Slug

A slug or land slug is a common name for any shell-less land gastropod mollusc. Unlike snails, slugs do not have a hard shell to protect them. Some slugs have a very small shell, or a tiny internal shell that is hard to see. Slugs are found all over the world. They live in many places, like gardens and forests.

The word slug is also used to describe other types of gastropods without shells, like sea slugs and semi-slugs. This is different from the word snail, which usually means gastropods that have a large, coiled shell.

Even though slugs look similar, they are not all closely related. Different groups of slugs come from many evolutionary lines. Some of these lines also include snails. This means slugs have lost their shells many times in history. Because of this, slugs are very diverse and not from just one family.

Taxonomy

There are six groups of air-breathing snails and slugs, called Pulmonata. Two of these groups, Onchidiacea and Soleolifera, only include slugs. Another group, Sigmurethra, has many types of snails, some snails with very small shells, and slugs. Scientists are still studying this group using DNA.

The Sigmurethra group includes smaller groups, such as Orthurethra and superfamilies like Acavoidea, Achatinoidea, and Arionoidea, among others. Each of these has its own families of snails and slugs.

Description

Slugs have special parts on their bodies that help them move and feel their surroundings. On their heads, they have two pairs of tentacles. The upper pair can sense light and has tiny eyes, while the lower pair helps them smell. These tentacles can be pulled in when needed.

On the top of a slug, there is a part called the mantle. Some slugs have a small opening on the right side of the mantle for breathing, while others have the opening at the back. The flat bottom part of a slug is called the foot, and it helps the slug move by squeezing and stretching. As they move, slugs also put out a slippery substance called mucus, which helps protect their foot. Some slugs even have a small, hidden piece of shell inside their bodies.

Physiology

Slugs are mostly made of water. They don't have a big shell to protect them, so they make slippery stuff called mucus to stay safe. They are most active when it is wet, like after rain or at night, and hide in damp places when it is dry.

Slugs make two kinds of mucus: thin and watery, and thick and sticky. This mucus helps them move without slipping and can keep them safe from animals that might want to eat them. Other slugs can follow the slimy trails left behind to find friends or food.

Reproduction

Slugs have both female and male parts. When they find a mate, they wrap around each other and share special cells called sperm.

In some banana slugs, the male part can get stuck. The slugs can free themselves and still have babies using just the female parts.

Ecology

Slugs help keep the ecosystem healthy by eating dead plants and fungi. Some slugs also eat other slugs and dead animals.

Slugs eat many things, like leaves from living plants, lichens, mushrooms, and even dead animals. They like to eat vegetables and herbs such as petunias, chrysanthemums, daisies, and fruits like strawberries. Some slugs enjoy eating fungi, like certain types of mushrooms.

Many animals eat slugs. This includes reptiles like snakes and lizards, birds such as common blackbirds and starlings, and mammals like foxes and hedgehogs. Even some insects and small creatures can be parasites or predators of slugs.

Behavior

When slugs are attacked, they can make their bodies smaller and harder to grab. They stick tightly to surfaces and produce slippery mucus that makes it hard for predators to hold onto them. The mucus also tastes bad to predators, which helps keep the slugs safe.

Some slugs, like the Kerry slug, react differently when threatened. Instead of sticking tight, they let go, roll up into a ball, and stay that way until the danger passes. This is special to the Kerry slug and not common in other slugs. Certain slugs can also drop part of their tail to escape from danger. In cold weather, some slugs go underground to stay safe, while others die when autumn comes.

Slugs sometimes act aggressively toward each other, whether they are the same kind or different kinds, especially when fighting over food or shelter. This aggressive behavior is more common in summer when resources are scarce. In winter, slugs tend to stay together in groups instead.

Human relevance

Most slug species are harmless to people and do not cause any problems. However, a few types can be pests for farms and gardens. They can eat leaves very quickly, sometimes harming plants. They also eat fruits and vegetables before they are harvested, which can ruin the crop’s appearance.

In very rare cases, eating raw slugs can lead to illness in people. This can happen if slugs are accidentally eaten on vegetables that were not cleaned properly, or if they are not cooked correctly.

Prevention

Main article: Molluscicide

To control slugs, people often use baits. Recently, safer baits made from iron phosphate have become popular because they are less harmful to animals than older types. Another natural way to control slugs is by using tiny organisms called parasitic nematodes (Phasmarhabditis hermaphrodita). These nematodes can find and infect slugs, helping to reduce their numbers. This method is safe for organic farming.

Other methods, like using beer traps, diatomaceous earth, crushed eggshells, coffee grounds, or copper, can help a little in small gardens but are not very effective on a large scale. Salt can kill slugs, but it is not used on farms because it can harm the soil. Certain farming practices, like conservation tillage, can make slug problems worse by leaving more organic matter on the ground where slugs like to live.