Arion lusitanicus

Arion lusitanicus is native to the Iberian Peninsula and southern France, but has been spread throughout Poland, Germany, Denmark, Sweden, Norway, Finland and the Faroe Islands and Iceland. This species has the potential for establishment in all of the countries of the region. (Weidema, 2006)

Arion lusitanicus inhabits areas that are moist, including deciduous forests and grasslands. However, the slug is often found in human-made habitats, such as gardens, parks, croplands, ditches, and wastelands. Certain vegetable crops such as cabbage, lettuce, beans, flowers, bulbs, strawberries, and raspberries are occupied the most frequently in agricultural areas. Areas with low relief relief are typical. (Weidema, 2006; Koztowski, 2007)

Arion lusitanicus (the Portuguese slug) is a large slug, from 70-150 mm. This species is often a reddish brown, and in some cases a yellowish color. Adults tend to be uniform in color pattern, but juveniles will sometimes have dark bands along their sides. This snail has an obvious respiratory hole (pneumostome) on the right side of the anterior end of the mantle. The foot of A. lusitanicus has the appearance of a having been stitched on, analogous to a very thick seam. Arion lusitanicus was long misidentified as the Spanish slug, Arion vulgaris. The best way to tell the two apart is by comparing their internal morphology, primarily the shape of the respective spermatophores. This comparison is also the only definitive way to discern A. lusitanicus from A. rufus. (Weidema, 2006; Schultes, 2011; Anderson, 2008; Koztowski, 2007)

Three phases of development described for A. lusitanicus are infantile, juvenile and mature. The infantile phase is from the time of hatching to when the slugs reach sexual maturity. The juvenile phase is defined by copulation and egg laying. As the slugs age and begin laying smaller batches of eggs, they are considered mature.

Eggs laying is temperature dependent and ranges from 2.5 to 3.5 months. Round or oval eggs, 4.2 by 3.5 mm are laid in batches held together by a mucous strand. Hatching is temperature dependent and 54-86% of the eggs will hatch after an average of 40 days (30 to 80 day range). Eggs laid in late September hatch the following spring from mid-March through late April. Juvenile snails usually merge in April through May, with different generations (fall or spring hatching) both present. After mating and after a year of life, the slugs usually die. (Koztowski, 2007)

Arion lusitanicus is hermaphroditic. When a slug sexually matures, around 5-8 months, it is ready to begin copulating. This usually takes place at the end of July and the duration of the copulation period relies solely on the temperature of the environment. If the ambient temperatures fall below 5◦C, mating will not take place. Slugs who are not yet fully grown are often the ones who mate, although it is not uncommon for smaller slugs to seek out larger ones with which to copulate. Mating takes place in a shaded moist area in a horizontal position and proceeds through four distinct phases. The first stage is the identification of a suitable partner. In the second stage the slugs pair and evert their genital openings. The third stage involves a continuous and motionless position where the actual copulation takes place. During this time (approximately 4 hours) the transfer of the spermatophore happens. It is formed by the slug and then inserted into the base of the bursa in the spematheca. The atria is then everted, so individual sperm masses can be transferred as well. Once complete the genitalia are inverted again and the slugs go their separate ways. (Weidema, 2006; Koztowski, 2007)

Approximately one month after copulation, A. lusitanicus begins to lay eggs. Mating is temperature dependent, and so is the duration of the egg laying. The peak of the egg-laying occurs in late September. The eggs are covered in a protective mucus coating and can be placed on top of the soil or under it. The eggs are stuck together in groups of on average of 68 within each bundle. Up to 400 eggs can be laid within one summer. About one month after the first set of eggs was laid, the slugs begin hatching. Only half of the eggs laid will hatch before winter, the rest hatch that following spring. Both the juveniles and embryos are able to survive throughout the winter. The entire life cycle of this species is usually complete in a year. Typically, the slug dies in the fall after it has laid eggs, and shortly after it has reached adulthood. Very rarely do adults survive through the winter. When this does occur, they die the following spring. (Weidema, 2006; Koztowski, 2007)

There is no parental investment after eggs are laid. (Weidema, 2006)

The adult slugs normally die in autumn after reproduction is complete. Mild winters and summers with plenty of rainfall increase the survival of the juveniles that overwinter in compost heaps and soil crevices. On rare occasions, some adults may even survive winters, e.g. an adult specimen was reported to survive winter under protected conditions near a greenhouse wall in Finland. (Weidema, 2006)

The slugs are the most active when the weather is rainy and cloudy or during mornings with moisture on the ground. Activity of the slug is most often observed at night and in the morning, but especially around two to three hours after sunset. Some studies have suggested that each slug behaves differently in terms of home range. Homing behavior, enabled by chemoreceptors that can sense mucus trails, has also been observed. (Grimm and Paill, 2001; Koztowski, 2007)

The slug’s home range is negatively correlated with population density and can vary with weather conditions. Home ranges of 45 square meters have been noted. (Grimm and Paill, 2001)

Copulation requires intraspecific communication. The slug has two types of sensory organs on its head, an olfactory organ and an eye on the end of each tentacle. The olfactory organ is used to detect mucus trails. (Barker, 2001)

Arion lusitanicus uses it radula to rasp away food particles from decomposing organic matter and vegetation, especially green leafy vegetation. This species has an affinity for plants grown in loamy soil, rather than sandy soil, because of their higher nutrient content. Maize, oilseed rape, sunflowers, flower gardens, house plants, beans, raspberries, and cabbage are just a few of the many plants on which A. lusitanicus will feed. The type of crops it eats depends on the area. For instance, in the Faroe Islands it feeds on rhubarb and potatoes. In gardens in Germany and Sweden it often feeds on strawberries. (Weidema, 2006; Koztowski, 2007)

Arion lusitanicus secretes a large amount of mucus which is used for a variety of functions, such as locomotion. This slime serves as a deterrent for predators as well because of its unappetizing viscosity. (Weidema, 2006)

Arion lusitanicus is beneficial in the carbon cycle because it can consume a large amount of decaying plant material, and even carcasses.

Arion lusitanicus, along with many other gastropods, is often the intermediate host for a variety of parasite species. An example is Angiostrongylus vasorum, the French heartworm, which can sicken or kill wild and domestic canids. Arion lusitanicus has an intricate role in the reproduction and dispersal of the parasite. When the slug feeds on the feces of an infected canid it ingests the nematode's first-stage larvae and acts as the intermediate host. When a healthy canid eats the infected snail the cycle begins again.

That gastropods are easily infected by parasites is sometimes used to control the slug population. Phasmarhabditis hermaphrodita is a parasitic nematode that is often embedded in a gel that can be added to water and sprayed onto the areas with slugs. The larvae will disperse in the infective stage and will burrow into the slug. It will move through the slug and reproduce, and cause the slug to reduce its feeding habits dramatically. In about 1-2 weeks, the slug dies from the infection and the worms are released, thereby becoming available to infect other slugs. The full effects of this treatment on the ecosystem are not known. (Conboy, 2000; Ferdushy, et al., 2010; Wilson, et al., 2010)

This species does not have a positive economic importance for humans.

Arion lusitanicus is considered a crop pest as well as a garden pest. This slug feeds on many different kinds of food (vast range of crop species, garden fruits and vegetables, carcasses, and even sick or dying slugs). They can inflict severe damage to crops and subsequently to the livelihood of the farmers. Depending on the density of the slug population in the area, crops can be decimated. The specific economic consequences of the injury to the wide range of horticultural crops have not been calculated, but in some areas there have been reports that over 50% of the yield has been lost due to the voracious appetite of A. lusitanicus. The majority of molluscicide pellets in the home and garden market of Central Europe is directly ascribed to A. lusitanicus. (Weidema, 2006)

Arion lusitanicus has not been listed as a threatened or endangered species. On the contrary, many efforts have been undertaken to reduce the numbers of this species. Methods include pesticides and the collection and destruction of adults and egg masses. (Weidema, 2006)