Defending the small and slimy

The worrying decline of many of Earths charismatic vertebrates has been reasonably well documented and highlighted by the popular media. Ecologists are acutely aware of the plight of beautiful animals such as polar bears, dolphins, and great birds of prey, for example, and many of us have probably been inspired to pursue careers in wildlife research largely due to these appealing animals. The general public are often equally captivated and saddened by the potential loss of these once-mighty, now imperilled creatures, which is hopefully a reflection of an increasing appreciation for biodiversity. Vertebrates however, probably comprise just over 1% of all animal diversity [1]. What about the other 99%?

Most animal species are invertebrates and most invertebrates are insects. The decline of some insects such as bees and other pollinators has been recently recognised and there is now thankfully a focused effort to conserve them. Apart from insects, the second largest group of invertebrates are the molluscs. There are possibly more than 200,000 different mollusc species currently alive on earth [2], which includes well-known animals like squids, octopuses, mussels, clams, slugs, snails and many other bizarre lifeforms. Most mollusc species are entirely marine and only one group, the Gastropoda (slugs and snails) have succeeded in crawling out of the sea and adapting to a fully terrestrial mode of life. Unfortunately, non-marine molluscs – gastropods and freshwater bivalves (clams and mussels) – are one of the most imperilled groups of organisms on the planet. In fact, almost half of all animal extinctions recorded since the year 1,500 have been non-marine molluscs, and almost 90% of these have been gastropods [3]. Non-marine molluscs in Europe have rapidly declined since the industrial age [4] and Irish molluscs show a similar worrying trend towards oblivion: one-third of our native terrestrial molluscs are threatened with extinction to one degree or another [5].

The lapidary snail Helicigona lapicidus became regionally extinct in Ireland in the 1970s. (Photo: Wiki Commons)
The lapidary snail Helicigona lapicidus became regionally extinct in Ireland in the 1970s. (Photo: Wiki Commons)

Ireland’s Land Molluscs

The Irish land-mollusc fauna isn’t particularly species rich. If we exclude molluscs that live in the sea (such as the squids, octopuses, oysters, mussels and cockles with which we’re so familiar), and other aquatic molluscs that live in our rivers and lakes (such as the critically endangered freshwater pearl mussel), we have approximately 80 species of land-snail, and 30 or so different slug species. Of those 100+ land-mollusc species however, only six are legally protected, despite the overall trends suggesting that many more ‘unprotected’ species are vanishing rapidly.

For most people, slugs and snails don’t conjure images of the beauty and splendour of the natural world, at least not in the same way as the vertebrates mentioned above do. It is unsurprising then, that our terrestrial molluscs, especially snails, are viewed as little more than an annoyance; getting in the way of ‘progress’ by holding up the construction of essential public infrastructure like roads and, of course, exclusive luxury 5-star golf courses in Doonbeg. As such, these slimy, slow-moving nuisances have famously caught the malign of many well-known Irish and international politicians from Bertie Ahern [6] and the Healy-Raes [7, 8, 9] to U. S. President Donald Trump [10, 11]. The reasons for political outrage against the snails and slugs are often the same: they are perceived as useless animals which get in the way of things being built.

Kerry slugs Geomalacus maculosus have been solely blamed for holding up roadworks in Co. Cork, despite the fact that the planned route would have sliced through a woodland site designated for the protection of many other organisms. (Photo: Aidan O’Hanlon©)
Kerry slugs Geomalacus maculosus have been solely blamed for holding up roadworks in Co. Cork, despite the fact that the planned route would have sliced through a woodland site designated for the protection of many other organisms. (Photo: Aidan O’Hanlon©)
Minute snails such as Desmoulin’s whorl snail Vertigo moulinsiana grow to only 2mm in (shell) height. A related species, V. angustior (which is legally protected) held up the construction of a sea wall in Donald Trump’s golf course at Doonbeg in Co. Clare. Vertigo angustior is listed as Threatened on the IUCN red list due to population declines across its global range, and Irish populations are considered to be of international importance. Despite this, Clare County Council granted planning permission to construct a wall which will inevitably damage the functioning of the SAC dune system at Doonbeg, and the V. angustior populations that they harbour (Photo: Wiki Commons)
Minute snails such as Desmoulin’s whorl snail Vertigo moulinsiana grow to only 2mm in (shell) height. A related species, V. angustior (which is legally protected) held up the construction of a sea wall in Donald Trump’s golf course at Doonbeg in Co. Clare. Vertigo angustior is listed as Threatened on the IUCN red list due to population declines across its global range, and Irish populations are considered to be of international importance. Despite this, Clare County Council granted planning permission to construct a wall which will inevitably damage the functioning of the SAC dune system at Doonbeg, and the V. angustior populations that they harbour (Photo: Wiki Commons)

 Of Molluscs and Men

Humans have a long and interesting (if not slightly weird) relationship with snails and slugs. They have been utilized since ancient times as food, for traditional medicines, as bait food for rearing other animals (such as poultry) and the humble snails have played a prominent role in the myths and traditions of many cultures across the globe. Due to their often striking colouration, snails and slugs have historically been sought after for the pigments which can be extracted from their shells and skin, and which have been used to create all manner of beautiful art from the earliest European cave paintings through to Renaissance masterpieces [12]. Molluscan shells have also historically been highly-prized symbols of status, which was just as true of primitive man using sea snail shells as decorative beads and currency, as it is of 21st century man using oyster pearls to signify wealth and social status. In Ireland, snails and slugs (seilide) pop up from time to time in our folklore and a host of old wives tales purport ways in which we can make great use of them. For example, slugs should be used as a cure for warts by letting them crawl over the warts, and snails can predict who you’ll marry if you let them spell out your potential paramour’s initials in a tray of flour or clay[13].

What are they good for?

It’s easy enough to demonstrate the value of marine molluscs such as oysters, cockles and mussels to Irish people because they are harvested from the seabed, Molly Malone sells them, and the rest of us eat them. Freshwater bivalves (such as the pearl mussel) play an important role in filtering our water[14], so many people with an interest in river or lake fishing don’t have to stretch their logic too far to understand that these organisms do a good job, if left undisturbed, of cleaning our rivers and lakes (and they do it for free). Sadly their ability to clean our water is often overwhelmed by the vast concentrations of fertilisers and industrial run-off that reaches our myriad water bodies in Ireland. But that’s a story for another day.   

Answering these kinds of “what do they do” types of questions about terrestrial snails and slugs is a trickier task because most peoples’ interactions with snails and slugs is at best “gross” or at worst quite negative (e.g. for voracious pest species that sometimes wreck garden plants or even whole fields of crops). Furthermore (and probably for similar reasons) land snails and slugs are an under-studied group of organisms about which we still know relatively little. The majority of snails and slugs in Ireland are herbivorous and probably contribute significantly to the break-down leaf litter and the cycling of above-and below-ground nutrients, therefore increasing soil fertility [15]. In this regard, snails and slugs are beneficial and are not too different from surface-dwelling worms in terms of their ecological “job”. Interestingly, the services provided by Irish worms has been valued at a whopper €700 million per annum [16]. I won’t attempt to put such a figure on the potential services provided by Irish snails and slugs because (a) we still don’t know enough about their functional roles within ecosystems and (b) it does little to help resolve the commonest anti-conservation argument of “snail vs. motorway”.

The general perception is that, surely it’s not going to mean the end of the world for anybody if one little snail or slug disappears, and that the economic and societal gains from constructing roads etc. always outweigh the economic and societal values of keeping these tiny animals at bay from extinction. This argument however, is a flawed one for a number of reasons. Firstly, if a particular site is designated on the basis of the occurrence of a rare snail or slug, it effectively means that everything else in that environment falls under the protective umbrella of that particular animal species. In this sense, the ecological service or utility of one or two mollusc species is not as important as the net benefits provided by a robust, species-rich ecosystem.

Apart from the likely importance of snails and slugs in breaking down leaf litter, they also form a major part of the diet of many other animals. Very few predators have evolved to specialise in malacophagy (eating molluscs) yet they are consumed by a range of predators including beetles, spiders, amphibians, lizards, hedgehogs, foxes, badgers and many birds[17]. Snails and slugs might be particularly important as a food source for generalist predators in late autumn and early winter, when more favourable prey sources such as earthworms and insect larvae are unavailable [18]. In fact, snails appear to be such an important food source for some birds that, where snails are absent from areas due to high levels of heavy metal pollution, birds lay deformed eggs that have a terrible chance of survival, which is due to the breeding females obtaining insufficient levels of calcium owing to the absence of snail shells[19]. So, at the very least, snails and slugs add some level of robustness to complex food webs and, by extension, overall ecosystem functioning.

Secondly, snails and slugs (like many other invertebrates) hold within them the potential for new medicines, materials and technological advancements. Such discoveries – of a special type of protein or special type of molecule with important medical / mechanical properties – can only be uncovered by biologists if these species hang on in our environment in sustainable populations. I listed some examples above of the sorts of quirky things people have extracted from snails throughout history but many species are still important to this day. For example, analgesic painkillers, (such as ziconitide) are derived from cone snail toxins [20] (cone snails are marine gastropods which capture their fish prey using an envenomed, harpoon-like feeding structure). You may have also unknowingly smeared snail slime all over your face – mucus harvested from the humble garden snail, which is used widely in the cosmetic industry for the treatment of wounded tissue, wrinkles and acne[21]. The old Irish wives tales about slugs as a cure for warts were clearly way ahead of the curve on that one. Snail and slug mucus also has antimicrobial properties, but the physical structure of the mucus (it is a liquid-like gel at rest and a stiff glue when irritated) is now inspiring scientists to develop medical adhesives for use in heart surgery and sealing lacerations in other internal organs[22].

Aside from this, Irish slugs and snails hold great potential as examples to illustrate some key concepts in evolution and ecology. Why study the famous peppered moths when you can study colour-changing Kerry slugs [23] to highlight phenotypic plasticity? Why study Darwin’s finches when you can study Irish grove snails [24] to highlight processes of ecological adaptation? Garden snails are probably the first ‘wildlife experience’ for most children. Why not teach children about the complexity of ecosystems using these ubiquitous snails and their arch nemeses (hedgehogs and blackbirds) as illustrations of a simple natural food web, and an introduction to how ecosystems work?

The most famous snail in the world: The garden snail Cornu aspersum is extremely abundant in most gardens throughout Ireland, making it a great mascot for land-molluscs. (Photo: Wiki Commons)
The most famous snail in the world: The garden snail Cornu aspersum is extremely abundant in most gardens throughout Ireland, making it a great mascot for land-molluscs. (Photo: Wiki Commons)

Who cares?

Of the 150 non-marine mollusc species native to Ireland, 51 are threatened with extinction to some degree or another and 2 have already gone extinct [5]. Perhaps more importantly, we may not know what ecological “job” snails and slugs have been doing quietly, diligently and free of charge for us for millennia in Ireland until they vanish. Terrestrial snails and slugs are the second-most imperilled group of organisms in Europe, with over 50% of all species threatened with extinction. One of the key drivers of molluscan species loss in Europe is climate change – particularly in warm, dry regions where land snails and slugs have evolved delicate life-histories to avoid drying up. In Ireland, climate change is predicted to bring about wetter, cooler summers and as such may actually be favourable for Irish terrestrial molluscs. Unlike the continental cousins of our land-molluscs, the most immediate and enduring threat to Irish biodiversity is and has always been habitat loss. Afforestation with non-native conifers appears to be particularly damaging [5], and I have no doubt that the insane frequency and intensity of illegal upland burning is likely an emerging and serious additional threat to our land-molluscs, given their limited mobility.

The ash-back slug Limax cinereoniger is the largest slug on Earth and reaches a maximum body length of 20cm. The one pictured above is perhaps less than half of its potential size. Irish populations are listed as ‘Vulnerable’ to extinction under IUCN assessment criteria [5], with declines attributed to forestry practices. (Photo: Aidan O’Hanlon©)
The ash-back slug Limax cinereoniger is the largest slug on Earth and reaches a maximum body length of 20cm. The one pictured above is perhaps less than half of its potential size. Irish populations are listed as ‘Vulnerable’ to extinction under IUCN assessment criteria [5], with declines attributed to forestry practices. (Photo: Aidan O’Hanlon©)
The ash-back slug Limax cinereoniger is the largest slug on Earth and reaches a maximum body length of 20cm. The one pictured above is perhaps less than half of its potential size. Irish populations are listed as ‘Vulnerable’ to extinction under IUCN assessment criteria [5], with declines attributed to forestry practices. (Photo: Aidan O’Hanlon©)

From this perspective, saving a few snails from extinction at the expense of rerouting a road is surely a good investment in the survival of a whole collection of species which aren’t doing well at all on a global scale. A big problem facing mollusc conservation in Ireland is disinterest and a poor understanding of the ecology of these fascinating animals. Little is known about snails and slugs because, just like non-scientists might rarely if ever think about the animals, zoologists have traditionally focused their research efforts on terrestrial slugs far less often than the charismatic fauna. So the next time you hear the ill-informed rantings of a gombeen politician with vested interests in the construction / planning sector, or you feel yourself becoming cynical at the thought of construction being temporarily halted so that some tree-huggers can save the snails, think instead of the habitat – and potential – which would otherwise be destroyed if not for the maligned molluscs.

Our 100+ native land-mollusc species have survived in wetlands, on sand-dunes, in forests and grasslands since the ice-age and most still hang on, by a thread, in tiny fragmented patches of suitable habitat along roadsides and behind housing estates. Will they be able to survive the next incarnation of the Celtic Tiger now that the “recovery” is apparently in full swing? Or will mad development schemes continue to get the better of these endearing, misunderstood yet strangely charming creatures?      

You can contribute to better monitoring of the conservation status of many of our Irish molluscs by submitting sighting records to the National Biodiversity Data Centre (http://www.biodiversityireland.ie/record-biodiversity/).

 

References

[1] Ponder, W. F., Lunney, D. (eds) 1999. The Other 99%: The Conservation and Biodiversity of Invertebrates. Mosman (Australia), Royal Zoological Society of New South Wales

[2] Scheffers et al. 2012. What we know and don’t know about Earth’s missing biodiversity. Trends in Ecology and Evolution, 27,501-510

[3] Lydeard et al. 2004. The global decline of nonmarine molluscs. BioScience, 54(4), 321-330

[4] Cuttelod et al. E. 2011. European Red List of Non-marine Molluscs. Publications Office of the European Union (Luxembourg)

[5] Byrne et al. 2009. Ireland Red-List No. 2: Non-Marine Molluscs. National Parks and Wildlife Service, Department of the Environment, Heritage and Local Government (Dublin)

[6] Hickey, D. 2007. ‘Ahern should stop complaining about snails, says top biologist’. Irish Examiner, 29 October [https://www.irishexaminer.com/ireland/politics/ahern-should-stop-complaining-about-snails-says-top-biologist-46488.html]

[7] Anon., 2007. ‘Bypass route is altered to avoid the Kerry slug’ Irish Independent, 4 December [https://www.independent.ie/irish-news/bypass-route-is-altered-to-avoid-the-kerry-slug-26336378.html]

[8] Downing, J. 2016. ‘‘Kerry snail’ blamed for 45-minute traffic jams – Healy Rae’. Irish Independent, 7 July [https://www.independent.ie/irish-news/politics/kerry-snail-blamed-for-45minute-traffic-jams-healy-rae-34864831.html]

[9] McNiece, S. 2016. ‘“We must put the people before the slug” – Danny Healy Rae on Macroom bypass delays’. NewsTalk, 8 July [http://www.newstalk.com/We-must-put-the-people-before-the-slug]

[10] Keogh, B. 2014. ‘Donald Trump hoping to rebuild Doonbeg and take on the snails’. The Irish Times, 6 March [https://www.irishtimes.com/sport/golf/donald-trump-hoping-to-rebuild-doonbeg-and-take-on-the-snails-1.1713871]

[11] Deegan, G. 2016. ‘Tiny snail may topple Donald Trump in barrier battle at Doonbeg’. Irish Examiner, 5 July [https://www.irishexaminer.com/ireland/tiny-snail-may-topple-donald-trump-in-barrier-battle-at-doonbeg-408465.html]

[12] Eastaugh et al. 2008. Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments. Routledge (Abingdon)

[13] Daniels, C. L., Stevens, C. M. 1903. Encyclopaedia of superstitions, folklore, and the occult sciences of the world; a comprehensive library of human belied and practice in the mysteries of life. Gale Research Co. (Detroit)

[14] Lopes-Lima et al. 2017. Conservation status of freshwater mussels in Europe: State of the art and future challenges. Biological Reviews, 92(1), 572-607

[15] Meyer et al. 2013. Influence of terrestrial molluscs on litter decomposition and nutrient release in a Hawaiian rain forest. Biotropica, 45(6), 719-727

[16] Bullock et al. 2008. The Economic and Social Aspects of Biodiversity Benefits and Costs of Biodiversity in Ireland. Report for: Department of the Environment, Heritage and Local Government (Dublin)

[17] Barker, G. M. 2004. Natural Enemies of Terrestrial Molluscs. CABI Publishing (Cambridge)

[18] Jelaska et al. 2014. Prey detection in carabid beetles (Coleoptera: Carabidae) in woodland ecosystems using PCR analysis of gut contents. European Journal of Entomology, 111(5), 631-638

[19] Graveland et al. 1994. Poor reproduction in forest passerines from decline of snail abundance on acified soils. Nature, 368(6470), 446-448

[20] Skov et al. 2007. Nonclinical safety of ziconotidae: an intrathecal analgesic of a new pharmaceutical class. International Journal of Toxicology, 26(5), 411-21

[21] Liu et al. 2017. Snails and skin-care: an uncovered combination. JAMA Dermatology, 153(6), 650

[22] Li et al. 2017. Tough adhesives for diverse wet surfaces. Science, 357(6349), 378-381

[23] O’Hanlon et al. 2017. Quantifying phenotype-environment matching in the protected Kerry spotted slug (Mollusca: Gastropoda) using digital photography: exposure to UV radiation determines cryptic colour morphs. Frontiers in Zoology, 14(35)

[24] Burke, D. P. T. 1989. Variation in body colour in western Irish populations of Cepaea nemoralis. Biological Journal of the Linnean Society, 35, 55-63

Aidan O'Hanlon
About Aidan O'Hanlon 1 Article
Aidan is a PhD student with an interest in the conservation, behavioural ecology and evolution of animals - especially invertebrates of conservation concern.

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