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.

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.

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.

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

A handsome snail

After a light shower yesterday, garden snails came out of their shells and went on a walkabout in search for food. Snails get some bad press in gardening circles, but most of my garden snails are feeding on the ground, on dandelion leaves or fallen vegetal material - OK, I am an untidy gardener now you know. I found this beautiful individual and thought I would take its portrait. It behaved quite well on a white plate and here you have it, on its best side.

Tiny snails on cockle shells

My daughter found some tiny snails under a cockle shell in a pot yesterday. I took some photos but I could not identify them so today I went to find them again and try and take some shots of the more informative mouth of the shell. There were five or six of them, that were now active after last night's rain.

I looked in the pot and I found dozens of them. I had never seen them before and we had a whole pot population of these 2 mm or so snails happily living in the garden! Fauna, from WAB, identified my photos of the shell mouth today as Lauria cylindracea, a very common and widespread snail in the U.K. A favourite habitat is on stone walls covered on Ivy, but also lives in moss, leaf litter and crevices of old trees, habitats not particularly wet. It feeds on fungus growing on the dead leaves.
 As many snails, individuals of L. cyclindracea are hermaphrodites, which mate with each other to fertilize their eggs. This species, however, is relatively unusual for a snail in that it is ovoviviparous. It retains its eggs - 4 to 6 -inside its body until they hatch, and they are born into miniature snails, even tinier than their parents. Some of these juveniles were also present under the shell near the adults. The snails reach maturity at two years old and can live over 5 years, reaching a maximum size of a mighty 3.5 mm.

 In their study of life history and reproduction of L. cylindracea in Israel, Heller and collaborators discussed what evolutionary forces might have led to the evolution of ovoviviparity in this species. They hypothesize that a combination of small size - which limits the total number and the size of the eggs that can be produced - and high egg mortality, for example due to drought or lack of appropriate egg laying sites or inability to bury the eggs, are the key selective pressures. That way, offspring are born right next to their parents at the conditions most suitable for survival. The snails can also determine up to some point when the birth happen, retaining the juveniles longer when the conditions are very dry, and quickly giving birth when favourable moisture conditions return, that way maximising the chances of their offsprings survival. Heller and collaborators state:

Active hatchlings are advantageous, as compared to eggs, in that they can immediately start to feed, grow, fight off fungi, cope with brief periods of desiccation by moving into deeper layers of litter or temporarily retreating into their shell, and avoid drowning (or being washed away) as a result of occasional flooding of the riparian litter habitat, by moving into higher layers.

They compared this reproductive strategy in minute snails with the situation in larger species:

Large snails can produce eggs in vast numbers. Further, lack of size constraints enables the large adult to lay large eggs, generously coated with thick layers of albumin and mucus to tide the developing embryos over varying periods of drought and hunger. In Helix texfa, a species of 40 mm and 4.3 g (found in Israel very close to our present study site), each single egg averages 5.5 mm in size and 70 mg in wet weight (bigger and heavier than an entire adult Lauria), an adult produces 60 eggs per clutch, and clutch weight represents about 10% of body weight (Heller & Ittiel, 1990). In large snails with large clutches also sib cannibalism may occur, thereby increasing survival chances of the first individual to hatch (Baur, 1994). Further, large oviparous snails frequently deposit their eggs into pockets in moist soil. A deep cavity lined with mucus can protect the eggs against bacteria and fungi (Baur, 1994); it also enables the deposition of eggs in soil layers that remain damp for longer periods of time, and thereby reduces risks of desiccation. The cavity is dug out by the snail’s foot, so its depth is broadly correlated to foot size. Helix can dig 5cm into moist soil and Arclzachatina can dig a 15 cm cavity in one night (Tompa, 1984; Baur, 1994). Minute snails, however, do not have the potential to dig much beyond 0.2cm (except for subterranean genera, see Heller, Pimstein & Vaginsky, 1991). They can either dig minute cavities, or deposit their eggs in damp litter, where they would be exposed to fungi. Ovoviviparity may thus be a mode of parental care for snails that can neither lay many eggs, nor big eggs, nor dig deep egg cavities.

I will keep looking for these snails to see how widespread they are in the garden. I wouldn't be surprised to have overlooked these fascinating creatures due to their minute size. Were not for the white background of the shell where we found them we would have probably missed them altogether.

Reference
Heller, J., Sivan, N. & Hodgson, A. (1997). Reproductive biology and population dynamics of an ovoviviparous land snail (Pupillidae). Journal of Zoology, 243 (2), 263-280 DOI: 10.1111/j.1469-7998.1997.tb02781.x

Waking up

I came across this snail on Friday, the first active snail of the year. It was covered on cobwebs; a fragment of its epiphragm - a sturdy yellowish membrane made up of layers of mucus that the snail produces to close their shell opening during their winter dormancy period - still attached to its shell. I am not sure if this snail emerged spontaneously, as I am doing a lot of pruning in the garden these days, but even if I disturbed, it moved all the way down the wall before settling again.

Cameraless

I made the mistake of taking photos in a beach on a windy day (what was I thinking?!). You don't need to have much imagination to guess what happened. Some tiny grain of sand got in the camera and it stalled with the objective open, then, an ominous bleeping and a warning 'lens error! restart camera!'. After that, I did some frantic searches in Google to find a simple solution and failure to restart the camera after trying various tricks. The consequence, my Powershot G10 is being now sent to a repair centre, and while I wait, my dear old G6 was rescued from a cupboard and put to action. Now, when you get used to an upgrade you often do not notice the jump in quality, probably as you are often frustrated trying to get used to the new buttons and getting aquainted with you new toy. But going backwards, that's tough. I miss my G10 so much! I miss the image stabilization, I miss the large, bright LCD screen, the optical zoom, the top dial to adjust the aperture...

 However, I am so glad this happened at the beginning of november, when Bugblog is feeling a bit sleepy and cold, almost ready to hide under a curly leaf and go into hibernation. I would have been straight to the shops to get myself a G12 if this had happened in the summer. And still, some bugs still refuse to believe that, yes, it is cold and time to hide! While gardening this weekend, I was accompanied by the surprising buzzing of honeybees foraging on the fuschia, I disturbed a healthy female Tegenaria when moving a compost bag. I also came across this tiny snail, probably born only this summer, having a walkabout. Despite all of the above, when I saw the photo above I realised I can survive without my G10...at least for a few days.

Snail season starts

After a few dry sunny days, rain fell non-stop yesterday. A few Garden snails (Cornu aspersum = Helix aspersa) had woken up from their winter rest and were slithering around the garden. The start of snail season.

Sleeping Snails

A sunny day like a jewel in a dull, cold winter. For the first time in months I venture out to do some gardening. There are little signs of bug activity. A few 7-spot ladybirds sunbathing; a cloud of winter gnats; a bluebottle which rapidly gets into the house through the back door...I moved an old tile left leaning against a south-facing wall and found a gathering of overwintering snails stuck to the tile and the wall. A few small ones, but mostly of them grown up snails, shell against shell. In this spot they keep relatively warm and dry during the winter. The snail secretes a specially tough mucus to seal its shell opening which can glue it to walls, or other snails.

A dormant snail operculum.

More on Garden Snails at BugBlog:

Darts for love on the strange mating tactics of your garden Snails.

Darts for love

I must apologize as so far my posts have been solely about insects. I hope this one will change the trend. A couple of days ago I saw the first mating Garden Snails (Helix aspersa or Cornu aspersum) of the year. It's been quite a long, cold winter and a dry spring, so, garden plants have been enjoying quite a long snail-free season. Snails have now come out of their winter dens under stones, behind tiles and in general dry places around the garden and are meeting each other ready for reproduction. Garden snails are hermaphrodite, that is, they produce sperm and eggs, but they only rarely self-fertilize, instead they mate several times over spring and summer (in my garden I've seen copulating snails from early May until the end of August. Mating is a reciprocal long-winded affair, the pair of snails exchanging sperm simultaneously. An remarkable aspect of courtship in snails is that it involves the shooting of a relatively large, calcareous, thin and sharp 'love dart' from one snail to its partner before actual copulation. The purpose of this violent affair was poorly understood and debated for a long time. The dart is not a requisite for successful mating (apparently, snails often 'miss') and not all the individuals shoot or a physical stimulant for copulation, as previously assumed. It became apparent that snails shooting their darts effectively had higher success fathering the offspring of the other snail. Recently, research has shown that the dart should be instead called 'the needle': the dart allows mucus to be injected into the other snails head: this mucus contains a chemical that inhibits sperm digestion (apparently, without the dart most sperm is digested), and therefore allows the shooting snail to father more offspring. Therefore the dart is an evolutionary result of sperm competition.

Mating garden snails, the snails on the right has been darted.

Snail laying eggs

Snail eggs