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

  Crabs will wrestle each other, checking out whether their opponent’s shell really is worth the effort. If it is, one crab will climb on to the other and repeatedly hammer the shell with its claws. Eventually, either the attacker runs out of energy and gives up or the defender has enough and relinquishes its shell.

  Things are different for the dozen or so species of hermit crabs that live on land. Being high and dry, they accept that seashells are in especially short supply, and these hermits will sometimes have to make do with whatever they can find, perhaps a piece of wood with holes in or a discarded plastic bottle. In Madagascar, land hermit crabs have been seen waiting at the base of a crumbling cliff and picking up hollow fossil shells that occasionally drop out. On beaches, land hermits bustle to the tideline in the hope of finding a new home among the flotsam and jetsam, but there can be so many crabs around that most of the suitable shells will already be occupied. The severe housing shortage forces these crabs to socialise.

  Whenever a land hermit crab is lucky enough to come across an empty shell (sometimes because a behavioural ecologist put it there) and if no one else is around, it will stop, take a closer look and probably try on the new shell for size. If it likes what it finds it will keep the new home and continue on its way. However, if the shell is too big the crab won’t pass on by, but will sit quietly next to it, sometimes for as long as 24 hours. In that time other crabs will probably amble past and wonder what’s going on. Then a spontaneous hermit party breaks out. Don’t get too excited, though, because the main thing that happens when hermit crabs get together is they start forming queues.

  A gaggle of hermit crabs clustered around a big empty shell will sort themselves out into a size-ordered line with the biggest at one end, leading to the smallest at the other. This orderly formation is called a vacancy chain, and people form them too, of jobs and houses. The crabs work out who goes where by clambering around and feeling up each other’s shells. Sometimes, if there are lots of hermits in the area, several queues will form around a single, large vacant shell and then things get a bit more interesting: a tug-of-war ensues.

  The biggest crabs will wrestle over the coveted empty shell while the little ones further down the line will shift queues like supermarket shoppers speculating on which checkout will move fastest. Eventually, one queue will win control of the empty shell and, in a flurry of claws, everybody in the successful line moves house. Each crab slips out of its old shell and into the newly abandoned shell of the crab one place ahead of it in the queue. They all get a new shell, one size bigger, and quickly scuttle off, once again going their separate ways. Behavioural ecologists have worked out that forming vacancy chains provides benefits for all the crabs involved; adding just one new shell can efficiently provide new homes, of just the right sizes, for a whole gang of hermits.

  Behavioural ecologist Mark Laidre took on the enviable task of studying hermit crabs on the beaches of the Osa Peninsula on Costa Rica’s verdant Pacific coast. In one experiment he coaxed the hermits out of their homes and gave them either new seashells or old ones previously worn by other crabs. From the outside, these two shell types seem to be similar in size, but second-hand shells have a larger entrance, and they’re bigger on the inside because previous occupants have excavated them (they secrete chemicals that soften the calcium carbonate, then scrape away layers inside). When Laidre gave crabs new shells, they were usually too big to fit in and part of their bodies stuck out, leaving them vulnerable to attack by predators. In contrast, the hermits given previously occupied houses were mostly doing just fine tucked up inside their shells. As well as being more spacious, the remodelled shells are lighter and easier to carry around. Laidre put hermit crabs on little treadmills and measured how out much energy they use up carrying new and old shells. He found that crabs have a much easier time strutting around wearing second-hand shells.

  The nub of the problem is that only the smallest, youngest crabs are able to move into new shells and begin the long task of digging out the interior, and they will only do this as a last resort, when they can’t find a pre-used shell. On the beaches, there is a booming second-hand market in remodelled shells that become ecological heirlooms, passed on between many successive hermits.

  In the sea, hermit crabs don’t bother remodelling their shells. For one thing, seawater buoys up their shells, effectively making them lighter. Marine hermits also need their adopted homes to be as strong as possible to protect them from all the ocean predators, including plenty of other crabs, that have become specialists in cracking their way into tough molluscs. Excavating a shell and making it bigger, but weaker, just isn’t worth the effort.

  By keeping seashells in circulation and stopping them from getting buried and ground down into sand by the waves, hermit crabs are what ecologists refer to as ecosystem engineers. When beavers build dams and create ponds they are engineering ecosystems, as are woodpeckers drilling holes in trees, and European Bee-eaters digging nests in the ground and in steep cliffs; other bird species will move in after the bee-eaters have left. All of these engineers are creating, modifying and maintaining habitats that other species take advantage of. In the case of hermits, it’s not just the crabs themselves that live inside the salvaged shells; they pick up other hitch-hikers along the way and become miniature, mobile ecosystems.

  There are hundreds of species that creep and crawl inside hermit crab shells, or hang on to the outside and go for a ride. An up-to-date list of the things that live with hermit crabs goes on for almost 50 pages. They include worms that twist their way inside the shell and position their heads at the entrance, ready to steal morsels of food from the mouths of their hosts (they will also nibble hermit crab eggs). Sponges, sea squirts, barnacles, bryozoans, corals and shrimp all take up residence in and on hermit crab shells. There are even some gastropods that have flattened or concave shells that fit neatly inside a hermit’s repurposed mollusc shell. Often two or three of these doubled-up shells will live together inside the same hermit cave.

  All of these assorted hangers-on gain protection from predators and a hard surface to stick to, something that’s generally difficult to come by on the boundless plains of soft, muddy seabed where many hermits roam. The hermits themselves also stand to gain from their cavalcade. Some will deliberately grab anemones and fix them to their shells, making a stinging line of defence; they will even bring their favourite anemones along with them when they move to a new house.

  One unusual variety of sea anemone helps out by building an extension to the hermit’s shell as it grows. Stylobates was originally found in 1895 by American naturalist William Healey Dall, who identified it as a peculiar type of deep-sea snail. Twenty-five years later he changed his mind and realised the golden spiral was in fact an anemone that grows around a snail shell; it makes a gleaming, papery model of the shell, rather like covering a balloon in layers of newspaper to make a papier-mâché bowl. When the anemone reaches the open mouth of the shell it carries on growing in the same spiral shape, as the snail did when it was alive. The anemone gains a solid, secure place to stick to, and the hermit crab will never grow too big for its shell. It seems to be an ideal partnership.

  It’s several months before I hear from Andy Woolmer again. The transplanted oysters are settling in for their second winter off the Mumbles, and Andy has been out to check on them. He emails, telling me about the video cameras he dropped down, but he couldn’t see much through the silt and minerals washing in from the nearby Tawe and Neath Rivers that turn the water into a flocculated snowstorm. He did a few tows with the dredge and brought up a good collection of healthy Scottish oysters that still seem to be getting on well in their new lodgings in Swansea Bay. Some were growing bigger with a white frilly edge to their shells, a line of new growth.

  I click to open a photograph Andy has sent me. On my screen I see a hand holding a large adult Native Oyster. Stuck to it is another, much smaller oyster: an oyster spat. Andy tells me he can’t be quite sure that it was born in
Wales, and hadn’t been brought down from Scotland already clamped to the adult’s shell. Either way, it’s definitely a sign of good things to come for the return of these oysters, which have been missing from the Welsh coast for such a long time.

  CHAPTER SIX

  Spinning Shell Stories

  Many stories have been told about a strange and fabled cloth called sea-silk. Some say that when Jason and his troop of mariners set sail aboard the Argo they may have been hunting for a golden fleece made of sea-silk. There are stories of Roman emperors bearing robes trimmed in shimmering sea-silk, with dancing girls clad in see-through dresses of this same fabric that apparently left little to the imagination. A pair of gloves made of sea-silk has been said to be so dainty, they fit inside half of a walnut shell. Ancient Egyptian kings had boats powered by sea-silk sails, and Egyptian mummies were thought to be wrapped in sea-silk cloaks. Sea-silk was commonly associated with the ‘cloth of gold’ mentioned many times in the Bible. When Henry VIII met the King of France in 1520 at the ‘Field of the Cloth of Gold’, the story runs that the field was decked out with sea-silk flags and bunting, with Henry’s men dressed to match in fine golden tunics. As for the source of this fine fabric, a peculiar set of stories emanated from Chinese traders in the second and third centuries. Water sheep, they said, lived beneath the waves in the Roman Empire and occasionally clambered onto the shore, where they scratched themselves against rocks and left behind clumps of wool; people gathered these tufts and wove them into fine cloth. Similar stories emerged among Arab traders in the tenth century, who told of a beast called abu qalamun that would emerge from the sea at certain times of the year and shed its golden hair along the shoreline. This hair was made into a cloth so rare and valuable that its export was forbidden. Later, a twelfth-century Moorish writer declared the source of these fibres to be a creature that resembled a small sheep with webbed feet like a duck.

  All this may be starting to sound rather far-fetched, and it is likely that the water-sheep stories were really just a joke that got a bit out of hand. From Roman times onwards, other writers based closer to the Mediterranean mention another possible source of sea-silk. They wrote about fine silken threads that came from giant seashells with gleaming beards. It’s here that the stories of sea-silk begin to edge closer to the truth.

  Since antiquity, a large species of Mediterranean bivalve has gone by the name of the pinna. Today they are called Noble Pen Shells, Pinna nobilis. They look like huge mussels standing alone on the seabed, at least as wide as a man’s outstretched hand, up to a metre (three feet) tall, and often covered in a fleecy cloak of seaweeds. They can live for 20 years or more, and while there are several other Pinna species, none are as large as the Noble Pen Shell; these are the biggest seashells in the Mediterranean.

  A net of silky threads with sticky ends sprouts from the shell of this towering mollusc, to stop it from tumbling over in brisk underwater currents; the threads root the Noble Pen Shell to the seabed. Other bivalve species produce similar strands; if you have ever cooked mussels you may have had to clean them first, pulling off their mossy beards.

  These fibrous anchors are formed in a process similar to the production of injection-moulded plastics. An internal gland secretes liquid collagen proteins that trickle along a groove in the mollusc’s foot. The proteins take a few seconds to set hard into a narrow strand while the mollusc presses its foot against the seabed; an adhesive pad at the end of each thread then sticks to seagrass roots, sand grains or other fragments in the seabed. Once the new thread is ready the mollusc lets go, and it will continue making more until it has a beard of 1,000 or so hairs poking out from its shell, and fastened to a central stem lodged deep inside its body. Roughly the width of a fine human hair, the threads can be up to 20 centimetres (eight inches) long. They are known as ‘byssus’, a word often used for sea-silk. Are these delicate filaments the source of the ancient golden fabric? The answer to that is yes. And no.

  It was an American biologist and science historian, Daniel McKinley, who in the 1990s decided to try to find out exactly how pen shells came to be pulled up from the depths and thrust into so many myths and fables. He picked up many strands of sea-silk stories and followed them back in time to see where they began. Through hundreds of manuscripts, books and museum specimens, he hunted for evidence to separate the truth from accumulated layers of mythology. What is sea-silk? When people wrote about pinna shells and byssus, what did they mean? Have these fine fabrics really been around for thousands of years? McKinley gathered together his findings and in 1998 published a monograph called Pinna and her silken beard: a foray into historical misappropriations, which already gives you a good idea of what he had to say.

  A major snag in the sea-silk stories that McKinley encountered is the changing meaning and spurious translation of words. The modern-day meaning of the word ‘byssus’ is clear-cut. The fibres many bivalves use to fix themselves in place on the seabed are called byssus, and they are made by the mollusc’s byssus (or byssal) gland. It follows that a fabric woven from those filaments should also quite reasonably be called byssus. The problem is that the term hasn’t always referred specifically to fibres made by molluscs. Tracing the word ‘byssus’ back in time, McKinley saw that the solid definition begins to get hazy until all certainty evaporates, and it becomes impossible to know what writers were actually writing about.

  Similar words in several ancient languages including Latin, Greek, Hebrew and Phoenician were used as general terms for a range of fine cloths that could have been made from linen or cotton or sometimes silk; the particular material is not always specified. In the Old Testament, for instance, the Hebrew words būş and šeš have been variously translated at different times into the Latin word byssus as well as ‘fine linen’ and ‘silk’ in English and bisso in Italian.

  An important waypoint in the story of byssus is Aristotle. He was supposedly the first person to connect the word ‘byssus’ with Noble Pen Shells and their luxuriant beards. However, when we delve into the details of what he actually wrote, and how his words have been translated, a different story emerges.

  In his book The History of Animals, written in 350 bc, Aristotle mentions pinna, and numerous translations have been made from the original Greek. In 1910, for example, zoologist and shell-shape ponderer D’Arcy Wentworth Thompson translated some of Aristotle’s text as ‘the pinna grows straight up from its tuft of anchoring fibres in sandy and slimy places’. Much earlier, a thirteenth-century Latin translation described the shells as growing ‘upright out of the depth in sandy places’. This phrase comes from Aristotle’s original Greek word, βυσσου, interpreted in this instance as βυσσός, meaning ‘depth’ (from which the words ‘abyssal’ and ‘bathysphere’ stem). This is probably what Aristotle was originally getting at (where Thompson got his ‘slimy places’ from isn’t clear). The real problems arose in the second half of the fifteenth century, when Theodorus Gaza, a Greek translator living in Italy, undertook a major re-write of Aristotle’s book. One thing he changed was the translation of that key Greek word. Instead of ‘depth’, Gaza read βυσσου as ‘byssus’ or ‘fine linen’. The difference comes down to the subtlest of slip-ups, shifting an accent from the last syllable (βυσσός) onto the first (βύσσος), which transforms its meaning from one word to the other (accents were a later addition to Greek that weren’t used in Aristotle’s time). And so, as easily as that, the pen shells were now growing upwards from their fine byssus, much like a tree growing up from its roots.

  Gaza’s translation of The History of Animals was published in 1476 in Venice, and it was immensely popular, far outselling all the previous versions. By making this misleading connection between pen shells and byssus, though, he sparked a game of Chinese whispers that has gone on ever since. Stories were reshaped and new ideas became fixed until most writers and historians uncritically came to assume that any mention of byssus, no matter how far back in the past, could have referred to sea-silk woven fr
om the Noble Pen Shell’s fibres.

  The true story, now well hidden and seldom told, is that up until the fifteenth century there was no reason to link byssus and pen shells. All the various ancient mentions of byssus – in the Bible, on the Rosetta Stone, on ancient papyrus scrolls and elsewhere – most probably referred to linens, or mulberry silk made by moths.

  Given all this, Daniel McKinley remained sceptical about many of the ancient stories of sea-silk. He was sure that the idea of Jason and the Argonauts chasing after a fleece made of sea-silk, however tempting, was just one of many embellishments added to the myth throughout centuries of storytelling. Analyses have shown that Egyptian mummies are wrapped not in sea-silk but in linen. And in McKinley’s view, the links of sea-silk to the biblical cloth of gold were equally shaky; Henry VIII and his men almost certainly never dressed head-to-toe in sea-silk.

  Nevertheless, sea-silk has been around for a long time, although not as widely or with as much significance as many still claim. In reality, sea-silk has always been incredibly rare.

  From myths to reality

  The earliest authentic written mention of sea-silk, one not based on hearsay or mistranslation, comes from the turn of the third century AD. ‘Nor was it enough to comb and sew the materials for a tunic. It was necessary also to fish for one’s dress.’ This quote is attributed to a man known as Tertullian, from Carthage in the African provinces of the Roman Empire. He goes on to describe how fleeces are obtained from ‘shells of extraordinary size’ that have tufts of mossy hair. He was clearly talking about pen shells and their byssus beards.