Supercool snails

In the last couple of months, I've regularly come across active Girdled Snails on my way to work. Damp, but often very cold even during frosty mornings I see these small snails on they way back to their day retreats on the pavement by a front garden, presumably after having been active, feeding? during the night (above, on the 23rd of January at 8:48 am). This nonchalant cold hardiness is in stark contrast to common or garden snails (Cornu aspersum), which have been dormant for a good while, and won't become active until March or April. Why is that? how cold resistant are snails? do they differ in their cold hardiness?
  Land snails are a very useful indicator species for ancient environments, as their shells fossilise very well and can be often identified to species level. Despite this, surprisingly little is known on their cold tolerance. Amazingly, 35 snail species live north of the arctic circle and 44 species over 2,000 m of altitude. How do they survive ice-cold temperatures? As other invertebrates unable to migrate snails have two strategies to survive sub-zero temperatures: freezing avoidance and freezing tolerance. Species that engage in freezing avoidance can actually be active in sub-zero conditions by supercooling. A supercooled snail will be at a temperature under 0 oC, but it won't be frozen, that is cool indeed! They can do this by producing large amounts of small sugar molecules that bind water and make their tissues more dehydrated, and also large antifreeze proteins, which inhibit ice formation even further, allowing them to remain active at sub-zero temperatures. Smaller snails (of shells up to 15 mm) appear to be more freeze avoidant than tolerant, and therefore, they are better at supercooling.

A favourite overwintering spot, with dozens of garden snails of various sizes under a tile lined against a wall in my garden.

We know a bit more about the cold tolerance of garden snails, thanks to the research of Armelle Ansart, from Rennes University and her colleagues. The garden snail has limited supercooling abilities, it is a partial frost tolerant species (they can only survive to a minimum of -5 oC). The are partially freeze-tolerant, avoiding freezing by emptying their guts - gut contents can start the formation of deadly ice crystals, reducing the water content of their body (which makes soluble chemicals more concentrated and decreases the temperature at which ice crystals form) and producing an epiphragm, a hard, thick calcareous layer of mucus that seals their shells shut, keeping the deadly moisture out. As an aside, the epiphragm is also produced in very dry weather, during aestivation, another dormant state in snails, but then the epiphragm keeps the moisture in.

An early waking young garden snail (28 Feb 2011), still carrying its epiphragm attached to its shell.

The preparation for overwintering seems to be kickstarted by the decreasing photoperiod of autumn, rather than temperatures dropping. Garden snails also seek high and dry microhabitats to overwinter and congregate, sometimes in very large numbers in favourable spots, such as the underside of logs, stones or holes in tree trunks. Large garden snails are more resistant to the cold than small ones, as they are better at avoiding the formation of ice crystals, so adults are more likely to survive a hard winter than immature snails.
 As for the Girdled snail, sadly I found nothing, although a comparative analysis of cold hardiness by Ansart and colleagues found out that its congeneric species Hygromia limbata freezes at -7 oC, not too impressive when compared to the tiny Columella edentula, also a British species, which doesn't freeze until the temperature descends to -17 oC, but probably enough to allow it to survive in the mild frosts of Hull.

References
Armelle Ansart, Annie Guiller, Olivier Moine, Marie-Claire Martin, Luc Madec. 2014. Is cold hardiness size-constrained? A comparative approach in land snails. Evolutionary Ecology, 28: 471-493. Here.

Ansart, Armelle, and Philippe Vernon (2003) Cold hardiness in molluscs. Acta Oecologica 24.2: 95-102.

Ansart, A. & Vernon, P. (2004). Cold hardiness abilities vary with the size of the land snail Cornu aspersum. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 139: 205-211.

Strawberry Snail and Girdled Snail

Continuing today with the snail theme due to the wet weather. My 5 yr old daughter has a keen eye for little bugs, and today, all on her own found these two snails on a front garden in my street. Two species that we haven't found in our garden.

This is the Strawberry Snail, Trichia striolata a relatively small snail (about 10 mm wide), with a dull, finely sculptured round shell and dark body. The shell colour is variable, but this is the most common in my area. The shell has got an 'umbilicum' underneath, an open space between the shell worls. It genus name refers to the fact that the snail is hairy when young.

This is a similarly sized snail, the Girdled Snail. This angle shows the white keel that makes them easy to recognise. I have written about them before.

The courtship of garden snails

While having a cup of coffee in the conservatory during breakfast, I noticed a pair of snails, one of them with its genital pore a bit everted. Aroused snails! It was 10:09. I quickly finished my coffee and came closer. Despite coming across tens (hundreds?) of slumbering mating snails for years, I had never witnessed the actual preliminaries of it, which was now taking place just behind the conservatory glass, on its wooden frame. The series of events was fascinating, by its slow, but surely developing tempo, its tactile nature, its synchrony, and the dance-like quality of it. First the snails approached each other, with their heads up, appearing to avoid exposing their gonopore (an opening behind the right hand side of their heads from which their penises emerge and where their vaginas open) to their partner, while at the same time touching each other's gonopores with their mouths in long, slimy kisses, and tentacles touching, occasionally retracting their heads. One of the snails appeared more keen, while the other regularly turned round and then back, each time coming closer to its partner.
Then courtship became more intense, both snails everting their penises and aligned their bodies toward each other, exposing their right sides to each other. Finally, after a couple of hours, they were sleeping in their copulating embrace, and their eyes retracted. I missed the darting and I couldn't see any protruding darts. Here is a slide show.

Every now and then, I checked on the snails. They slept on the frame of the conservatory most of the day (at 16:45 they were still mating). I just checked (21:40), and only one remains. The duration of mating in this species is about 7 hours.
I have uploaded a video with an early and a later sequecing of the courtship.

No matter how long you've been acquainted with a particular species, there is always something surprising and wonderful you might still witness.

Garden Snail Parade

Since I started this blog, I have been surprised by the diversity of land snails about in the city. As I have managed to get most of them on white background, I decided to write a post and display them all together.

Moss snail, Lauria cylindracea, a tiny, easily overlooked species which gives birth to live offspring. 

Garden Snails, Helix aspersa (=Cantareus aspersus), unashamedly mate in the middle of your garden  path throwing darts at each other. 

Girdled Snails, Hygromia cinctella, like walls. An introduced species since 1950, still expanding across Europe from the Mediterranean.

Kentish snail, Monacha cantiana prefers drier places. This species, introduced in the UK during Roman times, is a very common snail in my local Wildlife Garden. They can be darker with pale speckling.

Glass Snails, Oxychilus draparnaudi, are carnivorous snails that have caused havoc on native snails when introduced outside the UK.

Amber Snails, Succinea putris usually live in very damp places, but can be also found away from water. They cannot completely retract their bodies inside their shells.

Brown Lipped Snails, Cepaea nemoralis are very polymorphic in colour and pattern.You might be lucky to have these beauties in the garden.  

Have you noticed all the snails are facing to the right? This is because most (90%) of all snail species have right-handed shells (dextral). Occasionally a left-handed individuals appear in populations of right handed snails. These face a problem when trying to mate, as the genital opening will face away from most other snails in the population. The genetics of shell handedness has been elucidated in some species and appears to be determined by mutations in a single gene, but the left-handedness trait is expressed not in the mutant, but in the resulting offspring.

More information

Visit the Molluscs posts in Bugblog.

Terrestrial Mollusc Tool. A wonderful resource for US molluscs. Contains also lots of info on introduced European slugs and snails.

Another girdled snail

I spotted this Girdled Snail, Hygromia cinctella, the second ever I have seen, climbing up a wall in my street a couple of days ago. The white whashed wall made a white background for it and I was pleased to get to photograph the live snail as it went about its business.

Two large slugs

The current dry weather means I am not coming across as many terrestrial molluscs as usual. While moving some pots I came across these two beauties, the Yellow slug (Limacus flavus, top) and the Irish Yellow Slug (Limax maculatus). After a while they relaxed on a wet white bowl and let me take their portraits before being returned to the plastic pot pile. Until recently, they were considered the same species, but these two look very different, not only in coloration, but in body shape, despite being a similar length. Both are very common out and inside my house, and are active at night.
I found this useful slug key in the field studies website.

Silent aliens

Have a look at these:

Harlequin Ladybird

Lily Beetle

Western Conifer Bug

They are invasive insect species in the UK. They are also so bright, or large, that they are hard to miss. You'll come across them even if you don't look. Many, many other alien invaders are harder to notice, they arrived, and before anybody other than specialist taxonomists noticed, they have spread across sizeable portions of the country. Many such silent invasions involve invertebrates that are small and or hard to identify.
  Clutches of snail eggs in the soil of pot plants, or dormant adults in cracks in stones or other cargo are often sent as inadvertent stowaways across countries thousands of miles away. They arrive in the destination, thrive and begin a quiet invasion. As many snails are tiny, and to the untrained eye they look identical to other snails, they are transported about very often. A sizeable fraction of the snails species of Central Europe is now thought to be of alien origin (about 15%), most of Mediterranean origin and the trend is of a recent increase, aided by increasing temperatures and commerce. In some extreme cases the ability of the snails to disperse is extremely limited, or their requirements for specific substrata so high that they scarcely move after introductions. Such is the case of Papillaria papillaris, a minute snail native of Italy, Sicily and Malta. These snails were likely to have been introduced in the UK in the 18th century with Italian ornaments or stonework, then prized by affluent stately home owners. Just two populations (in Brownsea Island Castle and Cliveden House) have been found so far. Amazingly, they have barely moved a few meters in this time, although they form self-sustaining colonies. Likewise, in Spain, this snail is restricted to walls and ruins from the Roman period, two millennia living practically where they were placed!
 Of course, there are many examples of the other extreme of the spectrum. Species that quickly spread on arrival, or after a lag period. An example is the Girdled Snail, Hygromia cinctella, a snail of Mediterranean origin which now is rapidly expanding in the UK. It was first noticed in the South in 1950, and it has now reached Glasgow, that is fast - for a snail. There have been suggestions that the snail might travel as stowaways in cars! Although this might seem outrageous, snails' habit of climbing up vertical surfaces and attaching itself firmly for aestivation or overwintering might facilitate this and it has been documented that this behaviour increases transport by cars. The girdled snail is 1 cm across and triangular when looked at from the side, with a pronounced keel that sports a pale line. At a distance looks like a juvenile garden snail. My daughter, however, noticed this snail on the pavement in my street a few days ago and it was necessary to rescue it from being crushed by passers by. I only noticed it wasn't a garden snail when I picked it up, after it had retreated into its shell.  I will keep a close eye and see if it has already arrived in my garden.

Side view showing the keel and pale edge.

Underside showing the lack of umbilicus

More information

Burçin Aşkım Gümüş and Henk K. Mienis (2010) Records of Papillifera papillaris affinis in continental Spain and their connection with walls and ruins from the Roman period. The Archaeo+Malacology Group Newsletter, 18: 1-4. here.

Janet Ridout-Sharpe (2010) Papillifera papillaris: a second colony is discovered in England. The Archaeo+Malacology Group Newsletter, 18: 1-6. here.

Alena Peltanova, Adam Petrusek, Petr Kment, Lucie Jurˇicˇkova (2011). A fast snail’s pace: colonization of Central Europe by Mediterranean gastropods Biological Invasions : 10.1007/s10530-011-0121

Sexual roles and gender conflict in pond snails

I caught these pond snails mating in my indoor tank today. The bluish, tongue-like structure between both snails is the penis of the snail on the background, entering the other snail's mantle cavity. So, that snails is acting as a male, (the one with a walrus-like head resting on its partner's shell) and the other as a female. Pond snails are simultaneous hermaphrodites, all individuals have functional ovaries and testis. The snails can self-fertilise, but if sperm from other individual is present, they prefer to use it. Although reciprocal sperm exchange might appear as a logical reproductive strategy, each individual snail might benefit more from acting as a male or as a female in a particular encounter with a potential partner. For example, large snails are able to invest in energetically demanding eggs, and might prefer to act as a female. Individuals acting as females are limited in the frequency at which they produce eggs, so they might prefer not to mate as often. Male reproductive efficiency might depend more on how many sexual encounters he has had, so snails preferring to be males might want to mate more often.

Sexual encounters of pond snails can result in just one of them acting as a male and the other like a female, or in subsequent reciprocation.  But, the snails could disagree, what if both mating partner insist on providing, but not receive sperm? This is called gender conflict, and the resolution depends on one of the individuals compromising and adopting its less preferred role, at least initially.

Petra Hermann and collaborators studied the effect of age on sex role preferences in the great pond snails, Limnaea stagnalis, disentangling it from the effect of size.  They reared three batches of snails that had known ages at the time of the experiments (young, Y, middle age M, and senior, S) and matched them by size.

First, they looked at the effect of age on mating interactions in age-matched couples. The chances of copulation decreased strongly with age, with young snails copulating with much higher frequency (80%) than senior snails (30%). Encounters between young snails normally ended in a reciprocal intromission (the individual acting as a female initially, then acting as a male), middle aged and senior snails, in contrast, had mostly unilateral sperm exchanges.

 Then they set encounters between snails of different ages. When pairing young and middle aged snails, also size-matched, about 25% ended in no copulation, and unilateral encounters were more common than reciprocal. The primary role in the copulation (the snail acting as a male first) was adopted by the young snail in about 80% of the encounters, indicating that the male role is the coveted one for the young snail, and that the fact that reciprocation is rare indicates that the middle ages snail is content acting as a female in unilateral encounters. There is an age related shift in sexual role preferences from male to female.

Fig. 6. Sexual interactions between young and middle-aged Lymnaea.

(A) Most couples of a young and a middle-aged snail (YM) performed a

copulation. The majority of these interactions were unilateral, i.e. the snails

do not reverse roles after finishing the first copulation. Note that in the

cases in which the middle-aged animals acted as primary male all young

partners reciprocated. By contrast, role reversal occurred in only a small

minority (18%) in the couples in which the middle-aged animals acted as

female. (B) Younger snails act significantly more often as primary male than

their middle-aged partner. (modified from Hermann et al 2009)

They concluded:

Animals in the early phase of the species’ life cycle tend to assure that they act as male (either primary or secondary), independent of the age of their partner (Fig. 7A). Senior animals, by contrast, tend to act preferentially as females. The sexual behaviour of middle-aged animals depends on the age of their partner. Combined with similarly aged partners they either act as male or female. Combined with a younger partner they tend to act as female only. Combined with an older partner, they tend to execute both gender roles and will act as males with a similar probability as younger animals

Interestingly, the snails resolved conflict by engaging in reciprocation. When both agreed on a role the encounters tend to be unidirectional. Note that as these snails store sperm and the partners were not virgins, the lack of interest in copulation of the older snails might stem from the fact that they might have accumulated enough sperm to fertilise their eggs. So although the authors removed the confounding effect of age from their experiments, they failed to account for the effect of sperm storage.

 The snails' efforts are resulting on these lovely egg masses appearing attached on the walls of the tank.

More information

Hermann PM, Genereux B, & Wildering WC (2009). Evidence for age-dependent mating strategies in the simultaneous hermaphrodite snail, Lymnaea stagnalis (L.). The Journal of experimental biology, 212 (19), 3164-73 PMID: 19749110

A little red underwater planet

 I have brought a few animals from my half barrel (=mini pond) to see close up to a small tank. Most of them are detritus feeders or vegetarians, and regularly I top up the tank with fallen leaves. Today's water was stained red and the reflection of the light in the water and the leaves made the photos take a surreal tint. A Great Pond Snail (Lymnaea palustris) fed on the algae growing on the tank walls (above).

Another snail, Planorbis planorbis

A group of ostracods, small bivalved crustaceans, and a copepod towards the left of the image.

There are also a few water slaters and planarians, but they didn't show as well today.

UPDATE: Thank you to Richard Comont for the pond snail ID, now corrected.

More information
Online aquatic snail identification guide from the Conchological Society. here.

The Great Ramshorn Snail

There is still a lot of activity in ponds. We haven't had a frost yet and Pond Snails, water-lice and ponds skaters go about their lives as usual. This is a Great Ramshorn Snail, Planorbarius corneus, inhabitant of still or slow moving waters with a high calcium content and plenty of vegetation. Unlike land snails, Ramshorn Snail's eyes are at the base of their long tentacles. They graze algae and plants or feed on dead leaves. They are hermaphrodite, although they prefer to cross-fertilise one another. They will lay their gelatinous egg batches on the underside of leaves. I photographed it on the white bowl with some rain water.

How to get a snail to go aaaaah!

I have spent quite a lot of time lately tidying up my photo library and tagging photos and I have come across several worth posting about even if they won't be as timely as usual. I wanted to see if I could take some shots of snail's mouths. To do this, I mashed up a few dandelion leaves and painted an area of the outside of one of the conservatory window with the resulting concoction. Then I found a few active snails and this one delivered. It started climbing up the window, its foot showing the muscular ripples that power its slow advance, mouth shut tight.

As soon as the snail felt the dandelion mixture, it opened its mouth and started licking it, showing its tongue (the radula) and the chininous, dark hardened ridge in front of it. The radula is a muscular organ covered in rows of hard little teeth that can scrape surfaces and it was very evident how the snail used it to eat the bits of the dandelion leaves once it got to them.

Dandelion leaves, yum!

These are the marks left by snails or slugs while grazing algae growing on a pot

The snail also showed nicely the opening of its lung, on the right side of its body.

A bit of slug romance

As their snail relatives, slugs are hermaphrodites, each individual producing eggs and sperm. Although some species regularly self-fertilise, in many others, individuals  - given the chance - will trade sperm with other individuals they encounter. I came across this pair of courting slugs under a fallen apple an evening last week. They are Deroceras panormitanum (thanks to Fauna, from WAB for the ID). At the right time of the year, and when meeting a potential partner, these slugs crawl in circles, producing and eating chemicals mixed in their respective mucus trails, before mating happens. The outcomes of courtship and mating are diverse: the slugs are able to mate repeatedly and use sperm from several donors to fertilise their eggs, or they might decide not to give sperm to its partner, or digest the sperm obtained and self-fertilise. This means that, despite being hermaphrodite, a slug can decide if to act only as male (digesting the partners sperm but providing its own), as female (refusing to give sperm, maybe preferring to save it for a better partner) or as both. This can create conflict between the partners - each with their own interests - and has been hypothesized to spur an "arms race" - or "genitals race" if you wish, and has led to the evolution of a range of bizarre copulatory structures and complex mating courtships.
 Heike Reise has reviewed and described the behaviour of these diverse genus of slugs and summarized courtship and mating as follows:

(i) Precourtship phase: the partners encounter and investigate each other.

(ii) Courtship phase: both partners have their sarcobelum protruded from the genital opening and assume a position with their genital pores facing each other, forming a circle or yin-yang configuration. 

(iii) Copulation phase: the slugs evert their penes, entwine them, and mutually transfer the ejaculates from penis to penis (there is no intromission). 

(iv) Withdrawal phase: the penes are retracted together with the attached sperm masses.

After coming across a potential partner or trail, some behaviour follows. This species has a flattened tail, which is enlarged during courtship and waved from side to side. These movements increase the chances of contact with the partner's tentacles. If the leading individual is ready to mate, it eventually turns round and starts following the second individual, forming a circle. Both individuals then evert their sarcobellums - as seen in the top shot - a solid, mobile structure in the genitalia which produces chemicals, that are exchanged during their circling behaviour. The sarcobelum is very active during the courtship, tracking closely the partner tail end and stroking it, at the same time that smears the partner with chemical secretions. In this species, courtship can last over an hour, followed by a relatively quick copulation (a few minutes), in which the slugs genital openings become very close, actual penises - bluish, transparent masses - are everted and entwined with the partners and sperm are exchanged externally. These slugs also have a penial, multifingered gland, that they also evert after insemination and lay over their partner's back, transferring some secretions. The function of this gland has been suggested to be equivalent to the snails' dart, the smeared chemicals on the partner might increase the chances of paternity. The slugs above were in the courtship stage, but unfortunately, by the time I went out again to check on them, they had disappeared.

References
Heike Reise (2007). A review of mating behavior in slugs of the genus Deroceras (Pulmonata: Agriolimacidae) American Malacological Bulletin, 23, 137-156
Benke M, Reise H, Montagne-Wajer K, & Koene JM (2010). Cutaneous application of an accessory-gland secretion after sperm exchange in a terrestrial slug (Mollusca: Pulmonata). Zoology (Jena, Germany), 113 (2), 118-24 PMID: 20202803

An impressive slug

A showery day, I come across this enormous Arion slug crossing the garden path. It must be close to 15 cm. Identification to species level is difficult to impossible as they are distinguishable by examination of internal genitalia characters - or molecular genetic analysis, and hybrids are common. British large Arion slugs are made of several species complexes, Arion ater/rufus A. lusitanicus and A. flagellus. Both groups are very polymorphic slugs, from almost white to black, including all shades of brown yellow red and orange. I have never found the the black morph in the garden, but a good representation of the others abound. The polymorphism of these slugs in the U.K., not only in colour, but also in genital characters, might stem from common hybridisation between several subspecies that colonised the British Isles from Europe, postglacially but also in more recent human introductions. Distribution changes might also come about due to expansions as a result of climate change. In addition, the slugs - except for A. flagellus - are able to self-fertilise and form relatively homogeneous populations, which might give the impression of being a separate species.

 Arion slugs had deep tubercles on their backs, and no keel. At the end tip is the mucus gland, producing an extremely sticky mucus. Unlike the yellow slug, which is strictly nocturnal, Arion slugs are active all around the day, provided that it is damp. When disturbed they show a characteristic behaviour: they contract their bodies into an almost round shape and retract their tentacles inside their mantle. Inspired by a post in Myrmecos, I set the photo vertically. I had to use a white plate - instead of bowl - and allow the slug to relax and start to crawl out. 

A very yellow specimen

A disturbed Arion

A selection of colours found in the garden

The black morph of Arion sp.

References

Noble, L. R. Jones, C. S. (1996). A molecular and ecological investigation of the large arionid slugs of North-West Europe: the potential for new pests The ecology of agricultural pests: biochemical approaches Ed. William O. C. Symondson, Systematics Association, special volume. Clarendon Press., 53, 93-132
Evans, N. (1986). An investigation of the status of the terrestrial slugs Arion ater ater (L.) and Arion ater rufus (L.) (Mollusca, Gastropoda, Pulmonata) in Britain Zoologica Scripta, 15 (4), 313-322 DOI: 10.1111/j.1463-6409.1986.tb00232.x
UPDATE 30/08/11: reference added and Arion ater in a photo caption changed to Arion sp.

Brown-Lipped Snails

The Brown-lipped or Grove snail, Cepaea nemoralis has received a lot of attention by evolutionary biologists for more than a century, due to their strikingly variable shell colour - what is called colour polymorphism. In the decades of the middle of the last century it was a very popular research organism. The shiny shell can be yellow, pink or brown. Over each of these background colours there can be no bands, one band or five bands, and the bands can also be fused and be of variable width. The snail above, which we found yesterday feeding on the fallen leaves on the garden path, is a yellow/one banded one. This polymorphism happens within the same population, but what puzzled biologists was the occurrence of sharp changes in the frequency of colour forms from one population to the next, and these differences seem to persist with time. This phenomenon was called "area effects". Many explanations have been proposed through the years to explain how the polymorphism is maintained and how area effects come to be, from differential predation (especially by song thrushes), adaptation to microhabitat, or other forms of selection to chance effects due to colonization after the glaciations, genetic linkage, dispersal between populations, etc. Many of these factors are not mutually exclusive and seem to have different importance depending on the population.
 We found the shells below in the beach in Spurn Head a few years ago, all in a small area. They are a bit bleached by the sun, but you can see yellow and pink snails and three types of banding patterns.

The Brown Lipped snail can be found from dunes to roadsides, gardens and closed woodland. It can live up to 8 years old. They prefer to feed on dead vegetation than fresh, and on average, only 9% of its diet is fresh vegetation, although this percentage can increase during dry spells.

References
Cain AJ, & Sheppard PM (1954). Natural Selection in Cepaea. Genetics, 39 (1), 89-116 PMID: 17247470
Davison, A., & Clarke, B. (2000). History or current selection? A molecular analysis of 'area effects' in the land snail Cepaea nemoralis Proceedings of the Royal Society B: Biological Sciences, 267 (1451), 1399-1405 DOI: 10.1098/rspb.2000.1156
Paul J. Mensink & Hugh A. L. Henry (2011). Rain event influence short-term feeding preferences in the snail Cepaea nemoralis Journal of Molluscan Studies. DOI: 10.1093/mollus/eyr011

Amber Snail Puzzle

While removing an old pot containing a lot of grass and a dead Agapanthus, next to a rainwater filled pot, I stumbled upon this little snail. I was quite surprised as initially, I thought it was a pond snail, but closer inspection revealed the tell-tale eyes-on-top-of-tentacles characteristic of land snails and slugs, while aquatic snails have their eyes at the base of their tentacles. After sifting through a Molluscs guide I found out it was a Common Amber Snail, Succinea putris. Although not aquatic, it usually lives near water or in waterlogged habitats, and it is often found on the stems of aquatic plants. It cannot retract its body completely inside the shell, and the lower pair of tentacles is vestigial. I have no idea how this snail got into our garden, but snails, despite being slow and strongly dependent on humidity, are known to disperse widely. In the Origin of the Species, Charles Darwin believed birds were the most common long range dispersal agents of snails and other aquatic animals and plants. In 1893, Harry Wallis Kew reviewed the dispersal of land and water molluscs, and discusses the evidence for external transport on the feathers of birds:

Sir C. Lyell, remarking on the wide range of Succinea putris, a land-shell which inhabits moist places on the borders of pools and streams suggested that water-fowl might have distributed its ova entangled among their feathers and it seems quite likely that ova of certain terrestrial kinds may be occasionally thus carried, either in the feathers or on the feet of birds; indeed, we have a near approach to proof of such transportal, the Rev. Canon Tristram, as we have seen, having once found ova, believed to be those of a Succinea, upon one of the feet of a mallard shot by him, on the wing, in the desert of Sahara. It is doubtful, however, whether Succinea, from the nature of the localities they often or usually inhabit, ought not, for the present purpose, to be classed with fresh-water, rather than with land-shells. Mr. Darwin suggested that the just-hatched young, possibly, might sometimes crawl upon the feet of ground-roosting birds, and thus get transported; and it certainly seems in the highest degree probable that such is the case, but, as far as I know, no observations in support of such a supposition have yet been made.

A tantalizing possibility, also first put by Darwin, is that of internal transport of organisms in the digestive tract of birds. Many bird species feed on snails, and given that no gastric juices occur in bird's crop, they can potentially survive for a while and maybe be discharged later elsewhere by the bird, or regurgitated by raptor if the bird falls prey to it. Kew stated:

 Twenty specimens of a Succinea, peculiarly packed together, and four of Pupa viuscornvi were once found by Mr. W. H. Dikes in the crop of a bearded titmouse (Parus biarmicus); all the shells, it is said, were uninjured, but it is not stated that any were observed lo be alive.

In 1968, Biggs reported on the recovery of a living Succinea putris from a pigeon's crop at least 8 h after the bird had died. Indeed, although it seems even more unlikely, some snails can survive passage through the whole digestive tract of birds provided the shell is more or less unbroken. This has recently been shown to happen to a small estuarine snail, Hydrobia ulvae, which can survive passage through the digestive tract of Shelducks, and also in some small Japanese terrestrial snails, Tornatellides boeningi a proportion of which were recovered alive in the feces of two species of terrestrial bird they had been fed to.
I don't think any of these forms of dispersal apply to the particular little snail in the above photo. Maybe it travelled on a pot plant we bought some time back in a garden centre, or, stuck to our clothes or shoes during an outing into some wetlands. Maybe, but just that any of the forms of transport Darwin and Kew discussed actually happen to snails shows you don't need wings to fly high.

References
Biggs, H. E. J. (1968). Succinea putris (L.) in a pigeon's crop Conchologist Newsletter, 24: 36.

Gerhard Cadée (2011). Hydrobia as "Jonah in the whale": shell repair after passing through the digestive tract of shelducks alive. Palaios, 26 (4), 245-249 DOI: 10.2110/palo.2010.p10-095r

Darwin, C.R. (1959) On the Origin of Species. Read the book here.
Kew, H.W. (1893) The dispersal of shells: an inquiry into the means of dispersal possessed by fresh-water and land Mollusca. K. Paul, Trench, Trübner & Co., Ltd. Read the book here.
Shinichiro Wada, Kazuto Kawakami and Satoshi Chiba (2011). Snails can survive passage through a bird's digestive system. Journal of Biogeography : doi:10.1111/j.1365-2699.2011.02559.x

A blue glass snail

When lifting a pot to plant out today, I found this Glass Snail. There are four British species, and I think this is Oxychilus draparnaudi, given its relatively large size (a mighty 15 mm). Glass snails are named after their shiny, fragile and translucent shells when alive. After weathering the shells rapidly lose their lustre. Oxychilus shells are rather flat, which makes it easier for them to hide under pots or stones.

Given their small size - and their association with pots, nurseries and greenhouses - it is not surprising that several European Glass Snail species have been inadvertently introduced in temperate or subtropical regions around the world, including New Zealand, Australia and the U.S. and Canada. What perhaps is unexpected is that many Glass Snails, including O. draparnaudi are predators and their diet include other snails and slugs and their eggs, in addition to worms, and dead insects. They have even been reported to eat Garden Snails. When predating on snails, they attack the snail's body first with their radula and when they cannot access more flesh from the outside, they drill a hole on the shell to finish off their meal. The introduced populations, however, can be a threat for the native snail fauna and reductions or losses of local snail populations have been reported in areas with these predatory snails when compared with patches where they were absent in the U.S and New Zealand.

References
Barker, G.M. (2004). Natural enemies of terrestrial molluscs. CABI. DOI: 10.1079/9780851993195.0279. Here.
Karin Mahlfeld, Karin. Impact of introduced gastropods on molluscan communities, northern North Island. here.