Slugs and Slug Slime - Garden, Banana and Leopard Slugs

Slugs get little respect from most people. They are usually thought of as slimy, very unappealing creatures that are annoying garden and agricultural pests. However, not all slugs are pests, and after careful observation and study a person may decide that slugs are actually interesting animals. Some people even keep slugs as pets.

A slug has a soft, elongated body. The head has two pairs of tentacles, which can be retracted. The longer top pair have eyes at their tips, which can detect light but can’t form an image. The lower tentacles are sensitive to smells. The tentacles can be regrown if lost. A slug's tentacles wave gently through the air as the slug moves, sensing the environment as it travels.

Behind the head is a fleshy lobe known as the mantle. The mantle has an opening called the pneumostome, which leads to the slug’s single lung and is used for breathing. The pneumostome is usually on the right side of the mantle. Also on the right side, underneath the mantle, are the genital opening and the anus. These openings aren’t located near the rear of the body, as they are in most vertebrates, because of a phenomenon called torsion. In torsion, the visceral mass of the slug, which contains the internal organs, is twisted 180 degrees. Slugs are close relatives of snails, but they lack a shell, or have only a small shell embedded in the mantle or tiny remnants of a shell under the mantle.

Slugs don’t have a single structure that can be called a brain, but do have ganglia distributed around their body. A ganglion (the singular of ganglia) is often described as a “knot” of nerves. The ganglia of a slug are connected to each other, forming a nerve network.

Slugs eat fresh or decaying plant parts and may also eat insects, worms, carrion, animal droppings, kitchen scraps and pet food. Some slugs even eat other slugs. A slug’s mouth contains a structure called a radula, which is covered by rows of sharp teeth. The teeth are used for cutting and scraping, or for grabbing hold of active prey like worms. There can be up to 27,000 of these tiny teeth on a radula.

Slugs may be active in winter in mild climates but hibernate if it gets too cold. Some slug species die at the end of a season, living for only a few months, but others can live for six or seven years and need one or two years to mature.

Slug Slime Composition

Slug slime contains water, mucus and salts. Mucus is made of mucins. These are proteins with attached carbohydrates which are able to form sticky, moisture-trapping gels when they are added to water. Slug slime is hard to wash off our skin due to its stickiness and ability to absorb water. Slugs make both a thick and a thin mucus.

Functions of Slug Slime

The soft slug body dries out quickly if it is not protected. Slugs deal with this dilemma by secreting copious amounts of slime from skin glands, which keeps the skin moist and acts as a barrier against dessication. Even so, slugs are usually seen in damp environments rather than dry ones, and are most active at night. Many slugs spend a lot of time underground.

Slug slime also plays a vital role in locomotion. The lower surface of a slug’s body contains many slime-secreting glands. The slime released by these glands allows the slug to stick to surfaces – even vertical ones – as it moves by a series of muscular waves in the body. This movement is known as “adhesive locomotion”. A slug first sticks part of its body to the ground, then must expend energy to pull the body away from the adhesion. More slime is released and the process is repeated. Slug slime also helps prevent injury when a slug travels over rough surfaces containing stones or sticks.

A trail of glistening slug slime remains after a slug has passed through an area. The slime trail contains chemicals that can be detected by other slugs, indicating where the trail-layer has gone. This is useful if another slug wants to find a mate. In some slugs different chemicals appear in the slime during the mating season. The chemicals in the slime can also attract predator slugs.

The banana slug is found in the Pacific Coast region of North America from Alaska to California. Its slime contains an anesthetic. A predator picking up a banana slug would feel their mouth go numb and might drop the slug unharmed. Banana slugs are bright yellow to greenish yellow in color and sometimes have black blotches. They are the second largest slug in the world and may reach a length of 25 cm (9.8 inches), although most adults are six to eight inches in length. The largest slug in the world is Limax cinereoniger, which is found in Europe and may reach a length of 30 cm (11.8 inches).

Scientists and engineers working in the area of robotics are very interested in the relative roles of slug slime and a slug's muscles in controlling its movement, and in how these factors interact. The engineers are creating experimental biomimetic robots - ones that operate according to principles discovered in animals - based on their slug research.

In addition, researchers are studying the properties of slug mucus with the aim of creating a similar material for human use. Slug mucus has the unusual characteristic of changing its consistency as a slug moves over it. Its very adhesive nature allows slugs and snails to move over a wide variety of textures at a wide variety of angles, even while hanging in an inverted position in some cases.

A slug is a hermaphrodite - it contains both male and female reproductive organs. During mating the slugs entwine, exchange sperm and then separate. The leopard slug has an unusual mating ritual. The two mating slugs climb a tree or shrub and then lower themselves towards the ground on a string of mucus. Mating happens in midair before the slugs continue their journey to the ground.

Once sperm have been transferred from one slug to another, the sperm fertilize the eggs inside each slug’s body. Some slugs can fertilize their eggs with their own sperm. A few to hundreds of eggs are laid, depending on the species of slug. The eggs, which are generally white or transparent, are laid in sheltered areas such as in soil or under leaves or logs. Several batches of eggs may be produced in a year. The adult slug doesn’t guard the eggs once they are laid. The eggs have the ability to stay dormant until the environment is suitable for them to hatch.

In Bornea, a green and yellow slug called Ibycus rachelae has been found to use “love darts’ when it is preparing to mate. Some snails also use love darts. The darts are needle or harpoon-shaped structures made of calcium carbonate. A dart is released from one slug when it contacts another one, and injects a hormone into the second slug that increases the chance of successful reproduction.

There are various recipes and instructions on the Internet for killing or repelling slugs, some of which are effective and others not so effective. Although it’s understandable that farmers and gardeners would want to wage war on the slugs destroying their plants, many types of slugs do not attack domestic plants. Pest species can create a lot of damage, but unless slugs are interfering with human lives in some way there is no need to kill them.

Many slug species play useful roles in the environment. They break down and recycle plant and animal material in the soil, and provide food for some birds, frogs, snakes and even mammals such as raccoons. Studying the sticky mucus and movement mechanism of slugs may enable scientists to create new materials and devices with useful applications. In addition, slug behavior can be interesting to observe!