Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusk. The word slug is also often used as part of the common name of any gastropod mollusk that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semi slugs (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that the animal can fully retract its soft parts into the shell).
Slugs are best described as snails without shells. They are a type of mollusk, related to clams and oysters. Slugs are soft bodied, generally brownish or grayish, with eye stalks. They vary in size from 1/4 inch to two inches or longer. Slugs leave a silvery slime trail that they secrete as they move.
Slugs use file-like mouthparts to rasp and chew plant tissue. Because of their mouthparts, they create irregularly shaped holes. Feeding damage can be cosmetic, however extensive feeding can result in plant stress or even death.
Slugs’ bodies are made up mostly of water and, without a full-sized shell, their soft tissues are prone to desiccation. They must generate protective mucus to survive. Many species are most active just after rain because of the moist ground. In drier conditions, they hide in damp places such as under tree bark, fallen logs, rocks and man-made structures, such as planters, to help retain body moisture. Like all other gastropods, they undergo torsion (a 180° twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation—but externally, the bodies of slugs appear more or less symmetrical, except for the positioning of the pneumostome, which is on one side of the animal, normally the right-hand side.
Slugs produce two types of mucus: one is thin and watery, and the other thick and sticky. Both kinds are hygroscopic. The thin mucus spreads from the foot’s centre to its edges, whereas the thick mucus spreads from front to back. Slugs also produce thick mucus that coats the whole body of the animal. The mucus secreted by the foot contains fibres that help prevent the slug from slipping down vertical surfaces. The “slime trail” a slug leaves behind has some secondary effects: other slugs coming across a slime trail can recognise the slime trail as produced by one of the same species, which is useful in finding a mate. Following a slime trail is also part of the hunting behaviour of some carnivorous slugs. Body mucus provides some protection against predators, as it can make the slug hard to pick up and hold by a bird’s beak, for example, and the mucus itself can be distasteful.
The external anatomy of a slug includes the following:
Like other pulmonate land gastropods, the majority of land slugs have two pairs of ‘feelers’ or tentacles on their head. The upper pair is light sensing and has eyespots at the ends, while the lower pair provides the sense of smell. Both pairs are retractable.
On top of the slug, behind the head, is the saddle-shaped mantle, and under this are the genital opening and anus. On one side (almost always the right hand side) of the mantle is a respiratory opening, which is easy to see when open, but difficult to see when closed. This opening is known as the pneumostome.
The part of a slug behind the mantle is called the tail.
Some species of slugs have a prominent ridge running over their back along the middle of the tail (sometimes along the whole tail, sometimes only the final part). This ridge is called a keel.
The bottom side of a slug, which is flat, is called the foot. Like almost all gastropods, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus that it travels on, which helps prevent damage to the foot tissues. Around the edge of the foot in some taxa is a structure called the foot fringe.
Most slugs retain a remnant of their shell, which is usually internalized. This organ generally serves as storage for calcium salts, often in conjunction with the digestive glands. An internal shell is present in the Limacidae and Parmacellidae.
When attacked, slugs can contract their body, making themselves harder and more compact and more still and round. By doing this, they become firmly attached to the substrate. This, combined with the slippery mucus they produce, makes slugs more difficult for predators to grasp. Some slugs can self-amputate (autotomy) a portion of their tail to help the slug escape from a predator. Some slug species hibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.
Intra- and inter-specific agonistic behavior is documented, but varies greatly among slug species. Slugs often resort to aggression, attacking both conspecifics and individuals from other species when competing for resources. This aggressiveness is also influenced by seasonality, because the availability of resources such as shelter and food may be compromised due to climatic conditions. Slugs are prone to attack during the summer, when the availability of resources is reduced. During winter, the aggressive responses are substituted by a gregarious behavior.
Diet and eating habits
Most species of slugs are generalists, feeding on a broad spectrum of organic materials, including leaves from living plants, lichens, mushrooms, and even carrion. Some slugs are predators and eat other slugs and snails, or earthworms.
Slugs can feed on a wide variety of vegetables and herbs, including flowers such as petunias, chrysanthemums, daisies, lobelia, lilies, daffodils, narcissus, gentians, primroses, tuberous begonias, hollyhocks, irises, and fruits such as strawberries. They also feed on carrots, peas, apples, and cabbage that are offered as a sole food source.
Slugs will eat any kind of vegetation but prefer tender leaves. This means that particularly tender-leaved plants or seedlings are very vulnerable to slug damage. Slugs will also eat vegetables and fruits, causing unsightly damage to crops.
Slugs from different families are fungivores. Some slugs are selective towards certain parts or developmental stages of the fungi they eat, though this is very variable. Depending on the species and other factors, slugs eat only fungi at specific stages of development. Moreover, in other cases, whole mushrooms can be eaten, without any selection or bias towards ontogenetic stages.
Slugs and snails are hermaphroditic animals. Each individual has both male and female sex organs, but they usually require another individual to fertilize the eggs that each carries in its body. Each one is capable of laying a total of about 300 eggs. These are laid any time conditions are optimal, but peak times are the spring and fall.
The gel-like eggs are laid in clusters of 25, about an inch below loose soil in damp locations, and are covered with a layer of mucus. Eggs hatch in about 30 days. However, under dry conditions, the eggs may remain unhatched until adequate moisture has been absorbed. Slugs and snails reach adult size in 3 to 12 months and can live for several years. Temperature and moisture are the prime factors that influence their activity. In times of unfavorable environmental conditions, slugs can survive by burrowing as deep as 3 feet into the soil and snails will close up within their shells.
Slugs can be very damaging pests in moist, shady gardens. They feed on the leaves of many plants, especially seedlings. Later in the season they can feed on ripening fruits and vegetables. Slugs are especially numerous during rainy seasons and in well-irrigated gardens. If slugs are abundant one year, it does not mean they will be as common the following season; the relative number of slugs in a given season depends on how moist the growing conditions are.
Slugs as pests
Slugs are one of the most damaging pests in the garden. Given the proper environment, a family of slugs can devastate a vegetable crop in a matter of days. Understanding a few facts about slugs, like what do slugs eat, where do slugs live and what eats slugs can help you kill garden slugs in your garden. Slugs play an important role in the ecosystem by eating decaying plant material and fungi. Most carnivorous slugs on occasion also eat dead specimens of their own kind.
They will feed on almost anything in the garden—look for holes and ragged edges on leaves and stems. The holes should have irregular shapes due to their file-like mouthparts. Small seedlings can be consumed entirely.
Slugs can digest tissues from most plants, but you might find them especially liking your beans, lettuce, cabbage, and tomatoes.
Slugs and snails can be troublesome pests in the garden. Both eat large, ragged holes in the leaves of plants and may also completely consume young seedlings. They begin feeding early in the spring and continue through the growing season until frost. Slugs and snails feed on a wide variety of plant material and can be especially troublesome on hostas, violets, ageratum, lilies, cleome, strawberries, lettuce, and cabbage.
There are many species of these mollusks, but only a few present a serious problem. Some of these are the gray garden slug, Deroceras reticulatum, the tawny garden slug, Limax flavus, the spotted garden slug, Limax maximus (see photo above), and the brown garden snail, Helix aspersa.
Slugs and snails have limited protection against water loss and need adequate moisture levels to survive. They can absorb water directly through their skin or drink from puddles. Slugs and snails feed primarily at night, although they can also come out during foggy overcast weather, after rain, or after watering.
During the day, they seek protection from dehydration in cool, damp locations under leaves, mulch piles, rocks, stepping stones, wooden boards, porches, decks, crawl spaces, or flower pots. Large populations can sometimes be found under groundcover plants such as ivy, pachysandra, and vinca. Wood chip mulches also provide excellent habitat.
Slugs and snails travel by means of a large foot that glides over a trail of mucus or slime, secreted from glands located under their head. The slime provides a cushion over rough areas. This slime also serves as a trail marker. They return to their favorite hiding and feeding places night after night unless disturbed or the site becomes too dry. Slugs and snails tend to avoid crossing dusty or dry materials. The shiny dried slime trails left behind on plants or along the ground are good indicators of their activity.