Seasonal Update (July 2024)

Early starts during the summer, particularly after a clear night and in the vicinity of water, can produce some spectacular misty sunrises for the insomniacs among us. - Credit: Marc Baldwin

After an uninspiring start to June, most of southern and eastern England saw something approaching summer during the second half of last month, with temperatures back to where we'd expect them for the time of year, or even somewhat above given that some areas approached 30C (86F) in the penultimate week. Further north and west, however, low pressure continued to dominate throughout most of the month, and we saw a great deal of wet and windy weather.

News and discoveries

Contracting cetaceans. Wherever you sit with regards to the root cause of climate change, there can be little doubt that we're experiencing significant global changes in key factors such as temperature, precipitation, and ocean acidification. There's a growing pool of data pointing to how these changes are impacting the wildlife with which we share the planet, and a recent study adds grey whales (Eschrichtius robustus) to the list. The dataset, published in Global Change Biology last month, provides a statistical analysis of measurements from drone photos of 130 individual grey whales over a seven-year period between 2016 and 2022, which suggests a decline in body size of some 13% since 2000. Thirteen percent may not sound like much, but it means a whale born in 2020 reaches a maximum size 1.4 to two metres (4.6-6.6 ft.) shorter than it would've had it been born 20 years earlier. The researchers are concerned about whether being smaller makes the whales more susceptible to entanglement in netting and being hit by ships and if it impacts their reproductive potential.

Beavers are well known to shape the habitats in which they live, and new research suggests that the new growth they help stimulate is likely to influence local deer populations. - Credit: Marc Baldwin

Chastening cichlids. In the immortal words of Pink Floyd's Another Brick in the Wall, “If you don't eat your meat, you can't have any pudding.” The use of punishment to elicit desired behaviour is a well-known ploy of parents, governments, and employers alike, and we have data to suggest a similar situation is present in some quarters of the natural world. Back in April, researchers at Osaka Metropolitan University published a paper in Animal Behaviour describing how dominant breeders of the cooperative breeding cichlid fish Neolamprologus savoryi attacked idle helpers, including their own offspring. This encouraged helpers both to take a more active roles, such as territorial defence, and made them more likely to spontaneously participate in future, seemingly to avoid disciplinary attacks.

Beaver boost for deer. No species operates in isolation; their activities almost invariably touch other species within the ecosystem in which they live. Beavers are a particularly prominent example of this, as these ecosystem engineers radically change their environment from woodland to wetland. These alterations affect water passage across the land and, in turn, the presence and distribution of other species. We know quite a bit about how the presence of beaver populations impacts plant communities, but we're now starting to get a picture of how other animals might benefit. Kelsey Wilson at Stirling University and colleagues surveyed 20 plots in the Tay and Forth river catchments in central eastern Scotland and, in a paper published in Forest Ecology and Management last month, report that 62% of birch trees felled by beavers at the sites put out secondary shoots, and that at least half were accessible to deer (i.e., less than 1.5 metres above the ground). Not only did beaver-felled trees produce nearly one-fifth more shoots than standing trees, but the shoots also had a 13% higher nitrogen content. The authors suggest the response of deer to this increased food resource should be studied to understand how deer are attracted to and utilise this resource.

Seasonal highlight – slugs and snails

I appreciate that it's tough to get excited about slugs and snails. I suspect that, for most of us, slugs and snails elicit feeling of indifference at best, repugnance or pugnacity, the latter particularly among proud gardeners, at worst. For decades we have gone out of our way to find solutions to our 'slug problem' and the result has been a variety of pesticides and home remedies, many of dubious worth. Are slugs really all that bad, though, and do they do they deserve to be poisoned, stepped on, and launched over the garden fence of an evening? I can't promise that this brief exploration into the world of one of our most iconic garden nemeses will turn you into a 'slug-hugger', but I do hope that it gives you a newfound perspective on them and the role they play in our ecosystems.

Slug or snail?

Slugs and snails are invertebrates—that is, they lack a true backbone—and sit within the taxonomic phylum Mollusca alongside octopuses, cuttlefish, mussels, conches, clams, oysters, and so forth. The Mollusca is a huge group that contains about 85,000 species, 70-80% of which are grouped together within the class Gastropoda; these are the slugs and snails. The word “gastropod” literally translates to 'stomach foot' and alludes to the fact that the underside of these animals is a sheet of muscle that can be contracted and relaxed to pull them along. The low-level classification of the gastropods is rather complex, and we are far from a universally accepted scheme. I won't dwell on the taxonomy because it involves a lot of terms, such as “un-ranked clade” and “informal group”, that rapidly bore non-taxonomists, but the important point here is really their startling diversity. Some estimates suggest that there may be as many as 80,000 species of slugs and snails worldwide, and we currently know of at least 30,000 terrestrial (land-based) species. Within the UK, there are some 90 species of gastropod, about 20 of which we tend to find in our gardens.

In most cases, slugs and snails are relatively easy to separate owing to the presence of a shell in the latter, as in this banded garden snail. - Credit: Marc Baldwin

So, what is a slug or snail? Well, when it comes to trying to describe these animals, “small and slimy” will probably do for many people but, more comprehensively, slugs and snails have a fairly similar body plan. The body is generally small (there are some truly impressive slugs and snails to be found globally, but in the UK the largest gastropod is the great grey, or leopard, slug which grows to a maximum length of 20 cm/8 in.) and cigar-shaped, tapering to a rudimentary 'tail' at the rear and with a clearly discernible head at the front. Generally speaking, snails possess a shell into which they can recoil their body when threatened, or to conserve water, while slugs do not. There are, naturally, exceptions to this 'rule' and some slug species carry small shells, while some snail species have shells that can only really be considered vestigial. Furthermore, many slugs grow a small shell-like structure under the skin on their back which is used for the storage of calcium salts. On the back of the slug, just behind the head, is a thick fold of skin called a mantle; within this may be the subcutaneous shell but, more importantly, this marks the location of the 'lung', which is connected to the outside world by the small pore on its edge (usually on the right-hand side of the slug) called the pneumostome.

Slugs and snails typically possess two sets of antennae, more technically referred to as ommatophores, located on the head. The upper pair terminate in light-sensitive 'eye spots' and serve to distend the eyes from the body. Having eyes essentially out on independently adjustable stalks may allow the snail a better field of view, but it may also play a role in helping the animal's visual acuity. Slug and snail eyes possess a simple lens, but their eyes lack ciliary muscles, meaning that they cannot focus. Consequently, moving the eye back and forth on the stalk may allow the snail to focus on objects within their environment, without needing to physically move their body. The antennae are moved by a combination of hydraulic and muscular inputs; the snail pumps fluid (called haemolymph) into the antennae to extend it and uses a specialised retractor muscle, attached to the eyeball, to pull it back. The lower pair of antennae are shorter and highly sensitive to odours; there is some evidence that pheromones are used to locate potential mates and, when snails breed, they touch each other with these lower ommatophores. So, in essence, snails see with the top set of antennae and smell (and probably also touch/taste) with the bottom set. If the antennae are damaged or lost, they apparently grow back.

The protoconch, or embryonic shell, of Haliotis asinina, a tropical sea snail. - Credit: Jackson, Wörheide & Degna (WiliMedia - CC-BY-2.0)

Most snails possess a shell, the foundation of which they hatch with. This 'protoconch' is a small, translucent and very soft structure attached to the newly hatched snail and must be strengthened with calcium salts, which the snail initially obtains by eating the egg membrane. Snail shells are primarily composed of calcium carbonate (a form known as aragonite) and this is secreted from the mantle into a protein complex, called conchiolin, which acts like a frame. Now, calcium carbonate is a fairly rigid, hard material and offers the snail protection, but it is also susceptible to corrosion (or, more accurately, dissolution) from acids. Consequently, many land snails produce a thin 'organic skin', called the periostracum, and apply it to the outside of the shell as new material is added. The periostracum protects the shell from weathering and extends its useful life. As the snail grows, it adds to the (coiled) length and thickness of the shell, and the protoconch ends up in the middle of the spiral. Most snails grow dextral (i.e., clockwise spiralling) shells, although very infrequently (an estimated 1 in 30,000) sinistral (anticlockwise) spiralling develops.

Growing a shell allows snails to colonise a range of environments and travel potentially greater distances than slugs. Gastropods are very susceptible to dehydration, which means they must seek shelter during warm conditions. In the case of snails, this can simply mean retreating into their shell and sealing the entrance with a layer of mucus, forming a barrier to water loss, called an epiphragm. Slugs have no such luxury and are forced to find shelter in crevices, under rocks, etc. and wait for nightfall. Indeed, many slugs have excellent homing abilities and can find their way back to the same crevice each morning. As well as 'opting out' of the warmest conditions, some snails can reduce water loss by 'hopping' around to save mucus. Gastropod mucus is about 98% water and secreting it imposes significant demands on a slug or snail's water balance. (This, incidentally, is how slug pellets work. They stimulate excessive mucus production, which dehydrates the slug/snail and causes its death. This is also why slug pellets are less effective in wet conditions, where the gastropods are often able to rehydrate quickly enough from their surroundings to keep up with the loss generated by the pellets.) When a snail crawls, water is lost over the foot sole. Consequently, terrestrial snails can move by crawling on only parts of their foot, which minimises ground contact and thus water loss. This type of movement results in a broken slime trail.

Actual, physical contact!

I have mentioned that the name gastropod alludes to their mode of transport, but there's a bit more to it than just contracting this muscle to move. The sheet of muscle extends along the entirety of the slug or snail's underside that's in contact with the ground and a band of muscle along its length contracts from tail to head, pushing the animal forward. The alignment of the muscle fibres, however, means that slugs can only contract in this sequence; they cannot move backwards. The second aspect of gastropod motion is 'slime' and many of us have seen the dried slime trails left around our garden plants overnight. Not all mucus is created alike and, although the exact composition of the slime varies between species, it is generally the case that two types of mucus are created.

Residual slime trails left overnight by slugs and snails in our Hampshire garden. Slugs produce two types of mucus: a thin, watery secretion on which they glide, and a thicker glue-like slime that provides protection from predators and desiccation. - Credit: Marc Baldwin

A thin, watery mucus is secreted from a gland at the front of the foot and spreads out to the edges - it is upon this mucus that the slug moves. The thin mucus reduces friction and acts as a protective interface allowing the slug to move over rough and sharp objects without sustaining damage to the foot; it also contains fibres that provide some grip and prevent the slug falling off vertical surfaces. The second type of mucus is produced by glands on the slug's back and coats the whole animal—this much thicker mucus serves a dual function, reducing water loss and making the slug largely unpalatable to predators. Both types of mucus are hygroscopic (i.e., they absorb water), which is critical because as we have seen gastropods are very susceptible to desiccation, and contain a complex mixture of proteins, peptides, acids and metal ions, which appear also to have some antibacterial properties.

Slug and snail slime has what physicists refer to as non-Newtonian properties. In other words, it's essentially a solid glue-like substance at rest, but liquefies when a sufficient force (in this case, the slug moving forward) is applied to it. This explains why, in the garden last month, I was able to pick up a small (approx. 5 cm/2 in.) banded slug and with it came two pieces of flint over which it was travelling. Similarly, when attacked, a slug will contract its body under the mantle shell and the combination of a rigid body, thick back slime and glue-like thin mucus makes them very difficult for a potential predator to pick up. Gastropod slime is not, however, just a mechanism to speed up their progress (allowing some to reach a top speed of around 7 cm/3 in. per second), stop them dehydrating, or make them unpalatable. It may also serve a social and reproductive function. Observations of slugs and snails crossing the mucus paths of other individuals suggest that they can recognise trails produced by their own species and that this may help when it comes to finding a mate. Similarly, some slugs and snails prey on other gastropods and may use slime trails to help track down a meal. And finally, as we shall cover in more detail shortly, some species employ mucus to suspend themselves while mating.

Godzilla of the garden

Many of the avid gardeners I know believe that slugs and snails rampage through their gardens, destroying any vegetable matter in their path. When present in large numbers they can certainly cause their share of damage, but many gastropods have highly specialised diets. There's no doubt that, as a group, slugs and snails feed on a wide variety of vegetables and herbs, including flowers such as petunias, chrysanthemums, daises, lobelia, lilies, daffodils, narcissus, gentians, primroses, tuberous begonias, hollyhocks, irises, and fruits, with strawberries and apples being of particular interest to them. Gastropods will also feed on carrots, peas, cabbage and runner beans.

Slugs and snails can take their toll on gardeners, particularly when a mild, wet winter is followed by a cool and wet spring, because such conditions allow numbers to boom. - Credit: Marc Baldwin

Despite the long list of potential gastropod fodder, some plants are more appealing to slugs and snails than others, and are at greater risk at certain periods of their growth. In a paper published in 2014, for example, researchers at Leiden University in The Netherlands found that slugs could be a limiting factor in the survival of some strains of rapeseed; 26 times more seedlings of Brassica napus survived in enclosures where slugs were removed during the first 10 days after germination than in enclosures where no slug control was carried out. The biologists also noted that their slugs tended to avoid wild Brassica rapa, which appears to be because—compared with B. napus—this species of rapeseed contains higher levels of chemicals called aliphatic glucosinolates, which are precursors to the mustard oils that give plants such as mustard, cabbage, and horseradish their pungent taste. There are many home remedy slug control methods that involve providing alternative food (from cat food to rotten lettuce) to attract slugs away from your prized plants and these appear to be relatively successful at reducing slug damage.

As well as feeding on flowers, fruits, and vegetables, many slugs will also eat fungi, slime moulds and lichen, while others actively predate other invertebrates. The door snail (Clausila bidentata), for example, feeds almost exclusively on fungi and lichen, while the shelled slug (Testacella haliotidea) hunts earthworms below ground and is seldom seen at the surface, and pond snails feed almost exclusively on algae. Most slugs will feed on decaying matter (plants, fungi, carrion and even dung) and this makes them an important part of the nutrient recycling process within ecosystems. The microbes we commonly associate with the decomposition process find it hard work breaking down large lumps of tissue, but if you break it up into smaller pieces you increase its total surface area, and the microbes can decompose it faster. While feeding, gastropods do just this—they take large pieces of decaying matter and break them down into much smaller pieces, which are passed out in their faeces, and these smaller organic fragments are easier for microbes to liberate nutrients from.

Slugs and snails don't have teeth in the same way that mammals do and, as such, cannot chew their food. Instead, they possess a radula, which they use to scrape away at their food—the use of this radula explains why, when eating fungi, a groove/channel is created in the fungal cap as they feed. The radula is basically a cartilaginous tongue (called an odontophore) wrapped in flesh and covered in a membrane studded with rows of tiny chitinous teeth. Three muscles connected to the odontophore control its movement, allowing it to protrude and retract, and move from side to side, as the animal rasps at its food. As food is scraped up, it's swallowed and passes down the oesophagus and into the crop, which is a large pouch where food is ground into smaller pieces more suitable for digestion.

Slug love

Gastropods are hermaphrodites, which means they possess both male and female reproductive organs, both of which are accessed from just under the mantle. Contrary to popular misconception, the presence of both sex organs doesn't necessarily mean that these animals can self-fertilise. Indeed, although some species (the pond snail, for example) can self-fertilise, many slugs and snails experience a phenomenon called protandry. In other words, they start off being male and subsequently become female. So, in the case of the Roman snail, for example, the sperm cells develop more rapidly than the eggs; eggs are only fully developed once the sperm has been expelled and at this point the snail becomes female. Some slugs may go a step further and lose their penis after mating, through a behaviour known as apophallation. In apophallating species, the penis is curled and during mating it becomes entangled in the mate's genitalia. To separate themselves, one or both slugs chew off the other's penis; they're then able to breed using only their female apparatus.

A pair of European black slugs (Arion ater), which despite the name can be black, brown, or white, mating. The white blob in the middle is not, as is commonly assumed, sperm, but their reproductive organs. - Credit: Marc Baldwin

The process of mating in most of our garden slugs is fairly straightforward and involves positioning themselves side by side, facing in opposite directions, and aligning the right-hand sides of their bodies. One or both will extrude their penis and place it in the genital opening of the other. The genitalia are translucent white structures and are sometimes mistaken for the sperm itself. In some species, however, the mating ritual can be considerably more acrobatic. Reproduction in the great grey slug, or leopard slug, Limax maximus, for example, involves climbing a fence, tree trunk or wall and then lowering themselves down on a string of mucus. The slugs usually emerge at dusk on warm summer nights and set about looking for food (mainly fungi).

When two leopard slugs meet, they touch tentacles and then begin 'chasing' each other—this is actually more of a slow movement in circles, each following the other and 'nipping' at the rump of each other as they catch up. The pursuit can continue for up to an hour, apparently, and the area can be plastered in mucus. At some point, one decides enough is enough and breaks the circle, heading for high ground or, if already in a tree or on a fence, for a suitable overhanging branch. The second slug follows the first and when they reach a suitable spot they begin to wrap around each other, each exuding vast quantities of mucus as they intertwine. This twirling coils the mucus into a rope with a surprisingly high tensile strength and, before long, it is of sufficient length and thickness to support their combined weight. The couple now descend this 'mucus rope' like a pair of acrobats; at the bottom of the descent, hanging a few centimetres from their anchor point, copulation begins. The penis of each slug is situated on the side of the head and soon engorges into a large fan shape that's clearly visible below each of the writhing animals. The penis may be 10 cm (4 in.) long, which is about half the length of the animal's body, and sperm transfer is generally both precise and relatively quick. Once the sperm has been passed, the slugs remain suspended and writhing for several minutes. At some point, the pair retracts their genitals and disentangles. One slug climbs back up the rope, while the other remains patiently in situ. With the first slug gone, eggs fertilised, the second may climb back up the rope (eating the mucus as it goes) or may continue the mucus production and lower itself down to the ground.

Leopard slugs mate during the autumn in an elaborate courtship that involves dangling on a mucus thread. - Credit: Spleines at Wikipedia (CC BY-SA 3.0)

Snail courtship can also be complex and protracted. It starts, as it does in slugs, with a touching of the tentacles, but soon progresses to each rearing up on their soles and making mouth-to-mouth contact (kissing, if you like). The pair then begins releasing copious amounts of mucus, before lying side by side and (eventually) the sperm transfer process can begin. Transfer, during which packets of sperm (spermatophores) are passed from one individual to the other, may take hours to complete. Interestingly, when the genitals touch, each snail fires 'love darts' at the other. These darts are small white, chalky shards—composed of protein and calcium carbonate—that become embedded in the flesh of the other snail. It's not entirely clear why they do this, but the darts seem to be pivotal to the whole process and if either snail misses, or the darts fail to embed properly, the whole sperm transfer slows down significantly. It has been suggested that these darts in some way help with the timing of the procedure and may also add an element of spice! Some have also suggested that they are aimed at inhibiting sexual relations once the animals have parted, but snails are remarkably polygamous despite the darts.

Sperm can be stored for up to a year after copulation, but eggs are usually laid a few weeks after mating, some 30-50 over the course of a couple of days, in shallow holes dug in the soil or amongst decaying plant material. Fully formed baby slugs and snails hatch out after about four weeks and reach sexual maturity between 10 and 20 days old, depending on species. Once hatched, the baby gastropods are on their own; there is no parental care among gastropods.

Despite their mucus and shell defences, slugs and snails still face a myriad of predators and parasites. Trout, foxes, badgers, hedgehogs, frogs, toads, lizards, snakes, blackbirds, thrushes, crows, shrews, owls, ducks and beetles (particularly ground beetles) are among the long list of animals that predate slugs and snails. Slowworms and ducks are particularly voracious predators of slugs while, contrary to popular misconception, most hedgehogs eat very few. Shrews, glow-worm larvae, and thrushes are probably the most significant predators of snails in the UK, the latter beating the snails against rocks to break the shell and allow access to the soft flesh inside. Gastropods are also attacked from the inside by a variety of parasites, including mites, fly larvae, and nematodes. Indeed, slugs and snails are an important intermediate host for several nematode species, including the Angiostrongylus variosum lungworm, and it's currently thought that consumption of slugs may account for lungworm infection in both hedgehogs, foxes and domestic dogs. That said, a 2014 study in Greater London found lungworms in only six (1.6%) of the 381 slugs they analysed.

Mating snails - Credit: Professor Bizzarro (CC BY 2.0)

Slugs do also, apparently, fight with each other on occasion, particularly during the summer months when food and shelter may be at a premium. I've not found much in the literature for what constitutes old age in slugs and snails, but it seems that they may live for up to about five years in the wild.

Curse or cure?

Slugs and snails have had a somewhat chequered history with humans. Many people, particularly avid gardeners, actively dislike, even detest, slugs and snails. Others relish them as a foodstuff or see great potential in them as a cure for some medical conditions. Indeed, historically, slugs were widely regarded as a cure for ailments ranging from coughs and colds to tuberculosis. In his 2001 opus on Britain's natural history, for example, Stefan Buzacki writes:

“Snails in general, and Helix aspersa [now Cornu aspersum] in particular, have been used extensively in traditional medicine. It was widely believed that the broth obtained from the snail's mucilage was soothing to a dry throat, and the glass-blowers of Newcastle upon Tyne 'held an annual feast of snails to strengthen their lungs for the coming year'.”

Snails on toast served in a London restaurant. - Credit: Matthew Knight (CC BY-NC 2.0)

Looking back at some of the ailments slugs were supposed to cure, it is easy to see how these ended up treated as old wives' tales. Nonetheless, we do now know that slug mucus possesses antibacterial properties and there is ongoing research looking at the venom of the marine cone snail, which shows great promise as a new painkiller.

It seems people have been eating snails since at least the Stone Age and, in his Fauna Britannica, Duff Hart-Davies tells how dairymen in Victorian London used to beat up snails in milk and pass the frothy white product off as cream. These days snails are less popular as a menu item in the UK than on the continent.

When not pursued for medicinal or culinary ends, snails and slugs are hunted down for the damage they do to our crops. The plaintive cry of gardeners under siege from gastropods has prompted a considerable amount of research into methods of killing them, or at least keeping them away from certain plants. In many cases, this is unfortunately a chemical approach in the form of slug pellets that are put around the plants. Slug pellets are of variable effect and have been implicated in the declines of predators such as hedgehogs and thrushes, which may inadvertently feed on the poisoned slugs. Fortunately, in recent years there has been a shift towards more environmentally friendly methods of slug control, which range from beer traps and sand/egg shells around your plants (aimed at blocking the slugs by sticking to their mucus and preventing them from being able to move) to spraying plants with a garlic solution to make them distasteful, and even methods of biological control, including the introduction of a tiny parasitic nematode worm that seeks out slugs in which to lay its eggs, killing the slug in the process. There's also something to be said for patrolling the garden of an evening with a torch and collecting all the slugs and snails you can find, although we have some data suggesting even this may be rather futile. Duff Hart-Davies mentions a researcher in Hertfordshire who removed each slug he found in his garden every night (200 on some nights) over the course of a year; in total 16,000 slugs and snails were extracted from the garden, without any appreciable impact on the following year's population.

Larvae of the European glow-worm (_Lampyris noctiluca_) eating a snail on the New Forest. These insects are voracious predators of snails and can track them from several tens of metres. - Credit: Marc Baldwin

Ultimately, whether you find them fascinating or loathsome, slugs are an important part of your garden's ecosystem—they form a link in the food web and play a crucial role in the recycling of nutrients. Without gastropods to clear up the mess, dead and decaying animal and plant tissue would hang around for much longer and pose a greater disease risk. So next time you're doing your anti-slug patrol, rather than immediately stepping on them or launching the little critters over the fence into your neighbour's garden (from which they will most likely return, by the way), maybe take a few minutes to look a bit closer. You never know, an interest in malacology might be just round the corner.

For a round-up of Britain's seasonal wildlife highlights for mid-summer, check out my Wildlife Watching - July blog.

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