In the novel Dr. No, the titular villain explains to James Bond that he once survived an assassination attempt because his heart was in the wrong place. The good doctor had a condition called situs inversus – his organs were mirror images of their normal versions, found on the opposite side of his body. His heart, being on the right, was unharmed when his would-be murderer stabbed the left side of his chest. Having a mirror-image body can be useful when someone’s out to kill you and while that’s true for criminal masterminds, it also applies to snails.
In Japan, Satsuma snails have shells that mostly coil in the same direction. If you put your finger in the shell’s centre and follow the spiral outwards, you would probably move in a clockwise circle. And Iwasaki’s snail-eating snake knows it.
In the body of a snail, a war is waging. It’s so violent that the only reason there isn’t blood everywhere is that the combatants don’t have any blood. The fighters are flatworms, simple parasites that have taken over the snail. Its body is now theirs, a shell in which they mate, cooperate, and produce more flatworms. But they don’t have it all to themselves – other colonies, and even other species of flatworms can invade the same snail. When that happens, war breaks out and the flatworms wage it with something more commonly associated with ants or humans – a caste of soldiers.
Deep beneath the ocean’s surface lie the “black smokers“, undersea chimneys channelling superheated water from below the Earth’s crust. Completely devoid of sunlight, they are some of the most extreme environments on the planet. Any creature that can survive their highly acidic water, scorching temperatures and crushing pressures still has to contend with assaults from predatory crabs. What better place, then, to look for the next generation of body armour technology?
The scaly-foot gastropod (Crysomalion squamiferum) was discovered just 9 years ago at an Indian black smoker and it may have one of the most effective animal armours so far discovered. Its shell is a composite, made of three layers, each with different properties and made of different minerals. Together, they form a structure that’s completely unlike any known armour, whether natural or man-made. It can protect the animal from the searing heat of its habitat, stop its precious minerals from dissolving away in the acidic water and resist the crushing, penetrating, peeling claw-attacks of predatory crabs.
Animals have been protecting themselves with armour long before humans starting shaping steel and Kevlar. To create a protective covering, human designers must account for a mind-boggling array of physical traits including thickness, geometry, strength, elasticity and more. But evolution can take all of those factors into account without the guiding hand of a designer, putting thousands of structures through the test of natural selection and weeding out the best combinations. The results are the culmination of millions of years of research and development and they are striking in their effectiveness.
Haimin Yao from MIT works in the lab of Catherine Ortiz, a group that has been studying the defences of animals including sea urchins, chitons, a group of marine molluscs, to the Senegal bichir, a type of armoured fish.
Yao discovered the secrets behind the snail’s shell by slicing through it in cross-sections and studying its structure at a nanometre level. He even attacked it with a diamond-tipped probe, to simulate the crushing attacks of the crabs that frequent the black smokers. Using this data, Yao created a virtual simulation of the shell and put it through a digital crash-test, crab claws and all.
In Lake Alexandrina, New Zealand, a population of snails is under threat from a parasitic flatworm, a fluke aptly known as Microphallus. The fluke chemically castrates its snail host and uses its body as a living incubator for its larvae. But the snails have a weapon against these body-snatching foes – sex.
The New Zealand mud snail Potamopyrgus antipodarum is found throughout island’s freshwater habitats. They breed either sexually or asexually through cloning, and the two strategies vary in prevalence throughout the lake. In the shallower waters round its margins, sex is the name of the game, but in the deeper waters towards the lake’s centre, snails are more likely to opt for cloning.
Kayla King from Indiana University has shown that it’s the concentration of the local parasites that drives this gradient of sex. The flukes spend their adult lives in ducks and they rely on the birds inadvertently scooping up their larvae while feeding. In Lake Alexandrina, ducks only feed in the shallow waters around the lake’s margins so these areas are hotspots for parasites, and for co-evolutionary wars between them and their snail hosts. Sex provides the snails with the genetic ammunition they need to stay in the game.
The snails and their parasites beautifully support and illustrate the principles of the Red Queen hypothesis, which suggests that one of the chief benefits of sex lies in providing the genetic innovation necessary to outfox parasites in evolutionary arms races.
Yasunori Kano from the University of Miyazaki has found that the babies of Neritina asperulata, a tiny snail just 3 millimetres across, hitchhike on the back of a larger species Neritina pulligera. This living bus is about 2 centimetres long, and dwarfs its passengers by more than seven times.
The hitchhiking snail is a special sort of parasite, and one that Kano thinks has never been described before. They don’t use their hosts as a snack, a home, an incubator or a foster parent – they simply treat them as a vehicle. Other parasites may unwittingly migrate in the bodies of their hosts, but there’s no evidence that these travels are intentional. N.asperulata, on the other hand, is completely dependent on the movements of other host snails. Without them, it would never get to the small rivers it needs to complete its life cycle.
Conspiracy theories, TV thrillers and airport novels are full of the idea that the world is secretly run by a hidden society. We have come up with many names for this shadowy cabal of puppet-masters – the Illuminati, the Freemasons, and more. But a better name would be ‘parasites’.
Every animal and plant is afflicted by parasites. The vast majority are simple, degenerate creatures, small in size and limited in intelligence. They affect our health and development, and even our behaviour and culture. And by pulling the strings of key species, parasites can change the face of entire habitats.In a typical school textbook, an ecosystem consists of plants that feed plant-eaters, who in turn, line the bowels of predators. But parasites influence all of these levels, and as such, they can change the structures of entire communities.
The idea that nature is secretly manipulated by these tiny, brainless creatures is unsettling but manipulate us, they do. And by changing the behaviour of their hosts, parasites can change the face of entire habitats. Chelsea Wood and colleagues from Dartmouth College have found compelling evidence for this, by showing that a tiny flatworm can alter the structure of a tidal habitat by infecting small marine snails.