Dear Readers, I have always tried to be a champion of unloved species, and so I feel as if I have a soul sister in Imogen Cavadino, who has done a phD on slug diversity in gardens with the RHS and Newcastle University, and who is determined that we shouldn’t tar all our garden molluscs with the same brush. You can find her whole talk here, but I thought I would share some of the highlights with you today.
Cavadino starts off by asking ‘what is a slug’? Well, it’s a mollusc, specifically a gastropod (which means ‘stomach foot’, and encompasses both terrestrial and aquatic snails and slugs). I didn’t know that slugs have actually evolved from snails – over time, that big cumbersome shell which provides protection but also takes a lot of energy to produce has shrunk and simplified in most slugs. In some, there’s still an internal shell about the size of a baby’s fingernail buried in the mantle (the smoother bit at the front end of a slug), in others the shell has been reduced to a fine powder. In yet other species, there is a very simple shell right at the tail end.
Slugs only make up a third of UK terrrestrial gastropods, which means that species-wise they are much less common than snails.
The shell of the snail provides protection not only from predators, but also from drying out, so slugs have a disadvantage in this area. Most slugs hide away in cracks or beneath the soil if it’s too arid, and are also inactive below about 5 degrees Centigrade.
One reason that slugs are so considered so icky by some people is the slime that they produce (although of course snails do exactly the same, and are considered rather lovable). Cavadino points out that slugs produce at least two kinds of slime – one is laid down so that the animal can move, but they also produce a defensive mucus, and indeed this can be used to identify the slug in some species – the dusky slug (Arion subfuscus) produces an orange mucus if stroked, for example.
However, this defensive slime doesn’t seem to be much of a deterrent to the many, many animals that eat slugs, from specialised gastropod-eating flies, the larvae of glow worms (who don’t eat anything else) and ground beetles to foxes, badgers and many birds, to frogs and hedgehogs. All the more reason to go easy on the slug pellets!
Another reason to lay off the biocides is that many species of slug are actually beneficial to the garden. 28 percent of slugs are omnivorous, for example, and eat dead and rotting plants, carrion and faeces. A further 17% eat fungi. 12% are carnivorous, although these eat earthworms, which won’t help to endear them to gardeners. Only 26% are herbivorous, and while this group includes those pesky molluscs who eat our seedlings, some of these also prefer rotting plant matter. Finally, there are a full 17% of slugs who are so elusive and understudied that we don’t have the first idea what they eat.
How they eat is another matter. Slugs actually have ‘teeth’ (up to 7000 of them in fact) which are attached to a rasping organ called the radula, which is often compared to a tongue. The arrangement of these ‘teeth’ is slightly different according to the diet. Below is the radula of a ghost slug (Selenochlamys ysbryda) which feeds on earthworms – note the channel in the middle so that the slug can suck up the worm like a piece of spaghetti.
In the photo below, you can see the patterns left by gastropods munching on algae on a greenhouse window.
And the sex life of slugs is extremely complicated – not so much the actual mechanics (most slugs are hermaphrodites, and so they just have to find a willing partner of the same species) but the rituals that go with it. In the netted field slug, for example, the slugs stroke one another’s sides, circle around one another, bite the tail of their partner and bash heads before doing ‘the deed’.The leopard slug, in which the animals descend from a rope of mucus, has even appeared in a David Attenborough documentary.
Cavadino also mentions that some slugs will quite happily mate with a member of another species, producing offspring which she describes as a ‘taxonomic challenge’. Indeed.
And finally, some slugs, especially Arion species (those big black and brown slugs that seem to be very common in my garden) sometimes ‘dance’ if they’re threatened. They contract to about a quarter of their body length and then rock from side to side, like the one below. I might have to try a bit of gentle persuasion next time I find one.
So, there are approximately 44 species of slug in the UK and Ireland, and less than half are native – slug eggs and baby slugs are brought in with imported plants all the time, and they are extremely resilient creatures. Some are very rare. I was intrigued by the lemon slug (Malacolimax tenellus) for example – this little creature lives only in ancient woodland, where it feeds on fungi, and only emerges in autumn when the fruiting bodies of the mushrooms appear. I wonder if there are any in Coldfall Wood? I shall have to have a look before it gets too cold. For the rest of the year, it’s hypothesised that the slugs live underground, eating the ‘roots’ (hyphae) of the fungus. The slugs are totally dependent on ancient woodland – when it goes, so do they.
Another very rare slug is the ashblack slug (Limax cinereoniger) which is the largest slug in the world – it normally grows to about 15 cm but one a whacking 30 cms long holds the record. It again lives in ancient woodland where it feeds on fungi, though there is evidence that it isn’t quite as particular about its habitat as the lemon slug. Just as well, as ancient woodland currently only covers 2% of the UK. It is a rather splendid creature but has only been recorded in a few locations. Let’s hope that it hangs on.
Incidentally, as you can see this slug has a breathing hole on the right hand side of its body, near the head. Practically all slugs are ‘right-handed’. If you find one with a breathing hole on the left hand side this is something to get very excited about.
The only protected slug species in the country is the Kerry slug (Geomalacus maculosus). As its name suggests, this slug is found only in a few places in south-western Ireland, and also in north-western Spain and Portugal. There are two colour morphs – the black one is mostly associated with oak woodland, while the brown one lives, unusually, in blanket bogs and heaths. It also has a unique startle response – whereas most slugs when disturbed cling on to the substrate, the Kerry slug rolls up into a ball like a tiny armadillo. I think it’s a very attractive slug with its starry-spotted body. Let’s hope it will survive – it is also protected in its Iberian habitats, but is difficult to breed in captivity. As usual, preservation of the habitat is key.
And finally, if all this slug-related shenanigans has you interested in some citizen science, Cavadino is currently trying to find out about the distribution of a relatively new slug. The yellow cellar slug (Limacus flavus) has been around since at least the 15th century, but just lately a closely related species the green cellar slug (Limacus maculatus) has been making inroads. It’s thought that this latter slug has arrived from Ireland – whilst the yellow cellar slug has always been closely associated with human habitation, the green cellar slug historically was not, but the hypothesis is that climate change might be changing its distribution. You can find out all about the Royal Horticultural Slug Survey (and take part) here.
And finally finally, remember the ghost slug? This extraordinary creature is an all-white, blind slug that lives up to a metre underground and feeds on worms. It is also anatomically different from other slugs, with its mantle and breathing hole being found at the tail-end of the body, presumably an adaptation to its underground lifestyle. The slug was discovered from various sites in Wales, and its Latin name is Selenochlamys ysbryda, with the species name ysbryda coming from the Welsh word ‘ysbryd‘, meaning ‘ghost’. This is thought to be the first time that a species has been named from the Welsh language. Well overdue, I think. This splendid slug has not been recorded in London just yet, but it has been found in Welwyn Garden City, so it doesn’t have much further to crawl.
So I thoroughly enjoyed this talk by Imogen Cavadino. It was packed full of new information about slugs and their varied lives, and left me wanting to know more, which is surely the hallmark of a great introductory session. I am learning so much from this LNHS series, and can’t recommend them highly enough.
Photo One By Erik Veldhuis; modified by Tom Meijer – http://www.spirula.nl/images/nl_soorten/marien/Arion_subfuscus.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1835528
Photo Two By ©Amgueddfa Cymru – National Museum Wales – Attachment from email correspondence with Catrin Mears at the National Museum Wales., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10551268
Photo Three By Chiswick Chap – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15009732
Photo Four by By Lucie Juřičková – Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. (2010). “Komentovaný seznam měkkýšů zjištěných ve volné přírodě České a Slovenské republiky. [Annotated list of mollusc species recorded outdoors in the Czech and Slovak Republics]”. Malacologica Bohemoslovaca, Suppl. 1: 1-37. PDF. Online serial at <http://mollusca.sav.sk> 10-November-2010. Figure 84b., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=12266731
Photo Five by Rosemary Winnall from http://www.wbrc.org.uk/WORCRECD/Issue%2027/lemon_slugs_malacolimax_tenellus.htm
Photo Six By H. Krisp – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=20634600
Photo Seven By Aidannnuigalway – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=47060168
Photo Eight By John William Taylor (1845-1931; UK) – Taylor J. W. 1907. Monograph of the land and freshwater Mollusca of the British Isles. Testacellidae. Limacidae. Arionidae. Taylor Brothers, Leeds. page plate XXIV., figure 24. Cropped and background removed by User:Snek01., Public Domain, https://commons.wikimedia.org/w/index.php?curid=7555432
Photo Nine By ©Amgueddfa Cymru – National Museum Wales – Attachment from email correspondence with Catrin Mears at the National Museum Wales. catrin.mearsamgueddfacymru.ac.uk, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4456963