2 The Quintessential Snail
By its nobship sailing upside down,
by its inner sexes, by the crystalline
pimplings of its skirts, by the sucking on
lifelong kiss of its toppling motion,
by the viscose optics now extruded,
now wizened instantaneously, by the
ridges grating up a food path, by
the pop shell in its nick of dry,
by excretion, the earthworm coils, the glibbing,
by the gilt slipway, and by pointing
perhaps as far back in time as
ahead, a shore being folded interior,
by boiling on salt, by coming uncut over
a razor’s edge, by hiding the Oligocene
underleaf may this and every snail sense
itself ornament the weave of presence.
‘Mollusc’, Les Murray1
Les Murray shows us how effective poetry can be in conveying the essence of an animal in just one sentence. The snail’s ability to make us look to the past (‘pointing perhaps as far back in time as ahead’), for example, is a quality few creatures possess. Not only does the snail leave a visible trace of its presence in the form of a shining trail (Murray’s ‘gilt slipway’), it also provides us with tangible continuity with the past through the persistence, long after the animal inside has died, of its shell. Antiquity is implied in the phrase ‘hiding the Oligocene underleaf’ and brings to mind the opening lines of Ted Hughes’s poem, ‘Snails’:
Out of the earliest ooze, old
Even by stone time,
Slimed as eels, wrinkled as whales
As dog’s noses . . .2
The snail, like the tortoise, is a living fossil. It provides us with a reminder of how a particular body plan has endured over millions of years and is an example of a creature that has undergone diversity while keeping its body plan essentially unchanged.
The characteristic spiral imprint of a sea snail is a common feature of fossil-containing rocks, while the shelly remains of primitive man’s molluscan meals continue to provide us with an important source of palaeo-environmental information for those interested in unravelling our more recent past. Stonehenge, for example, was given a Bronze Age dating as a result of studying the assemblage of snail shells in its vicinity, while much of the prehistory of the chalk has been understood by analysing the remains of snail shells, which provide evidence of previous human activity and land use.
Nishiyama Hoen, Snail, early 19th century, ink on paper. |
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The palaeontologist Douglas Erwin accords snails ‘favourite’ status amongst fossils. His interest in them relates primarily to snails of the Permian period. Their presence in rocks demonstrates how capable snails have been of make a living in different ways, how they achieved a wide geographic distribution and how they have survived hostile times that saw other major groups, like the brachiopods and the ammonoids, wiped off the planet. Snails, because of their ubiquity and their persistence, proved particularly useful to Erwin in studying how animal numbers fluctuated over long periods of time. Snails, like other animals, have been subject to mass extinctions as well as periods of proliferation, but in contrast to many other groups they have survived and even flourished. As Erwin points out:
The essence of solving the problem of differential extinction is being able to compare similar winners and losers. A clade (or group of animals whose members share features of a common ancestor) where all species survive is not particularly edifying; nor is a group that almost completely disappeared. But with snails there are enough winners and losers to make useful comparisons.3
John Phillips, who in the mid-nineteenth century was responsible for dividing the fossil record into different eras (the Palaeozoic, Mesozoic and Cenozoic), was convinced that each represented a separate act of creation. The Palaeozoic had its brachiopods, the Mesozoic its bivalve molluscs, but it was the Cenozoic that boasted snails. While each era is indeed characterized by an apparent abundance of some creatures over others, it would be wrong to conclude that snails belonged to just one era. The first recognizable snails put in an appearance during the Cambrian period, at the start of the Palaeozoic. They underwent proliferation but, by the end of the Palaeozoic, they also underwent a considerable decline in numbers. The threat of extinction was to prove a watershed in their history. The snails of the Permian period, the majority of which fed on the detritus in the oceans, gave way to carnivorous, predatory snails that spread and diversified during the ensuing Mesozoic era. A further dramatic decline in snail numbers occurred at the end of the Mesozoic, a time when a staggering 95 per cent of all living species died out. But again snails bounced back in the Canozoic, emphasizing their survivor status. Today the snails constitute the largest, most diverse group within the molluscs, with over 80,000 described species. Many remain un-named and un-described, as I discovered when I gave the Natural History Museum several I had collected in Borneo. When I tentatively asked when they might be examined, I was told that they hadn’t quite finished with those collected on Captain Cook’s expeditions!
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The earliest land snails are to be found during the Palaeozoic era associated with insects and early land vertebrates. There is then a gap in the fossil record until the start of the Cenozoic era when, in the Palaeocene, land snails again put in an appearance. When they reappeared they were indistinguishable from present-day families of snails. In that interval, land snails had spread across the planet, occupying virtually every environment except the polar caps.
If survival and an ability to populate a wide variety of habitats is a characteristic feature of a snail, it is its shell that makes the animal instantly recognizable. Pleasing to the eye and satisfying to hold, fossilized or living, the spiral configuration of the shell picks it out from other animals, most of which have a bilateral symmetry. Formed inside the egg, where it is known as the ‘protoconch’, the rudimentary shell becomes the apex of the adult shell, with growth occurring as a result of material being added to its lip. The type of growth is unusual but also clever, in that it maintains its shape as the animal within grows bigger. The Revd Henry Moseley, commenting on the geometry of the snail shell in the early 1800s, remarked that ‘God had bestowed upon this humble architect, the snail, the practical skills of a learned mathematician’. It was later in the same century that D’Arcy Wentworth Thompson first used mathematical language to describe the snail shell’s spiral shape in his classic work On Growth and Form:
The surface of any shell . . . may be imagined to be generated by the revolution about a fixed axis of a closed curve which, remaining always geometrically similar to itself, increases its dimensions continually: and, since the scale of the figure increases in geometrical progression while the angle of rotation increases in arithmetical, and the centre of similitude remains fixed, the curve traced in space by corresponding points in the generating curve is, in all such cases, an equiangular spiral.4
Translating this mathematical language into pictures, the palaeontologist David Raup was the first to produce on his computer a range of hypothetical shell shapes based on a helically coiled pattern.5 Of all the theoretically possible shapes, only a minority could be recognized among either living or extinct shells. Certain shapes appeared to have been exploited in preference to others. Richard Dawkins in his book Climbing Mount Improbable talks of how he devised a computer program in which shells of different cross-sectional shapes were ‘bred’ by artificial selection, the human eye acting as the selector.6 His ‘zoo’ of computer shells shows an uncanny resemblance to those found in nature. When the variety of shapes is seen, it becomes easy to understand the attraction among collectors for amassing different types of snail shell.
Shell ornamentation proves to be even more extravagant than shell shape amongst snails and is seen not just among the large Pacific sea snails but among the much smaller land snails of South-east Asia. Jaap Vermeulen has executed some marvellous drawings of tiny snails of the genus Opisthostoma7 that conjure up certain lines from Tennyson:
See what a lovely shell
Small and pure as pearl
Lying close to my foot,
Frail, but a work divine,
Made so fairily well
With delicate spire and whorl,
How exquisitely minute,
A miracle of design.
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In ‘The Blind Snail-maker Program’ in Richard Dawkins’s Climbing Mount Improbable different snail shell shapes resembling real shells are generated by computer. |
William Paley, the eighteenth-century theologian, would doubtless have wondered at the complexity of design of some of these tiny Bornean snail shells. It is easy to imagine him using them as examples of God’s hand in Nature, but equally easy to imagine Richard Dawkins’s response: ‘Wrong, gloriously and utterly wrong, it is all the product of cumulative selection, by slow and gradual degrees!’ It is tempting to think that the ribs, flanges and other ornamentation seen in these small snails are a response to environmental demand based on a need for defence against predators, or a disguise so that they become more difficult to spot in the places they inhabit. One evolutionary biologist, Menno Schilthuitzen, who has made a special study of these snails, points out that these explanations may be insufficient to account for the full range of shell diversity in this highly diverse group.8 He hypothesizes that shell diversification may be the result of sexual selection acting on shell ornamentation. Mating snails, he believes, are able to sense each other’s particular shell ornamentation. The tactile clues provided lead to successful mating, with highly ornamented individuals being over-represented amongst the mating pairs. The hypothesis has yet to be tested, but is an interesting example of how a tiny snail may tell us something about the processes of selection.
In this series of drawings of Bornean land snails of the genus Opisthostoma resemblances to instruments from a Hoffnung orchestra are evident.
Returning to the fundamental spiral, the whorls of an individual snail shell can either be in contact with one another (the usual pattern) or disconnected (as in worm shells) or simply close to one another but not touching (as in the ‘precious wentletrap’). When they are fully in contact with one another, the lines of contact form the sutures of the shell. The largest whorl culminates in the shell aperture, bound by a lip, which, in some cases, is extended into a canal known as a siphon. A shell may have a variable number of whorls, be flattened so that all of the whorls lie in a single plane (as in the ‘ram’s horn shell’) or be in the form of a spire (seen to its fullest extent in auger shells). Limpets are snails that have lost their spire entirely, replacing it with a simple conical shape and a large aperture. In marked contrast to limpets are cowries where the aperture is reduced to a mere slit.
‘Worm shells’, from Filippo Buonanni’s 1681 Ricreazione dell’Occhio e della Mente. . . Buonanni’s early drawings show how the typical snail shell has undergone de-coiling.
The precious wentletrap (Epitonium scalare) was much coveted by early collectors. In this etching, Buonanni has unwittingly reversed the coiling. |
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It used to be thought that a feature like shell coiling, once lost in the course of evolution, couldn’t ever return; so-called ‘Dollo’s law’. We now know otherwise, as a result of work on a family of snails called calypraeids. Rachel Collin and Roberto Cipriani have shown that in this family of limpets, coiled shells re-evolved at least on one occasion, perhaps more than once.9
Perhaps Frodo Baggins was right to pity snails and all animals that carried their homes on their backs. On several occasions in the course of their evolutionary history snails have abandoned their shells in favour of a slug-like habit. A shell can seriously limit mobility. Loss of the shell meant that the animal could extend its range, find protection in corners where a shell would be an encumbrance and become less dependent on calcium, an element essential for shell growth. The loss of a shell has, in some cases, been partial and in others, complete. The naked slug is thought by evolutionists to represent the pinnacle of snail evolution. Gardeners might grudgingly agree, knowing how hard it is to eliminate them from the vegetable plot.
In the limpet the characteristic spiral shell of the snail has been lost and the body whorl expanded at the expense of the spire.
The fleshy part of the snail, rather than being an amorphous, extensible piece of muscle, is in fact a highly organized and complex example of animal architecture. It can be conveniently divided into a muscular foot, a visceral mass contained within the shell, and a space known as a mantle cavity into which the gut empties its waste. It is in this same cavity that the head, with its tentacles, finds shelter and that the breathing organs are normally located. In snails with translucent shells these and other internal features can be made out without resorting to dissection: sometimes a pulsating heart can be seen, often a kidney, intestine and, with optical assistance, reproductive organs of considerable complexity.
The head of a snail is particularly fascinating. It has a prehistoric, almost alien, appearance. The face of the roundmouthed snail, Pomatias elegans, isn’t too dissimilar to that of a miniaturized elephant with its grey skin, long trunk-like proboscis and eyes placed at the base of two tusk-like tentacles. Other snails have warty heads with eyes at the tips of their tentacles, but facial appearances can vary considerably. The tentacles, themselves a hallmark of the snail, are highly sensitive structures that can rapidly retract or quiver in response to chemicals in the air. Shakespeare once remarked that only love’s feeling was ‘more soft and sensible than the tender horns of cockled snails’, and certainly they are the features most emphasized in cartoons and illustrations of snails.
Lazzaro Spallanzani, an eighteenth-century Italian scientist, viewed the head of a snail from an entirely different perspective.10 He was interested in repair and regeneration and was responsible for the first snail transplant. Spallanzani removed part of the head of a snail and showed it could re-grow. He went on to transplant the face of one snail on to the body of another. So successful was the experiment that Voltaire, in a letter to a friend, expressed the hope that one day the same might be achieved in humans (he had in mind some of his less attractive acquaintances!). Little did Voltaire anticipate that in 2005 a French lady by the name of Isabelle Dinoire, whose face had been badly mauled by a dog, would have it partially replaced with the facial tissues of another person.
Consideration has so far been given to those external features of a snail that are immediately apparent to the human eye and which could be said to characterize the animal. There are of course others, less immediately apparent, but equally distinctive. One such feature offers a field day for dentists while another has exercised zoologists’ minds more than anything else in snail biology. Housed out of view within the snail’s mouth is a ribbon called a radula on which teeth are set in rows. It is a structure seen only in molluscs, and most snails, with the exception of some parasitic ones, possess one. Its licking motion was first noted by a German malacologist, Franz Troschel, in 1836.11 The teeth are renewed as they become damaged, much like those of a shark. Differences in dentition represent differences in diet. Those land snails that scrape algae from stones have numerous small teeth while carnivorous sea snails have fewer, larger teeth. The patterns produced by a radula on glass coated with a film of algae can be arresting, as can close-up views of radulas themselves.
The idea that a snail has teeth, in some cases hundreds of them, may come as a surprise, but a second feature, body torsion, proves even more surprising and somewhat puzzling. Again it is peculiar to snails and quite distinct from shell coiling. It is a twist in the snail’s body occurring early in the animal’s development. Back in the Cambrian period, 600 million years ago, when creatures resembling modern-day snails first appeared, torsion was already present. The shell and the mantle cavity could be seen to have rotated 180 degrees in relation to the rest of the body. Torsion has persisted, but the reason for its existence remains obscure, being present both in the body of the young larval stage of sea snails as well as in the adult. What torsion achieved was to bring the mantle cavity to a position in front of and above the head. Walter Garstang, a professor of zoology at Oxford, had his own theory about how torsion arose which he published in verse form.12 Among his many achievements Garstang was responsible for writing a parody on Gay, ‘The Student Opera’, applied poetic analysis to bird song, and was also an accomplished marine biologist who wrote several important scientific papers. What he is mainly remembered for is a book of zoological verses entitled ‘Larval Forms’. He was one of the first biologists to realize that natural selection acted just as powerfully on the early stages of development as on the adult and he applied this knowledge to sea snails and their larvae.
In the roundmouthed snail, Pomatias elegans, the eyes are actually at the base of the tentacles. The animal’s long, flexible proboscis can be seen. |
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Snails that live in the sea shed their eggs and sperm directly into the water. From the fertilized egg hatches a larva, not a mini-adult but a quite distinct creature with a two-lobed swimming organ or velum fringed with hairs and called a veliger. It is capable of swimming or being carried some distance by the ocean currents and it is in the veliger stage that torsion first occurs. Garstang argued that the twist gave more protection to the larva’s vulnerable head than if it hadn’t occurred. In a poem entitled ‘The Ballad of the Veliger or How the Gastropod got its Twist’, reproduced in full as an appendix, Garstang set out his argument. Selection in favour of the twist in the larva was, in his view, so strong that it outweighed contrary selection in adult life and so this larval feature became a feature of the adult snail too: an example of paedomorphosis. Whether or not Garstang was right is open to debate. What is clear is that, as a result of torsion, the animal couldn’t conveniently grow lengthways so that the viscera were forced to grow upwards in a compact, spirally coiled hump, giving rise to the characteristic snail form we all recognize.
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A scanning electron microscope image of a snail’s radula showing its rasp-like teeth. |
Of course, the essence of any creature isn’t just to be found in its structural detail and its development, but in its habits and behaviour. Some aspects of snail biology are worthy of separate consideration and I have devoted space in later chapters to snail courtship, for example, as well as homing activity and a land snail’s ability to change its behaviour according to local circumstances. One aspect worthy of immediate consideration, however, is movement. Snails are, by definition, slow creatures. Cartoons usually emphasize this feature above all others. To move ‘at a snail’s pace’ is to move very slowly indeed, as those who have visited the World Snail Racing Championship at Congham, Norfolk, will know. Congham is to snail racing what Newmarket is to horse racing. The event was the brainchild of Tom Elwes in the early 1960s. Part of the village fête, it proved especially popular amongst the local children. It gained momentum in the 1970s when the journalist Tony Scase involved the national press. Now over a thousand children and adults attend each year and over two hundred snails compete over a 13-inch course for a silver tankard stuffed with lettuce leaves. A garden snail called ‘Archie’ covered the course in two minutes and twenty seconds in 1995, according to the Guinness Book of Records. The present ‘snail master’ Neil Riseborough, a local farmer who breeds snails, arranges the heats and ensures no one gains an unfair advantage over the damp cotton sheet course. The pent-up energies of the participants are suddenly released when Neil shouts ‘Ready, steady, slow’. Giant foreign snails aren’t allowed, only the English common garden variety.
The slow adhesive progress of the snail relies on slime, another distinguishing feature of the animal. Land snails produce copious amounts of it, glistening, sticky and sometimes frothy. It lubricates their body movement and forms glue that allows the animal to cling to the underside of leaves. Sometimes, rather than form a continuous trail along the ground, slime is produced in a series of shining spots. This is because some snails ‘gallop’ rather than glide, lifting their heads, extending their bodies and finally bringing their heads back into contact with the ground in a looping movement. Even snails that live in water produce slime.
The composition of slime, its viscosity and colour, can vary. The foot of a land snail, for example, produces watery mucus that changes its physical properties when subjected to pressure. Unstressed, it behaves like a gel; stressed, it becomes a fluid. Anette Hosoi and her team at Massachusetts Institute of Technology became interested in the role played by mucus in locomotion and devised robotic snails to mimic snail movement.13 As Professor Hosoi points out, snails can manoeuvre over a range of complex terrains, even cross ceilings, and yet their movement is mechanically simple. She has been able to demonstrate how undulations of a rubber-coated body moving over a thin film of oil (here substituting for mucus) propel the body forward with waves travelling from front to back. The fluid is pushed backwards and compressed, creating large pressures within it. When the waves are made to pass in the opposite direction, from back to front, forward undulating movement is again generated. Many biological systems involve fluid movement within a flexible boundary, not least blood flowing through veins, and her research may produce benefit to man beyond a better understanding of snail locomotion.
Slime, when dried, can effectively seal the aperture of a shell and allow the animal to survive drought. Gilbert White in his journal describes how the land snail protects itself from dehydration and frost: ‘it throws round the mouth of its shell a thick operculum formed from its own saliva, so that it is perfectly secured, and corked-up as it were, from all inclemancies’.14 Desert snails have thick shells, small apertures and substantial mucus seals. Others, notably those snails that live in the sea, instead of having a seal made of mucus have a lid or operculum made of the same material as the shell. Its presence or absence conveniently divides snails into two groups: the ‘operculates’ that respire by means of gills and are dependent on water, and the ‘pulmonates’ that use their mantle cavity as a lung breathing air, a feature that allows them to live on dry land. Some freshwater snails are operculates and others are pulmonates, since some arose from the sea and others from dry land.
Snails approach the finishing line at the World Snail Racing Championship at Congham in Norfolk. |
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The lid or ‘operculum’ of this snail, Cyclostoma belari, has itself a spiral motif. |
Water is lost from the snail’s body as a result of exposure to the air or taken into the body through the skin if the liquid that surrounds it is more dilute than the snail’s own body fluids. Moisture is important to all animals, and snails are no exception since slime offers limited protection against drying out. The shell, being impermeable, offers substantial resistance to water loss but the animal, unless dormant, still has to emerge to feed and is therefore at risk of either losing water or taking more water on board than it needs. This goes some way to explaining why land snails usually prefer humid conditions since there is less risk of becoming dehydrated. Slugs, likewise, prefer a moist environment but show an even greater ability to withstand water loss from their bodies. Moisture and the living animal are intimately associated with one another. Seeing a dried, often bleached shell makes us think that the animal inside is either absent or dead. This isn’t always the case. Snails can survive in suspended animation during periods of unfavourable weather and appear externally dead when, in fact, they are alive but immobile.
The search for the quintessential elements that make up a snail has taken us from persistence in the face of adversity, to spiral shells, to body twists, to microscopic detail and, finally, to water and slime. Some of its other attributes form the subject matter of later chapters, but those qualities we ourselves project onto the snail are of equal importance in defining the animal. How we imagine the world of the snail and what a snail might be thinking about us forms the subject of the next chapter.