Everything about slugs
In spite of the fact that slugs are different from snails only having no shell, the majority of people has absolutely different attitude towards these shellfishes. Snails are often heroes of cartoons, fairy tales, and people usually treat them with certain sympathy. Seeing a slug, people feel disgusted most often. But along with slug’s hunters, regarding these animals only as vile vermins of plant crops, there are a significant number of people, admiring original beauty, grace and uniqueness of slugs.
Slugs of various sizes and coloring are seen all over the world, but they reach their largest number and diversity of species in areas with mild and humid climate – in forests of New Zealand and Australia, North America, in Central and Western Europe. Here the slugs occupy fields, gardens, meadows, forests. The choice of such occupying areas is not by chance – these shellfishes don’t have shell, protecting them from heat, drought and cold. The slugs have to hide their tender body from heating sun rays under leaves and in thick grass, showing activity in the morning, in the evening and at night. If the weather remains hot and dry for a long time, shellfishes reduce their body surface so not to lose moisture – they roll up into a lump. When the soil gets dry slugs may create some kind of a cocoon, surrounding themselves with clods of soil, pasted together by slime. Some species of slugs gather into large groups at such periods – biologists consider that such original “hostels” allow reducing significantly dehydration of the body and survive bad weather conditions.
But despite all this, in dry and hot years slug populations suffer great losses, only some individuals remain in wet lowlands. If summer happens to be quite wet and what is more wet fall precedes it, than you won’t avoid massive reproduction of slugs. The shellfish show their highest activity under temperature of 7-15°C (44-59°F).
The body of slugs is symmetrical, long, very much stretched, but due to muscle contractions may change its form almost to spherical. Like other gastropods their body consists of three parts – head, feet and visceral mass. Snails have their visceral mass placed into shell, and slugs have it on the back side of their foot.
Circular furrow, dividing foot from the body is well seen by slugs. One or two pairs of contracted tentacles are greatly identified – eye tentacles serve as organs of smell as well and lower ones fulfill the function of organ of touch. Skinny wrinkles around their mouth form lips, also slugs have one jaw and teeth. But teeth are located not the way we are used to – in jaws, but on the tongue. The whole tongue of slugs is covered with thousands of tiny horny teeth bent back, which work as kitchen grater. Zoologists call this grate a rasping tongue; layer by layer tearing off soft tissues of food, slugs become saturated.
Shellfish mainly use their sense of smell when looking for food. By that their sense of smell is developed rather well, so they can smell ripe strawberries or potato tubers in a distance of about 2 meters. Pulmonary unpaired opening (pneumostome) is on the right at head and leads to mantle cavity with very thick network of blood vessels, functioning as lung. There is also an anus opening on the head near pneumostome.
Behind head on the body of slug there is an oval thorax – it is a mantle. There is a thin semitransparent plate under the mantle – rudimentary shell left for these shellfishes from shell of their far ancestors.
The slugs move thanks to wavy contractions of their muscular foot. So to save their vulnerable body from rubbing at soil or other solid substrate, these shellfishes excrete large number of slime, which can be of two kinds. Watery, more liquid slime is spread from the center to the edges of foot, and more sticky and thick substance is stretched to the tail. The slime of shellfishes of various kinds is also different – some have muddy, whitish; others – almost transparent. In any case the slime, thanks to its water absorbency can retain moisture, so it helps not only to make easier movement, but also allows protecting the slug’s body from overdrying. The slime is excreted from a special gland, located under mouth. Besides, many single-celled glands placed on foot and somewhere on the body produce slime. Some shellfish use their slime as a protector from predators – unpleasant taste and stickiness sometimes are able to frighten away potential slug eaters – raccoons, hedgehogs, moles, storks, wild boars, toads, etc.
Slugs can’t boast of having slime as snail’s, which is nowadays widely used for making anti-aging face masks. Being thicker and sticky the slime of these relatives of snails is washed off the skin badly, causing very unpleasant feelings. Besides, the psychological barrier caused by dislike and disgust of majority of people to slugs holds back the studies of cosmetic and medical properties of their slime.
Slugs as well as snails don’t stand out for their quickness of movement. But when there are bad conditions when searching for food of moisture they may run the distance of several thousands of meters.
The majority of species of slugs are herbivorous; they are not nasty in choosing food and gnaw everything they find – above parts of roots, leaves, flowers, fruits. There are slugs specialized in feeding with mushrooms, or eating dead parts of alive creatures (carrion, moss, fallen leaves, excrements etc.)
Despite their seeming slowness, among slugs there are even species which successfully hunt for their tinier congeners or earthworms. Although these shellfish are very voracious, at difficult times they can stand starvation in a wet shelter for about 2 months.
Reproduction of slugs happens 1-2 times a year. Just like snails, slugs – are hermaphrodites, every specimen at the same time has got egg cells and sperms. In spite of this the maturation of sexual products doesn’t appear at once – at first come male (sperm, packed into “packages” – spermatophores), then – female. There is no self-fertilization, as egg cells begin their active development only after sperms leave sexual gland. Slug ready for reproduction begin excreting slime, containing pheromones, the same congener finds it by smell. After mating and exchange of spermatophores there is a maturation of egg cells and their fertilization in the body of slugs. Large white or transparent eggs shellfish lay into the ground. During 3-5 weeks there develop larvae, to the moment of hatching they are turned into completely formed tiny slugs. Little grow very fast, and can already be sexually mature to 2 months age. If summer is wet, than the live-stock of slugs will increase considerably. Such speed of growth and early maturity are not by chance – the average life span of slugs is only 1-2 years. The bigger part of these seashells dies when the cold weather approaches, and there are only eggs left for hibernation. Though there are few species which have successfully hibernating young and even sometimes adult specimen.
Everybody knows about the harm that slugs give to agriculture, by damaging cucumbers, strawberries, cabbage, watermelons, beetroot, and wheat. In hothouses slugs destroy tender vegetable and flower sprouts, in vegetable stores – they eat root and potatoes. At garden and country districts these shellfishes have time to gnaw crops and spoil the look of ornamental plants and flowers. They don’t only destroy shoots and sprouts of plants, but also spread many dangerous fungal, virus, bacterial diseases of crops, pathogen of which calmly go through digestive path of slugs, not destructing. Being intermediate or main owners of some kinds of helminthes and other parasitic worms, slugs may cause contamination of wild birds and poultry, sheep, goats. The bird becomes ill when slugs are sticked together, and domestic animals – by eating grass where shellfish crept.
But along with that, slugs play essential role in ecosystems of gardens, vegetable gardens, fields, eating fallen leaves, the rests of corpses, excrements and even poignant for other species mushrooms. Not knowing their skills, slugs help to fight with such pest insects as beet fly on beet fields – the shellfish eat first those leaves where legs are laid. So having revealed these intruders on your farmland you shouldn’t start chemical destruction of slugs at once. There are plenty of soft, organic ways to reduce amount of these shellfishes in the arsenal of summer residents – arrangement of special fences, protecting plants, traps with baits etc.
Unlike snails, which people breed to get tasty and nutritious meat, eating slugs by human is very much limited. Only several species of rather large slugs are used for cooking of exotic dishes. As these shellfishes are very voracious and don’t differ from choosing their nutrition much, before using them they are must be kept on starvation for a few days or are just fed with plants with a nice smell.
Some species of slugs possess such nice looks that have become objects of great attention among terrariumists. For example, the banana slug, occupying parks of the USA along coast of Alaska to California, growing up to 20-25 cm in length and possessing bright-yellow coloring of the body. Or these unique endemics – Carpathian slugs, though not growing so large, only 10-14cm, but possessing very unusual coloring. Adult sexually mature shellfishes may be of different unnaturally bright colors – dark-blue, violet, and sea-green.
The potential of breeding slugs as pets is very high. Unusual look, lack of problems with feeding, opportunity of keeping in small-sized terrariums – all this are surely advantages of these unique animals. To disadvantages we may write short life span (very often, having lain eggs adult specimen die) and difficulty with keeping slugs in hot summer months. For normal activity of seashells the temperature should be 10-18°C (50- 64°F), which is very difficult to provide within summer in conditions of a flat.
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