Dear Readers, I was planning to do the Big Butterfly Count this morning, but when I stepped out of the front door it became apparent that any self-respecting butterfly would be hiding under a substantial leaf to keep dry. However, half a dozen garden snails (Cornus aspersum) were gently gliding around on the wet stones, and so I sat down on the front step to watch them. I was much taken by the delicate tracery of burgundy-brown and caramel on the shell of each individual, the colours enhanced by the drizzle. What, I wondered, were the advantages and disadvantages of having a shell (after all, slugs manage without one)? Why did the shells seem to curl in the same direction on every snail in the garden? And does the shell tell us anything about the life of the individual snail? I reach for my New Naturalist ‘Slugs and Snails’ by Robert Cameron to see if he has any answers, and several hours later, I emerge, amazed.
The shell of snail performs two main functions: it protects its owner against predation and it acts as a shield against drying out. On the downside, however, a shell requires energy to build, and energy to transport. Slugs and snails are the only molluscs who don’t live in water: a water snail doesn’t have to contend with gravity in the way that a land snail does, because the liquid helps to support it. So, we have to assume that the costs of having a shell are offset by the value of not being eaten quite so regularly, and the value of not drying to a frazzle every time there’s a heatwave.
The garden snail comes originally from the Mediterranean, and there is little doubt that it was brought to the UK (and lots of other places) by the Romans, who enjoyed eating them. The climate of the snail’s native range would historically have been much hotter and drier than Northern Europe (though all bets are off with climate change), which may explain the robust shell, especially when compared to our smaller and more delicate native snails. Traditionally, the garden snail was a creature of the warmer parts of the UK because it couldn’t survive the harsher winters ‘oop north’. Watch this space, however.
The vast majority of garden snails have what is known as ‘dextral chirality’. This means that the mouth of the shell is on the right when viewed from above, and the ‘coil’ of the shell runs clockwise if viewed from the centre. The most important organs of the snail are within the shell, and they are in torsion: if the shell is ‘dextral’, the lung, stomach etc will be twisted in the opposite direction. Chirality is inherited from the mother snail, and in most species, including the garden snail, any individual unfortunate to be born with the opposite ‘twist’ will be unable to mate, owing to the way that these hermaphrodite creatures need to ‘line up’ in order to shoot one another with their ‘love darts’. The sex life of the garden snail probably needs a blogpost all to itself.
Incidentally, snails can do something directly that most animals have to rely upon microorganisms to achieve: they seem to manufacture the enzyme cellulase, which digests the fibrous cellulose that makes up the structure of plant cells. And, while we’re on the subject of eating, the garden snail is one of the few mollusc species in the UK that eats some live plant material (most of the others are detritivores and munch up dead and rotting leaves). Young snails appear to have a particular taste for new growth. However, Robert Cameron does point out that the damage done by garden snails is a tiny proportion of the damage done by the field slug (Deroceras reticulatum) so we can probably cut them a little slack.
The shell of a snail starts with a layer called the periostracum. This is the shiny, tortoiseshell-like ‘stuff’ that I was admiring earlier. It is made of proteins which resemble those that make our fingernails. It is, however, relatively delicate, and all that creeping under stones and rubbing up against flowerpots will soon remove it. Elderly snails can look rather bleached and dull, unlike those polished youngsters that are hanging about under my buddleia. Apparently garden snails who live on sand dunes are literally ‘sand blasted’.
The strength of the shell, however, comes from the lamellar layer, which is formed from several layers of calcium carbonate, laid in opposite directions much like the alignment of the layers in plywood. Calcium carbonate is not as ‘expensive’ for the animal to deposit as protein: Cameron points out that if 5% of the shell is made of protein, that has taken about 50% of the energy to make the whole shell. Unfortunately for the snail, calcium can be difficult to find:the snail eats soil in order to get the materials that it needs, and snails living on limestone have thicker shells than those living on acid soils. Snails might also been seen eating rocks, bones or even the shells of other snails in order to top up their calcium – I distinctly remember that I once saw the skeleton of a dead sheep that was absolutely covered in snails, and now I know why. There may be no snails at all in the most acidic environments, such as heather moors or sphagnum moss, but of course there will be plenty of slugs who don’t have to worry about such things.
One of the saddest sounds of a wet day is the muted crunchy ‘pop’ of a snail that’s been accidentally trodden upon. It’s clear that snails are not impregnable in spite of all that effort, but I was cheered to hear that, in the presence of sufficient materials, a snail can regenerate its shell, provided the damage is not too great. Indeed, you can sometimes spot a snail bearing a tatty, misshapen shell which looks as if it was stuck together with a glue stick. Ladies and gentlemen, what you see before you is a battered molluscan warrior, so respect is due. But wait! I just discovered this article which tells the worried pet owner how to repair the shells of any injured domesticated snails. Truly, the internet is an endless cornucopia of wonders.
The main advantage of a snail shell, however, seems to me to be the protection that it provides against drying out. It’s been estimated that a garden snail loses 8% of its body weight per hour while crawling around, which explains the huge number of snails that I find hiding in the overhanging lips of my garden containers when the weather gets hot. Snail shells are pretty much impermeable, and many snails can seal themselves up completely to wait for happier, damper times.
Incidentally, research across Europe has shown a clear correlation between the proportion of slugs to snails and the dampness of the climate: in Cyprus only 9% of land molluscs are slugs, whereas in lucky old Ireland it’s 31% (thanks again to Robert Cameron Fig 62 page 99). The benefit of having a shell, especially in hot dry climates, appears to be largely about keeping the fluid levels up so that the creature can survive, rather than protection against predators. When the climate is coolish and dampish, slug diversity and numbers increase.
I have always had a soft spot for snails. I love the way that their eye-stalks extend and contract independently, and I love the way that they ooze gracefully across the patio. I know that they can be a pest in the garden, but I suspect that they also do a fair bit of cleaning up. And on a wet night I will sometimes look up from my book to see a snail climbing up the window, silhouetted by the street light and looking for all the world like some kind of molluscan angel, ascending to heaven. The author and poet Munia Khan wrote
“The intriguing placidity from the slothful pace of a snail is truly very peaceful. Our world is in need of this calmness to pacify itself”
I couldn’t agree more.