Slugs need moisture above almost everything else. Without a shell to seal in water the way snails can, a slug’s soft, permeable body loses fluid constantly to the surrounding air. Studies on the Iberian slug show that eggs and juveniles begin losing water as soon as ambient humidity drops below roughly 99.5%, and survival depends on staying in persistently humid habitats. Beyond moisture, slugs require food, suitable temperatures, calcium for their internal structures and eggs, oxygen exchanged through a simple lung, and enough shelter to avoid predators and desiccation during daylight hours. Each of these needs shapes where slugs live, when they are active, and how long they last.
Moisture Is the Non-Negotiable
A slug’s body is roughly 80 to 90 percent water by mass, and unlike a snail, it has no thick shell to retreat into and seal off with a mucus plug. That makes water balance the single biggest constraint on a slug’s life. Research on the Iberian slug, Arion lusitanicus, found that juveniles could tolerate losing a surprising amount of body water before dying, around 72 percent of their water content, but the catch is that they start losing it almost immediately when humidity dips even slightly below saturation. Eggs were even more sensitive to drying out, though they tolerated a higher percentage of water loss (about 81 percent) before half of them died. The study’s authors concluded that despite this impressive tolerance for dehydration once it begins, the species still depends on humid habitats because evaporative loss is essentially uncontrollable for a slug in dry air.1PubMed. Drought tolerance in eggs and juveniles of the Iberian slug, Arion lusitanicus
This is why you almost never see slugs crawling in midday sun. They are overwhelmingly nocturnal or crepuscular, emerging after dark when humidity rises and temperatures fall. During the day, they tuck themselves under logs, stones, dense leaf litter, or into soil crevices where microhabitat humidity stays close to 100 percent. If you have ever flipped a board in your garden and found a cluster of slugs on the damp underside, you have seen this survival strategy in action.
Work on the invasive leopard slug, Limax maximus, has shown that individual slugs vary in how well they resist dehydration, and that this variation is consistent over time, meaning some individual slugs are measurably better at holding onto water than others. The researchers suggested that water balance could respond to natural selection, which helps explain how slug populations manage to colonize a range of environments from damp forests to suburban gardens.2Invertebrate Biology. Repeatability of energy metabolism and resistance to dehydration in the invasive slug Limax maximus
The Expensive Business of Mucus
Mucus is so central to a slug’s existence that it deserves its own discussion. Slugs produce two general types: a thin, slippery mucus they lay down as a trail for locomotion, and thicker secretions used for other purposes like defense. The locomotion mucus is not optional. It is the surface a slug pushes against to move, because slugs do not have legs and instead ripple their single muscular foot along a ribbon of slime.
The energy cost of this system is remarkably high. A classic study on gastropod crawling found that the metabolic cost of movement was about 904 joules per kilogram per meter, far more than what is typically measured for other forms of animal locomotion. The majority of that expense comes from manufacturing the pedal mucus itself, not from the muscular effort of crawling.3PubMed. Locomotion: the cost of gastropod crawling This means that a slug needs to eat enough not just to fuel its body’s basic functions but also to bankroll a continuous outlay of mucus every time it moves. It is one reason slugs tend to forage in a relatively small home range rather than roaming widely; the energetic price of travel is steep.
Some species have turned mucus into a weapon. The red triangle slug of Australia produces a defensive secretion from specific patches on its back that is thick, sticky, and elastic, nothing like the thin trail slime. Field researchers observed an adult tree frog trapped and incapacitated by this adhesive mucus, and lab tests showed the secretion actually becomes stickier and reactivates on contact with water.4Ethology. Adhesive defence mucus secretions in the red triangle slug (Triboniophorus graeffei) can incapacitate adult frogs Most garden slugs do not have such dramatic defenses, but the point remains: mucus production, whether for movement or protection, is a constant metabolic demand that shapes what and how much a slug needs to eat.
What Slugs Eat
Most terrestrial slugs are generalist feeders. They eat decaying plant material, living leaves, fungi, algae, and occasionally animal matter like carrion or other invertebrates. This flexibility is part of what makes them so successful. A slug does not need a single host plant or a specialized food source; it can make do with whatever is available in its environment.
Research in a mixed deciduous woodland measured the feeding rates of six slug species on various types of leaf litter. The smallest species tested, Arion intermedius, had the highest consumption rate relative to its body weight and the highest assimilation efficiency, extracting about 72 percent of the energy from what it ate. The slug community as a whole consumed an estimated 8.4 percent of the annual leaf litter input, making slugs meaningful players in decomposition and nutrient cycling in forest ecosystems.5Ecography. Litter decomposition by slugs in mixed deciduous woodland
Breaking down tough plant cell walls requires cellulase enzymes, and slugs get help from their gut bacteria. A study of the large black slug, Arion ater, identified multiple cellulase enzymes in the gut and isolated 12 distinct cellulolytic bacteria using genetic sequencing. These microbes allow slugs to extract nutrition from fibrous plant tissue that they could not digest on their own.6PubMed. Characterization of cellulolytic activity in the gut of the terrestrial land slug Arion ater: Biochemical identification of targets for intensive study This gut microbiome is not something a slug actively “needs” in the way it needs water, but without these microbial partners, its diet would be far more restricted.
Temperature Limits
Slugs are ectotherms, so their body temperature tracks whatever the environment provides. Most temperate-region slugs are active at temperatures between roughly 5°C and 25°C, and they become sluggish (in the non-metaphorical sense) at the extremes. What surprises many people is how well certain species handle cold.
Three European Arion species were tested under controlled freezing conditions. All three survived being frozen solid for two days at −1°C. Some individuals of Arion rufus and Arion lusitanicus even survived at −2°C. When their body fluids froze, the slugs accumulated glucose, lactate, and succinate, with glucose levels in A. rufus jumping from about 6 to 22 micrograms per milligram of dry tissue. The glucose appears to act as a cryoprotectant, helping prevent lethal ice damage inside cells, although the concentrations were lower than those seen in highly freeze-tolerant insects.7PubMed. Cold tolerance and freeze-induced glucose accumulation in three terrestrial slugs
A different study on the Kerry slug, Geomalacus maculosus, a species protected under EU law, tested tolerance down to −9°C. The slugs survived −3°C and −6°C exposures without significant mortality compared to controls, though immediately after thawing they were covered in frozen yellow mucus and had hardened bodies with retracted tentacles. At −9°C, mortality climbed significantly. Even so, some individuals revived within 24 hours of thawing at the lower temperatures, gradually resuming movement and responding to touch.8Journal of Molluscan Studies. Tolerance to extreme hot and cold temperatures in the EU-protected terrestrial slug Geomalacus maculosus
Not every slug species relies on cryoprotectants. Research on Ambigolimax valentianus found that this species is not strongly freeze tolerant but becomes more cold-hardy when acclimated to shorter day lengths, mimicking the approach of autumn. Rather than ramping up any particular protective metabolite, the slug showed evidence of metabolic suppression heading into winter, essentially dialing down its energy use rather than armoring up chemically.9PubMed. The cold tolerance of the terrestrial slug, Ambigolimax valentianus So different species use different strategies: some produce chemical antifreeze, others just slow down and wait it out.
How Slugs Breathe
Slugs do not have gills like aquatic mollusks. Terrestrial slugs breathe through a structure called the pneumostome, a small opening, usually on the right side, that leads to a vascularized chamber functioning as a lung. If you watch a slug closely, you can sometimes see this opening rhythmically dilating and closing. A detailed anatomical study of a tropical slug, Trichotoxon copleyi, found that the respiratory surface inside is richly supplied with blood capillaries and surrounded by a thick muscular layer that helps pump air in and out. The researchers concluded it functions as a true lung in every structural and functional sense.10Journal of Zoology. The morphology of the lung of a tropical terrestrial slug Trichotoxon copleyi
This lung needs to stay moist to exchange gases efficiently, which circles back to the moisture problem. A slug in dry air is not just losing body water from its skin; its respiratory surface is drying out too, making it harder to absorb oxygen. Waterlogged environments are not ideal either, though, because slugs can drown if submerged without access to air. The goldilocks zone is damp but not flooded, which explains why well-drained soil with organic cover is classic slug habitat.
Calcium, the Overlooked Mineral
People rarely think about calcium when they think about slugs, but it is essential. Slugs retain a vestigial internal shell, sometimes just a small plate or a few granules buried under the mantle, and that remnant is made of calcium carbonate. More importantly, calcium is stored throughout the body in specialized connective tissue cells, digestive gland cells, and skin gland cells. During egg laying, a wave of calcium is mobilized from these internal stores and routed through the reproductive tract to supply the eggshells and egg fluids, which the developing embryo then absorbs.11Integrative and Comparative Biology. Calcium Dynamics in Land Gastropods
Slugs obtain calcium from their food, from soil, and sometimes by eating snail shells or chalky surfaces. Gardens with alkaline, calcium-rich soil tend to support larger slug populations than acidic soils, and slug-rich habitats are often associated with limestone geology. If you have ever wondered why slugs seem to thrive in certain gardens and not others despite similar moisture levels, differences in soil calcium could be part of the answer.
Reproducing Under the Right Conditions
Temperature and soil moisture jointly dictate when slugs lay eggs. Controlled experiments on the grey field slug, Deroceras reticulatum, one of the most economically damaging species in agriculture, found that egg-laying rate peaked at about 18°C and 53 percent dry soil moisture, with slugs producing around 26 eggs per individual per week under those conditions. But the conditions that maximized the number of viable eggs that actually hatched were different: about 7.5°C and 46 percent soil moisture.12Annals of Applied Biology. The importance of temperature and moisture to the egg-laying behaviour of a pest slug, Deroceras reticulatum In other words, slugs lay the most eggs in warm, moist conditions, but the eggs are most likely to survive and hatch in cooler, slightly drier soil. This mismatch means that autumn and early spring, when temperatures dip, are often peak hatching seasons in temperate climates.
Many slug species are hermaphrodites, carrying both male and female reproductive organs, which means a single individual can potentially reproduce if it encounters no mates. In practice, most still prefer cross-fertilization, but the hermaphroditism gives slugs a population-level resilience: even a small founding group can establish a viable colony, which is one reason invasive slugs spread so effectively.
Surviving Drought Through Dormancy
While many people associate slugs exclusively with wet weather, some species have evolved strategies to endure dry spells. Aestivation is a state of aerobic dormancy triggered by high temperatures, drought, or food shortages, where the animal drastically suppresses its metabolic rate and waits out the unfavorable period.13PubMed Central. Aestivation in Nature: Physiological Strategies and Evolutionary Adaptations in Hypometabolic States Aestivation is better documented in snails, which can seal their shell aperture with a hardened mucus epiphragm, but terrestrial gastropods more broadly deploy a range of behavioral and biochemical adaptations, including seeking deep soil refuges, reducing mucus production, and activating protective enzymes.14PubMed Central. Snails in the sun: Strategies of terrestrial gastropods to cope with hot and dry conditions
For slugs specifically, the options are more limited than for their shelled cousins. Without a shell to seal off, a slug cannot create the same kind of sealed microenvironment around its body. Most slugs cope with dry weather by burrowing into soil or retreating deep into compost heaps and staying inactive until moisture returns. Some species lay drought-resistant eggs that can persist in soil through a dry summer, even if the adults die.
What Kills Them
Understanding what slugs need to survive also means understanding what most commonly kills them. Predation is constant. Birds, hedgehogs, beetles, frogs, and certain fly larvae all eat slugs. But one of the most effective natural enemies is a parasitic nematode, Phasmarhabditis hermaphrodita, which was first identified as a slug parasite in the early 1990s. The nematode penetrates the slug beneath its mantle, causing a characteristic swelling, and kills the host within about 7 to 21 days. It then reproduces inside the dead slug. Lab tests showed it killed every pest slug species tested, including multiple Arion and Deroceras species.15Biocontrol Science and Technology. The rhabditid nematode Phasmarhabditis hermaphrodita as a potential biological control agent for slugs
This nematode has been commercially developed as a biological control product. When mixed with water and applied to soil, the nematodes actively seek out slugs and infect them.16PubMed. Thirty years of slug control using the parasitic nematode Phasmarhabditis hermaphrodita and beyond For gardeners looking for alternatives to chemical slug pellets, nematode treatments are one of the few options with solid research behind them, though results vary with soil temperature and moisture (the nematodes themselves need damp conditions to move through soil).
Chemical molluscicides, particularly metaldehyde and iron phosphate pellets, remain widely used. Research on slug responses to various deterrents found that most tested products significantly reduced locomotor activity. The most effective substance tested, cinnamamide, cut snail movement by 94 percent and track length by 96 percent. Slugs showed strong avoidance of treated areas, with avoidance exceeding 95 percent for some products.17Pest Management Science. The behavioural response of slugs and snails to novel molluscicides, irritants and repellents Metaldehyde, once the dominant slug-killing chemical, has been banned or restricted in several countries due to water contamination concerns. Iron phosphate pellets are generally considered safer for the environment, though they work more slowly.
Why Losing the Shell Was a Trade-Off
Slugs evolved from shelled ancestors multiple times independently. The transition from snail to slug has happened in at least a dozen separate lineages across gastropod evolution, which tells us the shell-less body plan must confer real advantages, or it would not keep arising. The main benefits are access to tight spaces (a slug can squeeze into crevices a snail never could), reduced calcium requirements (building a shell is metabolically expensive), and potentially faster growth.
But losing the shell came with costs, and those costs map directly onto the survival needs described above. Without a shell, slugs lost their primary defense against predators and their best tool for preventing water loss. Phylogenetic analyses of sea slugs have shown that shell reduction correlates with the evolution of alternative defensive strategies: chemical defenses, sticky mucus, noxious secretions, and specialized body structures that deter predators. The acquisition of these new defenses in turn opened up new food sources and may have driven diversification in shell-less lineages.18PubMed Central. Opisthobranchia (Mollusca, Gastropoda) – more than just slimy slugs. Shell reduction and its implications on defence and foraging
For terrestrial slugs, this evolutionary trade-off explains the lifestyle you see in your garden: nocturnal activity to avoid sun and dry air, a dependence on sheltered microhabitats, a generalist diet that compensates for the inability to roam widely, and a heavy investment in mucus for both locomotion and protection. Every slug you find under a rock is living out an ancient bargain, one that traded the safety of a shell for the flexibility of a soft, squeezable body, and then spent millions of years evolving workarounds for the vulnerabilities that bargain created.
Photosynthetic Relatives and the Limits of Slug Biology
While terrestrial slugs are firmly dependent on food they eat and digest, some of their marine relatives have pushed the boundaries of what a slug body can do. Certain sea slug species in the order Sacoglossa steal chloroplasts from the algae they eat and keep them functional inside their own tissues for weeks or months, a phenomenon called kleptoplasty. Research on two of these species, Elysia timida and Plakobranchus ocellatus, showed that they fix carbon dioxide in light at rates 60- to 64-fold higher than in darkness, confirming the stolen chloroplasts are genuinely photosynthesizing inside the slug. Yet even these solar-powered animals do not actually need photosynthesis to survive: individuals kept in complete darkness or treated with a chemical that blocks photosynthesis survived months of starvation without losing weight any faster than control slugs in normal light.19PubMed Central. Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive
This finding matters because it highlights something fundamental about slug biology: even when equipped with what looks like a game-changing superpower, the basic requirements of moisture, shelter, and a functioning metabolism still dominate. Terrestrial slugs do not have stolen chloroplasts or any photosynthetic trick. They survive by being flexible, tolerant, and very good at finding the damp, food-rich microhabitats their soft bodies demand.