months the oyster spat grows into a mature male not much bigger than a thumbnail. Native Oysters start out life as males, then periodically undergo sex changes, gender-flipping several times during each spawning season, producing eggs, then sperm, then eggs again.

After three or four years the oyster will reach marketable size, seven centimetres (almost three inches) across, and if left alone in the sea for 15 years, it will continue to lay down layers of new shell until it reaches 11 centimetres or more. Long before then, its alluring smell will start drawing in other young oysters – including some of its own progeny, perhaps – so continuing the clustering of generations that builds up oyster beds and banks, spawn by spawn, year on year.

People have known about this part of the oysters’ complex life cycle for a long time. Noticing that oysters of many species are inclined to huddle together and stick to shells, oystermen figured out a simple way of boosting their catches. Back in the golden era of oyster fishing in the US in the late
1800s, people collected up empty shells known as ‘cultch’ from shucking houses and canning factories, and threw them back into the waters they came from. As long as there were enough adult oysters left in the sea, these empty shells provided more places for their larvae to settle on and grow.

Laying down cultch was sometimes carried out in conjunction with the clearing away of oyster predators, including starfish. In New Haven, Connecticut in 1879, the ‘starfish mop’ was introduced. These frayed cotton ropes were dragged over the seabed to gather up starfish, snagging their sticky tube feet. The laden mops would then be brought up and dunked in vats of boiling water. Oystermen then dropped bushels of empty shells into the sea along with some adult oysters to help nurture the next generation. So, just like farmers on land, these seamen were farming the resources of the seabed.

The idea of putting shells back in the sea has been adopted more recently by conservationists, who are trying to undo decades of damage. Rebuilding ecosystems is a painstaking business that is not always successful, but for oysters it does seem to pay off. Restoration efforts, especially in the US, are beginning to show that fully functioning oyster habitats can be put back. Both sides of the country have their own species of habitat-forming oysters: Eastern Oysters on the Atlantic coast and Olympia Oysters in the Pacific. In dense aggregations they form solid reefs that can stand several metres up from the seabed, and in times gone by could be major navigational hazards. Back when oysters were super-abundant in the US they did more than tear open the occasional boat hull; they did a lot of good too.

When oyster reefs flourished they protected coastlines from storm floods and erosion; they nurtured young fish and shellfish that would eventually grow up, wander off and get caught and eaten by humans; and all those gaping bivalves played a vital role in keeping coastal waters clean and clear. There was a time, around 100 years ago, when every drop of
water in many American estuaries, flowing from rivers and out to the sea, first passed across the gills of an oyster. Millions of oysters performed a crucial service, sifting and cleaning the waters, and all free of charge. Oysters remove suspended particles of mud and silt that can otherwise smother seagrasses and other sun-loving organisms. They also do a good job of slurping up excess nutrients from artificial fertilisers and sewage washing off the land and can curb outbreaks of harmful algal blooms. But in parallel to the European story of overfishing, pollution and disease, oyster reefs around American coasts faced a similar fate.

Today there is only one estuary in the US that is still known to have enough oysters to filter all of its water. That fact was uncovered by
Philine zu Ermgassen
from Cambridge University, who crunched a huge amount of data on oyster reefs past and present. Of 13 estuaries she studied, only in Apalachicola Bay on Florida’s panhandle are there enough oysters left to filter all the bay’s water before it pours into the Gulf of Mexico, and that incredible feat is largely thanks to the efforts of conservationists who have helped to put the oysters back.

Nearly every US coastal state has some form of oyster restoration programme under way. In many places, squadrons of willing local residents are volunteering their time because they want to see oysters growing once again on their watery doorsteps.

Various techniques for putting shells back in the sea are being tested. The shallow waters of Florida’s Canaveral National Seashore, with the John F. Kennedy Space Center visible in the distance, are protected from dredging and fishing, but oyster reefs have still been suffering. The wakes from passing boats create bald patches where there used to be oysters. To help heal these gaps, 10,000 volunteers have hand-tied individual shells to plastic mesh mats. Among the helpers were cruise-ship crews, who spent their down-time at sea drilling holes in millions of shells ready to be fixed onto reefs.
Restored areas have since been overgrown by new oysters and are apparently indistinguishable from undamaged reefs.

In Louisiana and Alabama, living oyster reefs are being tried out as a means of protecting coastlines from hurricanes and storm surges. Recycled shells are tied up into mesh bags and pinned to the coasts, where they will gradually be overgrown by new oysters. Concrete reef balls are constructed like giant footballs with holes drilled in them and with oyster shells embedded to encourage more oysters to settle and reefs to grow.

Following America’s lead, other countries are trying out oyster restoration. In the UK, plans are brewing to restore lost natives to several areas that had oyster fisheries in the past, including the Blackwater Estuary in Essex and the Solent on the south coast, but at the moment it’s just Andy Woolmer and the Mumbles Oyster Company who are giving it a go, and a giant pile of empty shells is a key part of their efforts.

As well as transplanting all those mature Scottish oysters to Wales, the team have also added four tonnes of empty cockleshells to the derelict Mumbles oyster beds. There are already old dead shells down there, but Andy wants to know if adding more will help things along.

The empty shells came from the nearby Burry Inlet, where cockles are still gathered from the muds and sands at low tide using hand rakes and sieves, just as they have been for centuries. Owners of the cockle-processing plant let Andy help himself to the huge mounds of empty shells they produce. It still costs thousands of pounds to hire boats and crews to take the free cockleshells out to the Mumbles and Andy is working hard to find ways of making the whole venture economically viable.

One idea he has is to find uses for the invading Slipper Limpets. He is considering a trial system of retaining as many of these gummy intruders as possible when they come up as bycatch in his dredges, then freezing them to make
sure they are quite dead, preserving them in salt and selling them to local anglers. Andy tells me they turn into little rubbery disks, which spring back into shape when soaked in seawater and apparently work well as bait for catching seabass and cod.

Restoring the Mumbles oyster fishery has become a three-pronged strategy: a new adult population has been introduced, shells have been added for newly hatched oysters to settle on, and unwanted molluscs are being removed. Now all the team can do is wait and see if their oysters will spawn successfully.

Oysters aren’t the only molluscs that create ecosystems; many other species do their bit. Blue Mussels are a common sight in shallow coastal waters in the Atlantic and Pacific. They glue themselves to rocks and each other with sticky threads, and form wave-resistant beds. You might have seen them covering boulders and rocks at the beach.

Horse Mussels look like a larger version of Blue Mussels, although they don’t taste as good, apparently. They colonise the seabed hundreds of metres beneath the waves, where they can live for 50 years. Solitary Horse Mussels are widespread and in just a few places they gather together and form thick carpets. Some of the most spectacular are in the Bay of Fundy in the Gulf of Maine where Horse Mussels pile up, forming banks up to three metres (10 feet) high, 20 metres (65 feet) wide and stretching for hundreds of metres. These habitats are highly vulnerable to the impact of dredging and may take decades to recover, if they ever do at all.

Perhaps some of the most surprising habitat-making molluscs are little clams called Flame Shells. They get their name from the bright ruffles of orange and red tentacles that stick permanently out from their shells (they can’t close them all the way). Unlike adult oysters, which live their lives fixed in one place, Flame Shells can swim around, clapping their
shells together and lifting off into open water when they get disturbed or feel threatened. Normally, though, when things are peaceful, Flame Shells get busy building nests.

In a similar way to mussels, each Flame Shell squeezes out a sticky net of silky threads that binds pebbles, gravel and bits of broken shell, forming a honeycomb structure that covers the seabed in a thick crust. The Flame Shells hunker down inside little tunnels, mostly keeping their gorgeous tentacles to themselves. When I first heard about Flame Shell reefs I imagined they unfurled an underwater red carpet that sets the seabed on fire. But in fact they are far more modest and secretive, and in a funny sort of way that makes me like them even more.