Spirals in Time: The Secret Life and Curious Afterlife of Seashells Read online

  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, a
nd 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.

  Dan Harries, from Heriot-Watt University’s Scientific Dive Team in Edinburgh, knows Flame Shells well. He tells me they can be easy to miss. ‘Occasionally they’ll come to the entrance to their nests and you’ll see them,’ he said. ‘But usually they’re hidden away.’ Instead, to get your eye in, you need to start looking out for suspicious bumps and lumps that shouldn’t really be there on a flat plane of tide-swept gravel and sand. If you give the seabed a gentle prod, you’ll notice it is soft and spongy.

  Then there are all the animals that live among the Flame Shells. Thickets of bristle stars (leggy relatives of starfish) will congregate on a Flame Shell reef, thousands of them waving their arms in the water, while an occasional worm called a sea mouse will snuffle past with its luxuriant coat of iridescent spines. Sea sponges, soft corals and sea firs (relatives of jellyfish that look like miniature evergreen trees) are all devotees of Flame Shell reefs. In the midst of an ever-shifting substrate, they help themselves to the stable surfaces created by the reef where it would otherwise be impossible to get a grip. Clustered together, the clams and their nests transform the seabed from a featureless expanse into a bustling community.

  A team of scuba-divers, including Dan, recently discovered the world’s biggest Flame Shell reef. Dodging the vessels in a busy shipping lane, the divers sank down beneath the waves of the sea inlet, Loch Alsh, that runs between the Isle of Skye and the heaving backdrop of the Scottish Highlands. As the tides rise and fall each day they suck water in and out of the narrow channel that links the loch to the deep open water of the Outer Sounds of Raasay, making this an ideal spot for current-loving Flame Shells.

  Dropping down and mapping out the Loch Alsh seabed, Dan and the dive team found Flame Shells everywhere. The reef they discovered covers roughly 75 hectares (almost 200 acres), an area equal to almost 3,000 tennis courts. It’s home to an estimated 100 million Flame Shells.

  ‘We were a bit curious as to why no one’s noticed them before,’ Dan admitted to me. A theory currently being tested is that these shelly reefs might naturally come and go. As the clams aren’t cemented in place on the seabed, there’s nothing stopping them upping sticks, moving on and building more nests elsewhere.

  To create homes for other creatures doesn’t necessarily require millions of molluscs, gathered together in great reefs and beds. Solitary seashells can also form important habitats. There are fishes and octopuses that lay their eggs inside empty seashells; on land, mason bees use snail shells as nests. The Belligerent Rockshell doesn’t wait around for the other snail to die before turning its shell into a nest. Its victims are vermetid snails (known also as worm snails) that fix their tubular shells onto coral reefs like a random squeeze of toothpaste, with no mathematical elegance and with the open end peering up like an alien eye on a stalk. First the rockshell will suck the vermetid snail out of its shell, leaving behind a smear of eerie blue goo; it then turns around and lays its eggs inside the newly vacated tube. Charming.

  Another group of animals that make use of second-hand shells are especially well known, perhaps because scientists and non-specialists alike find them endearing and fascinating in equal measure. These are crustaceans that seem to think they are molluscs, and have become experts at bringing empty seashells back to life.

  Quietly watch over a tide pool and you might spy a seashell behaving strangely. Instead of sitting very still or perhaps gliding slowly and smoothly along, it scuttles in bursts, dashing forwards for a short way, then hunching down when it thinks danger is near. And if you pick up one of these curious shells there’s a possibility that instead of an inquisitive soft tentacle peeping out you may be greeted by a sharp pinch.

  Most crabs make their own shells. They construct a suit of body armour, which they shed and replace throughout their lives, each time finding somewhere safe to hide while their new outfit dries and hardens around them. Nevertheless, close to 1,000 living species of crab don’t bother with that. They have permanently lost their shells, and have instead evolved ways to take advantage of empty seashells. These are the hermit crabs, and they’ve been borrowing shells for a very long time.

  In 2002 an unusual fossil shell was found in Speeton, a village in Yorkshire, England not far from tall cliffs that overlook the North Sea. The shell was an ammonite, an extinct cephalopod that swam through far more ancient seas, in the Lower Cretaceous around 130 million years ago. After it died it sank down to the seabed where a crab scuttled past, picked it up and climbed inside. It was Dutch palaeontologist René Fraaije who spotted the perfectly preserved body of this hitch-hiker inside its ammonite home with its claws peeping out. This is the oldest known hermit crab, and the only one found inside an ammonite so far.

  A naked hermit crab is a bizarre sight. It has a soft, extended abdomen that twists to a point, making it look like some sort of grotesque shrimp. A crab that lives inside coiled gastropod shells – as many species do – will push its bendy rear end into the empty shell and hold on tight, gripping the central pillar; it then retreats inside, plugging the shell opening with its claws, which have evolved to be just the right shape. These are trap doors that bite.

  Other hermit crabs will grab on to a single, disarticulated bivalve shell – a clam or a cockle perhaps – and hold it over their head like an umbrella. Some specialise in long, narrow tusk shells. Their pincers are rounded to form a perfect plug for the entrance of their tubular homes. The one thing that hermit crabs never do is kill the occupant of a shell before moving in. They only adopt vacated shells and never consume their hosts: they wait for other animals to do that first.

  The majority of hermit crab species live in the sea, and they have evolved a finely tuned sense of smell that draws them to the places where molluscs are being eaten. Particular peptides are produced when enzymes in predator-spit begin to digest mollusc meat; these waft through the water, and when a hermit crab picks up the scent, it marches off in search of a shell that will be abandoned any minute, ju
st as soon as the predator has finished its dinner. Finding new shells is a critical part of being a hermit crab, and they devote a lot of time to this single pursuit. By not making their own shells hermits avoid the costs of construction, but it means that as they grow bigger they will keep on outgrowing their homes. Like Goldilocks, hermits are constantly on the lookout for the perfect shell: not too small, otherwise they won’t fit inside, and not too big, otherwise the shell is too heavy and unwieldy to carry around.

  Curiosity about how an animal evolves to rely entirely on the leftovers of another has led many scientists to watch hermit crabs very closely. These scientists are behavioural ecologists, a bunch who devote themselves to understanding why other animals do what they do. Those who specialise in hermit crabs tend to spend their time tinkering secretly with shells, numbering them, swapping them over, offering new ones and all the while watching how the crabs respond. From detailed behavioural studies one thing is becoming clear: hermit crabs do often live up to their name, and they can be quite antisocial.

  For one thing, they have no qualms about stealing each other’s shells. Suitable shells can be in such short supply that hermit crabs are permanently at risk of being evicted, and when two of them meet, a number of things can happen. The etiquette of a crab-to-crab encounter usually begins with a ritualised show of claws as the duo try to settle things without a fight. A larger crab will hold out its claws so its opponent knows exactly what it’s dealing with (claw size is a good indication of overall body size and hence fighting ability and strength). This can sometimes end in surrender. The loser drops its shell and runs off naked; the victor can then take its time, inspecting the empty shell and perhaps trying it on for size before deciding if it wants to move house. If the situation is more evenly balanced, the slightly smaller crab might put up its dukes, thrusting its claws forwards repeatedly, probably in the hope that this will intimidate its aggressor and make it back off. But sometimes a scuffle is inevitable, and a hermit battle kicks off.