Spirals in Time: The Secret Life and Curious Afterlife of Seashells (35 page)

Hunting for answers to these questions has led researchers into the inner structure of seashells and the nano-world of the incredibly small. At this scale molluscs have evolved elegant solutions to the problems of attack, and found ways of not getting smashed. And it’s here we find inspiration for constructing protective shells of our own.

In the last few years there’s been a surge of interest among scientists and engineers in mother-of-pearl. This is the shiny layer on the inside of shells, also known as nacre, and the stuff that pearls are made of. It’s well known that pearls are the result of parasites or bits of grit that irritate a bivalve’s soft innards; the mollusc envelops these foreign bodies in
layers of smooth nacre to protect itself (a fact that gem merchants rarely admit). Material scientists are busy learning lessons from nacre; not, though, on how to make things iridescent and pretty, but on how to make things incredibly strong.

Adult mollusc shells are commonly made of aragonite, a tough form of calcium carbonate that offers good protection against crab claws, fish jaws and even attacks from its own kind as they try to drill and stab their way in. However, the layer of calcium carbonate is prone to fractures. When cracks appear on the outside of a shell they spread, but they stop as soon as they reach the shiny inner layer of nacre. This happens because of nacre’s unique architecture.

Seen through an electron microscope, nacre is composed of diamond-shaped crystals, stacked like bricks in layers on top of each other, made of 95 per cent aragonite. At around 300 to 500 nanometres thick, these layers are just the right size to make waves of visible light bounce around between them, creating the structural colours that give mother-of-pearl its gleam. These nacreous bricks are mortared together with much thinner layers of chitin, the protein component of insect and crustacean exoskeletons. When cracks spread through nacre they are sent on a tortuous path between those microscopic bricks, which saps their energy and stops them in their tracks. The bricks slide over each other and the chitin protein stretches, helping to dampen the growing fracture.

Layers of nacre are also somewhat kinked, which seems to make them even more crack resistant.
Mohammad Mirkhalaf and François Barthelat
from McGill University demonstrated the importance of this waviness in nacre by mimicking its nano-scale structure. They used a laser to etch wavy lines into small glass plates and showed, counter-intuitively, that the glass became tougher by scratching it.

Over recent years several research teams have used a range of ingredients to make synthetic nacre. A team from Manchester and Leeds Universities used calcium carbonate
mixed with polystyrene to make crack-resistant ceramics that could one day be used in building materials and bone replacements. Lorenz Bonderer and a team at ETH Zurich mimicked nacre using thin plates of aluminium oxide coated in chitosan (a material derived from shrimp and crab shells with added sodium hydroxide) to make a composite material that could be used to make aeroplanes and spacecraft. And in 2012, a group from Cambridge University were the first to make artificial nacre the way nature intended.
Alex Finnemore
and Ulli Steiner headed up a team that imitated the steps molluscs themselves take to produce nacre. They laid down layers of calcium carbonate and then sandwiched them between protein sheets pitted with pores. This artificial nacre looks, feels and behaves just like the real thing.

The next, eagerly awaited step is to start using mollusc-inspired materials for applications in the human world. In the pipeline are visors for motorcycle and space helmets, based on a mollusc named, appropriately enough, the Windowpane Oyster. These oysters, also known as capiz shells, were traditionally used in Asia to make windows, and more recently have been made into all sorts of decorations; a roaring export trade in lampshades, candle holders and Christmas lanterns called paról has wiped out stocks of these molluscs in the Philippines. The shells are not just decorative and see-through – they also happen to be incredibly strong.

Ling Li and Christine Ortiz
from MIT worked out that Windowpane Oysters, which are 99 per cent calcite, have a similar nanostructure to nacre, with layers of elongated hexagonal crystals. In the lab, Li and Ortiz whacked chunks of these shells with a diamond-tipped hammer, then inspected the damage under an electron microscope. They saw the crystals behaving in various complicated ways including so-called nanocracking, visco-plastic stretching and nanograin formation; suffice to say that at a nano scale, the crystals put a halt to the spread of damage and set up a no-go buffer zone that cracks don’t cross. It means that
unlike artificial ceramics, damaged shells stay largely intact and crystal clear. So, if their oyster-inspired helmets get cracked, astronauts will still be able to see out.

Composite ceramics, based on ideas borrowed from deep-sea snails, could one day show up in military body armour and vehicles. The Scaly-foot Snail was discovered on the Kairei hydrothermal vent field in the middle of the Indian Ocean, more than 2,000 metres (1.2 miles) beneath the waves. The scaly-foot gets its name from its strange covering that looks rather like the sclerites that covered the Cambrian creature
Wiwaxia
. That would seem odd enough for a modern mollusc, but even more bizarrely their scales are made of
iron
, and their shells are too. No other organisms are known to make skeletal structures clad in iron.

The Scaly-foot Snails make tri-layered shells that trump those of all the other molluscs. Sandwiched between hard layers of iron sulphide on the outside and calcium carbonate on the inside lies a spongy organic sheet. Back in Christine Ortiz’s lab at MIT, her research team tested the protective capabilities of these peculiar iron shells, again by bashing them with sharp probes, then bathing them in hot acid and programming computer simulations of predator attack. They worked out that each layer in the shell has its own distinctive role in protecting the squashable snail inside.

The inner calcium carbonate layer, as in all molluscs, provides an unbending scaffold that is strong but prone to fractures. The organic mid-layer is padding that dulls the blow from attacks; in the wild these snails are hunted by crabs that grab hold of them and can keep on squeezing for days. In addition, the organic layer protects the inner shell from overheating and corroding in the scorching, acidic waters that gush up through the hydrothermal vents. The outer covering of iron sulphide (in fact a form of the compound called greigite) has a nano-scale structure that, similar to nacre, stops cracks from spreading through the shell; it probably also blunts the claws of crabs that try to smash their way in.

The iron-rich scales that give Scaly-foot Snails their name help them to survive attacks from another mollusc species that inhabits the same deep sea vents. Turrid snails hunt in a similar way to their close relatives, the cone snails, firing out venomous darts. The Scaly-foot Snails protect themselves from the rain of arrows by cladding their feet in chain-mail armour. Compared to the cones, very little is known about the venom of turrid snails, and within these minute molluscs even greater pharmaceutical treasures may await discovery.

In 2008,
Baldomera Olivera
went searching for microsnails in the Philippines. He worked with a big team, including
Romell Seronay
, who tested a highly effective but simple collecting tool: two armfuls of knotted, broken fishing nets. These were tied to a weight and lowered 40 metres (130 feet) down into the clear waters off Balicasag Island in the central Philippines and left there for six months.

Known as
lumun-lumun
, this fishing technique was developed in the Philippines to meet a highly unusual demand. There are shell collectors, mainly in Japan, who devote their spare time to gazing at teeny tiny shells down a microscope. Fishermen worked out that placing their old nets in certain areas of the sea was an ideal way of gathering up these diminutive molluscs. The fine netting acts as temporary habitat for drifting mollusc larvae, which settle down and start growing. Other small but mature molluscs will creep in and seek refuge in the tangled mesh.

After waiting patiently for months, Seronay and the team hauled in and shook their net bundles, and got quite a surprise. Out dropped more than 200 mollusc morphospecies – that is unidentified, probable species. The haul included five new cone snail species and 30 turrids, all of them smaller than half a centimetre long.

The team dissected out the venom ducts (a fiddly job) of the most common turrid in their catch, a tiny thing called
Clathurella cincta
. Sequencing the DNA from
Clathurella
’s venom duct, they found genes for two novel peptides similar to conotoxins and presumably with some form of neurotoxic effect. This small project was proof of the concept that
lumun-lumun
fishing could open up a whole new window onto the pharmacological treasures of the deep.

Cone and turrid snails, super-strong nacre, iron-clad deep sea snails and sticky mussel glue together make a compelling case for protecting marine life. Even if it’s for no reason other than self-interest we should care about keeping ocean ecosystems as healthy and intact as possible, just in case there are more things out there that will one day be useful in solving human problems.

There is, however, a potential paradox in this argument. What if too many people want to get their hands on these useful species? For cone snails in particular, there is widespread concern that they are being taken from the wild in vast numbers to feed a growing demand from research labs around the world.

In the past, the only way to get hold of conotoxins for research was to grab a living cone snail (very carefully) and chop out its venom duct. Fishermen in the tropics came to specialise in catching cone snails for this very purpose. The exact volume of the trade is unknown, but a US laboratory reported buying consignments of venom ducts a kilogram at a time. Each kilogram would have contained the ducts from around 10,000 snails.

Since then, techniques to keep cone snails alive in captivity and milk their venom have been developed, but this is not for the faint-hearted. One of Olivera’s students was the first person to rub an inflated condom on a goldfish, then offer it to a cone snail. The snail dutifully obliged, launching an attack, and seconds later the condom was bobbing at the surface with a poison dart lodged in it and the snail dangling
down. More recently, advances in sequencing technologies, the ability to amplify DNA from tiny samples and to make peptides in the lab should see an end to the great piles of dismembered cone snail ducts. Still, though, cone snails face many other threats.

In 2013, a global assessment of 632 cone snail species revealed some key facts about their status in the wild. On the one hand, around three-quarters of all cone snail species seem to be doing reasonably well; they are widespread and abundant enough that they aren’t at risk of going extinct anytime soon. A question mark hovers over 87 species that haven’t been assessed due to lack of data. The remaining 67 cones – around one in ten known species – are considered to be at risk of extinction or likely to head that way in the near future. If we are to maintain the option of studying and using those cone snails and their complex conotoxins, all these species need protecting.

One reason for their threatened status is that many cone snail species have highly restricted ranges. There are species that are found only in the waters around one island or even in just a single bay. As was suggested for the extinct ammonites, the species with smaller ranges are often more likely to go extinct, especially when their habitat is at risk. The stories are sadly familiar. Two species of cone snails found only in Florida are losing their habitat to condominiums and tourist resorts; several Caribbean islands, including the Bahamas, Martinique and Aruba, have their own unique cone snail species, and these are at risk from collectors taking too many. The majority of the world’s endangered cone snails live in the eastern Atlantic, in the Cape Verde archipelago and on the coast of Senegal around the capital city, Dakar. These snails are at great risk from sprawling coastal development and encroaching urban pollution.