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

  Molluscs have always been an important food for people, and it’s easy to see why. Unlike many more mobile sea creatures they don’t tend to hurry away at great speed, and they live in shallow waters and between the tides, so even the clumsiest hunter can easily pick them up. Plus they come neatly packed into their own containers and cooking pots. Some varieties taste better than others and while there can be complications, as we will see, on the whole molluscs are nutritious and protein-packed.

  At the moment, people collectively eat more than 16 million tonnes of molluscs every year, worth approximately $5 billion, more than £3 billion (the total amount of fish and other sea life we eat annually is around 130 million tonnes). Most of those molluscs are bivalves, and most of them don’t come from the wild but are reared in seafood farms, with more than 70 per cent of them grown in China. The molluscs we eat tend to be grouped together with various other shelled marine animals – mainly crabs, lobsters, prawns and shrimps – and are referred to collectively as shellfish.

  In the past, people have had some rather peculiar shellfish habits. The Romans apparently liked to eat clams that glowed in the dark. Pliny the Elder wrote about people’s mouths shining like fire, with bright juices dripping over their hands, down their tunics and onto the floor. Piles of empty mollusc shells have been found at the site of a Roman bathhouse in southern England, including bioluminescent angelwing clams (also known as piddocks). Were these the leftovers from night-time bathers, feasting on a twinkling midnight snack?

  These days, mollusc-eating is usually a much less adventurous pursuit, especially in Britain. Many people have mixed feelings about eating food that can sometimes look rather like a sneeze, or has the dismal texture of a mouthful of rubber bands. And this is nothing new. In her 1867 book Edible Molluscs of Great Britain and Ireland (With Recipes for Cooking Them), Matilda Sophia Lovell laments the nation’s lack of interest in anything other than oysters and cockles, especially when compared to the rest of continental Europe where much more enthusiastic mollusc consumption was going on. Whelks, in particular, have long been one of the UK’s unloved edible molluscs. Today, thousands of tonnes of these large marine snails are caught around the British coast each year, and almost all of them end up being exported. Huge demand comes from South Korea where canned whelks, known as bai-top, are delicacies. Ever since their local whelk stocks collapsed from overfishing more than a decade ago, Korean importers have been hunting for new sources of this sought-after food, and the European snails from a ‘cool and clean sea’, as one online supplier puts it, are ‘100% natural’.

  Whelks are very easy to catch. Pots, often made from 20-litre plastic canisters that are punched with holes, are baited with crabmeat and lowered onto the seabed. The whelks catch a whiff of food, amble into the pot and are still there stuffing themselves a few days later when the pot is hauled back up. Fishermen traditionally operate a rotational system, harvesting whelks from one area for a while before moving on, and only returning to the same spot when the population has had a chance to recover. As long as there aren’t too many people working the same patch of seabed, this is a low-impact way of fishing. It causes none of the physical damage of dredgers that drag heavy metal machinery across the seabed to catch things like scallops, devastating fragile marine habitats as they go. Certainly, it seems to make sense for the British public to get over their squeamishness and eat more of these gently caught, home-grown whelks, rather than sending them to the other side of the world. But having seen a crate of live whelks on sale at a market in Swansea, Wales, and watched their squirming white feet with black freckles, I realised that I still needed convincing that eating them is a good idea.

  An excellent reason to eat molluscs is that some of them are among the most sustainable seafood available. With rampant overfishing stripping the oceans bare, there has never been a more important time to consider carefully the available choices when eating seafood. As a result, various conservation groups release advice on which species are the best options, the ones that come from well-managed fisheries that aren’t overfishing stocks or vandalising habitats. And the better options include plenty of molluscs.

  Rope-grown mussels are often top of the ‘good seafood’ lists. The process of producing them is simple and smart. Ropes are suspended in the sea or poles are pushed into the seabed downstream from a population of wild mussels. In the spring, as waters warm up and spawning takes place, wild mussel larvae waft through the sea and some will settle on the ropes and poles. Essentially, this exploits the fact that each adult mussel produces millions of offspring and only a handful from each batch will survive naturally. Even if a mussel farm intercepts thousands of youngsters from the rain of larvae pouring through, it will have virtually no impact on the wild population. The most important thing is to have clean water with strong currents that will sweep in larvae and provide oxygen and food for the growing mussels, while dispersing their droppings that otherwise pile up on the seabed and can cause local problems. Once they have settled, the growing larvae can be transferred to wire rafts or mesh socks suspended from the sea surface, where they are left for 12 to 18 months to reach marketable size. Then the mussel farmers come along and gather in their crop by hand.

  Farming mussels grown like this involves none of the villains of bad fishing practices. There’s no damaging fishing gear that tears up the seabed. There’s no bycatch of other, unwanted species that are thrown back, mangled and dead. A major bonus for growing mussels, and other bivalves, is that they feed themselves, sifting particles of food from the water. Many farmed fish are fed on other fish – usually caught from the wild – in particular salmon, as well as tiger and king prawns (known as shrimp in the US). And mussels, on the whole, are also quite a healthy bunch and don’t need to be doused in pharmaceuticals to ward off disease, another common practice in fish farms that use powerful drugs to keep diseases under control among animals kept in often cramped, overstocked conditions.

  Oyster farms run along similar lines, although a growing number use larvae that are reared in land-based hatcheries rather than gathered from the sea. Mature oysters are kept in aquarium tanks and encouraged to breed, then their larvae are siphoned off and released into nurseries at sea. As they grow bigger, the young oysters can be laid out on racks, placed in cages or glued to ropes. Like mussels, oysters feed themselves from plankton in seawater but they generally take longer to reach marketable size. The most common species, the Pacific Rock Oyster, will be ready to eat after two or three years. With a helping hand from people, this has become the most successful of all the oyster species. It was originally native to the Pacific coasts of Asia, and farmed for centuries in Japan, before other countries caught on. In the twentieth century, they were transferred to oyster farms across the globe and many populations have established in the wild, from Australia to South Africa, Europe to North America. If ever you shuck and slurp an oyster, wherever you are in the world, the chances are you’re eating a Pacific Rock Oyster. Which raises a question that I am often asked: is it cruel to gulp down living oysters?

  A similar question can be asked of boiling lobsters and crabs alive, and there is mounting scientific evidence to suggest that crustaceans can and do feel pain. However, this remains largely unstudied in molluscs. Basic biology tells us that molluscs in general, and clams, scallops and mussels in particular, are far simpler creatures than crustaceans. The bivalves’ lack of brain means they probably have only a limited capacity to sense and respond to the world around them. Among the shelled molluscs we eat, it’s the super-intelligent octopuses and squid, plus the itinerant snails, that have heightened senses of perception and are more likely to be able to feel pain – a good excuse, if you want one, to pass on the calamari and escargots.

  By contrast, there are some vegans who consider oysters and mussels to be so plant-like that eating them isn’t a problem. In reprints of his book Animal Liberation, vegan advocate Peter Singer keeps changing his mind about whether it’s OK to eat oysters. It
seems there is no concrete proof either way on whether they do or don’t feel pain. But certainly, I think there are fewer questions to be asked about farming and eating bivalves than there are about rearing mammals and birds in horrific factory farm conditions. However, ethical issues aside, there are a few other reasons why it’s not always a good idea to eat molluscs. For starters, eating shellfish comes with the risk of catching a dose of food poisoning, and occasionally something much worse.

  The whole shucking truth

  On the shores of Chichester Harbour in southern England, the small town of Emsworth was once home to one of the longest-running oyster fisheries in world. Records show that people were eating Emsworth oysters as far back as 1307. The fishery thrived throughout the nineteenth century, but came to a sudden halt in 1902, when tragedy struck. Two great banquets were held in the nearby cities of Winchester and Southampton, where guests were served oysters from Emsworth. In the days that followed, 63 people fell ill and four of them died, including Dr William Stephens, the Dean of Winchester Cathedral. They had all developed typhoid, caught from eating oysters contaminated with human sewage. It turned out that recently laid sewers and drainpipes were pouring effluent into the harbour, directly over the oyster beds. When this unpalatable reality was brought to light, the Emsworth oyster fishery was immediately abandoned, and hundreds of people lost their jobs.

  The fact that molluscs – in particular bivalves – can sometimes be very bad to eat is not the molluscs’ fault but ours, for polluting the seas they live in. Even in wealthier countries, where most human waste is now collected and treated, sewage still leaks into the sea, especially following heavy rains when sewers become overloaded. Other effluent comes from farm manure that runs off land, into groundwater and out to rivers and coasts. And whenever there are disease-causing bacteria and viruses floating around, it doesn’t take long for bivalves to pick them up; oysters can filter around 100 litres of water every day, a large bathtub-full. The molluscs themselves may not suffer from their noxious load, but they will pass it on to people who eat them. If you’re very unlucky you could catch norovirus (also known as the winter vomiting bug), E. coli, listeria or salmonella from sewage-infected bivalves. The biggest outbreak of shellfish infections on record was in Shanghai in 1988 when almost 300,000 people contracted hepatitis A from eating clams.

  There is a suite of other dangerous conditions people can catch from bivalves. Self-explanatory names describe the key symptoms of various diseases: there is paralytic shellfish poisoning, amnesic shellfish poisoning and diarrhoeal shellfish poisoning, along with neurotoxic shellfish poisoning and the so-called ‘possible estuary-associated syndrome’. For vulnerable people, and in high doses, these illnesses can be deadly. The problems stem, once again, from bivalves’ habit of filtering seawater for food. A major part of their diet is phytoplankton, the plant-like microbes that harness the sun’s energy on a colossal scale. Among thousands of phytoplankton species there are around 80 that can become extremely virulent, including some dinoflagellates and diatoms. They produce strangely named noxious compounds like yessotoxin, saxitoxin and domoic acid, which between them cause the various shellfish poisoning syndromes.

  These toxic phytoplankton will sometimes proliferate, triggering phenomena formerly known as red tides, though scientists now prefer the term ‘harmful algal blooms’ because they can turn water purple or green or dingy brown. When bivalves find themselves in the middle of a harmful algal bloom, they filter plankton from the water, along with the toxins. Again, the bivalves themselves don’t suffer, but the toxins build up in their tissues to levels that are dangerous for anyone or anything that eats them (including, perhaps, those ancient Andean shamans, who may have eaten them to commune with the spirit world). Harmful algal blooms can happen quite naturally without any influence from people. Recently, palaeontologists uncovered a nine-million-year-old graveyard of 40 or more whales in the Atacama Desert in northern Chile that were thought to have died after eating fish contaminated with toxic plankton.

  The really bad news is that harmful algal blooms seem to be on the rise. Several decades ago, they were only seen on a few coastlines, but outbreaks now appear worldwide. At the same time, shellfish poisonings are more common, and molluscs are generally becoming more dangerous to eat. In the 1980s, there were around 2,000 reported cases of shellfish poisoning each year. More recently, estimates have shot past the 60,000 mark. It may be that people have become more aware of the issues and report a greater proportion of cases than before. There are also a lot more people in the world, and more of them than ever are eating shellfish. In China alone, clam consumption has increased 400-fold in the last 30 years. However, the most likely explanation is the fact that people are poisoning the seas – although this time, indirectly.

  The exact triggers for harmful algal blooms are still being studied but one important factor is well established: nutrients. Wherever nitrates and phosphates wash into seas and lakes it increases the chances of a harmful bloom forming because phytoplankton absorb those nutrients and grow, just like plants on land when fertiliser is added to the soil. The extra nutrients can very quickly kindle a lot more plankton.

  The rise of artificial fertilisers and industrial-scale farming have played a big part in nutrient pollution. Since the industrial revolution, average phosphate levels in coastal waters have tripled, and nitrate levels have increased even further. Household cleaning products are also implicated. Environmentalists are campaigning for these nutrients to be banned, and in recent years the EU and US have set strict limits on phosphate levels in domestic laundry powders and dishwasher detergents (people living in hard-water areas will just have to make do with glassware that doesn’t gleam quite so brightly). The massive growth in fish farms in recent decades, especially for salmon, is also contributing to the flood of nutrients into the seas, from fish faeces and uneaten fish food.

  As well as encouraging plankton blooms, the torrent of nutrients pouring into the ocean triggers a second ecological disaster. When the blooms come to an end, usually after several days, weeks or months, the dead plankton sink to the bottom where bacteria break them down. This uses up oxygen from the water and creates so-called dead zones where few aquatic species can survive. Since the 1960s, the number of dead zones worldwide has doubled every decade. One of the largest and most persistent is in the Gulf of Mexico, fringing the US states of Texas and Louisiana, which is caused largely by the polluted waters of the Mississippi River. In 2014 it covered 13,000 square kilometres (5,000 square miles), an area roughly the size of Connecticut or East Anglia. And as climate change warms up the oceans’ nutrient-rich soup, the extent and duration of harmful blooms and dead zones will only get worse.

  Because of all these threats, many countries have introduced checks and balances to ensure shellfish is safe to eat. Early warning systems forecast and detect the onset of harmful algal blooms and, when they do strike, any nearby fisheries and fish farms will be closed until all risk of contamination has passed. Regular tests are carried out in many countries to check on levels of bacteria and toxins in shellfish. And while raw sewage may not always be pumped into the sea as it was in days gone by, coastal pollution still remains an issue.

  Across Europe, shellfish beds are assigned to strict classifications according to the levels of faecal coliform bacteria they contain. Grade A molluscs can be eaten straight from the sea (they have fewer than 230 E. coli cells per 100g of flesh). Meanwhile Grade B molluscs, with higher coliform counts (up to 4,600 E. coli per 100g), must be put through a process of purification (or depuration) before they’re eaten. Following the Emsworth oyster poisonings and various other typhoid outbreaks in Europe and the US, methods were developed for depurating bivalves. Now a well-established technique, it generally involves keeping live bivalves for 42 hours in tanks of fast-flowing clean water, often blitzed with UV light, to remove contaminants from their tissues. Some oysters are put through a depuration process even when they come from Grade A beds, just
to make sure. There is also a Grade C (for shellfish with up to 60,000 E. coli per 100g); these have to be moved to cleaner coastal waters before anyone is allowed to consider eating them. Molluscs with even higher coliform counts are strictly off limits.

  Today, the majority of molluscs that make it to market – at least in developed countries – are fine to eat but only because we’ve had to invent ways of protecting ourselves from the pollutants we pour into the natural world. When it comes to eating molluscs, the other major issue is that some species are rather too delicious for their own good. A long time before we came up with ways of farming them, humans already had a terrible track record of taking too many molluscs from the sea.

  Who ate all the clams?

  The very earliest known case of any wild species being driven to the brink of extinction by people was a giant clam, around 125,000 years ago. Giant clams are the largest living seashells on the planet. They can grow to well over a metre (three feet) in length, and live for longer than a century. I saw a living giant clam for the first time many years ago on Australia’s Great Barrier Reef and was amazed at just how huge it was. I smiled down at it and it seemed to grin back with its colourful, corrugated lips (their fleshy mantles acquire their bright colours from photosynthetic microbes called zooxanthellae living inside them, similar to those that live inside many corals). The clam sensed my shadow passing over it with hundreds of tiny eyes and hesitantly withdrew its mantle and closed its twinned shells. Their reputation as dangerous man-traps is utter nonsense with no record of anyone ever getting a part of themselves stuck inside one of these enormous bivalves. As with many legendary beasts, giant clams have far more to worry about from us than we do from them.