Arion ater, the black slug (also known as black arion, European black slug, or large black slug), is a large terrestrial gastropod mollusk in the family Arionidae, the round back slugs.
Many land slugs lack external shells, having a vestigial shell."Family summary for Arionidae". AnimalBase, last change 12-06-2009, accessed 23 May 2023. Most slugs retain a remnant of their shell, which is usually internalized, unlike other terrestrial mollusks (such as snails) which have external shells. Without such shells, slugs produce mucus—that may also contain toxins—to deter predators. Terrestrial slugs produce two other forms of mucus that facilitate locomotion and prevent death from drying. Such mollusks are hermaphroditic. Slugs most often function as decomposers but are also often omnivores. Arion ater is one such slug, decomposing organic matter, preying on other organisms, and consuming vegetative matter including agricultural crops. Native to Europe, the black slug is an invasive species in Australia, Canada (British Columbia, Newfoundland, Quebec), and the United States (Pacific Northwest, Alaska).
Subspecies Arion ater ater (Linnaeus, 1758) Arion ater ruber (Garsault, 1764) Arion ater rufus (Linnaeus, 1758)
Description
Arion ater varies from , reaching maturity at about and moves at a speed of up to Stroud, Angela. "Arion Ater." Animal Diversity Web. 2000. http://animaldiversity.org/site/accounts/information/Arion_ater.html .
The black slug is generally deep black, with some adults being brown or even white. Generally, pigmentation darkens directly with increasing latitude. Young specimens tend to be brown or ivory whitish, turning to grey before becoming characteristically black at maturity.Arion ater on Animal diversity web Rust-brown individuals are arguably classified as a separate subspecies Arion ater rufus or species Arion rufus (red slug). The two can only be distinguished by dissecting the reproductive anatomy.
The foot-fringe is black, the tubercles are large and elongate, and the sole is blackish grey. The atrium and vagina (genitalia) are considerably narrower than is the spermatheca (organ for storing sperm). The oviduct is narrow while the spermatheca is spherical.Schultes, F. "AnimalBase :: Arion lusitanicus Species Homepage." AnimalBase :: Arion lusitanicus Species Homepage. 2013. http://www.animalbase.uni-goettingen.de/zooweb/servlet/AnimalBase/home/species?id=1811 .
The black slug is omnivorous, and its diet includes fungi, carrion, earthworms, leaves, stems, dead plant material and dung. The food is shredded into tiny pieces by the radula and is then digested by enzymes.
Like other terrestrial slugs, the black slug is a hermaphrodite, preferring to find a mate—often several—but can self-fertilize. After mating, the black slug seeks a dark, moist environment such as beneath mosses—occasionally within topsoil—to lay its eggs about in diameter. Between August and October, an individual slug lays up to 150 eggs every one to three weeks—clutches diminishing to 20 eggs late in the season. Juveniles hatch after at least twenty-seven days, hatching later under cold temperatures. Maturation takes up to nine months, enabling mating in early summer. Black slugs die shortly after laying its last clutch, rarely surviving into a second year.
The ovotestis (gonad or hermaphrodite gland) produces gametes at the proximal end of the reproductive system. The hermaphrodite duct runs forward, connecting to the albumen gland and the common duct (spermoviduct). The male and female gametes separate before entering the common duct. The male portion of the reproductive tract contains the vas deferens and the epiphallus. Arion slugs lack a penis. The vas deferens connects the spermoviduct to the epiphallus. The epiphallus then opens to the genital atrium. The spermathecal duct (bursa copulatrix duct) is a sac for storing spermatozoa and also enters the atrium. The female portion contains the oviduct, transferring eggs from the common duct to the atrium. The atrium is further subdivided into an upper and lower atrium. A stimulating organ (ligula) can also be found in the atrium. Finally, the atrium opens through the genital pore. The genital pore is located on the animal slightly behind and below the right upper tentacle on the head.
Like other members of the family Arionidae, the black slug has a pneumostome (breathing hole) on the right side of its mantle through which it breathes. This mantle is the part in snails that secretes a shell, but in the black slug, the mantle contains a resilient protective structure of calcareous granules.
Arion ater produces three forms of mucus. The first two aid the animal in locomotion, with a thinner mucus coating the animal laterally, and a thicker, more viscous mucus secreted along the length of the slug. This combination facilitates wave like contractions of the foot that propel the animal forward. The third type of mucus is a vile-tasting substance which helps deter predation. All three forms of mucus help protect the slug from dehydration, as well as to locate and identify other slugs to mate with, or even prey upon.
Distribution This species is found in northern Europe (including Great Britain and Ireland), Canada, the Pacific Northwest, and most recently in Australia and some Caribbean countries:
Great Britain: often found in habitats more wooded and less human-dense than those in which A. ater rufus occurs Maladeta Mountains, Spain: often an agricultural pest Ireland, Italy, Germany,Deutsche Namen für einheimische Schnecken und Muscheln Scandinavia, and Gerês, Portugal Canada [invasive]: first document in 1941, established pest by 1960 British Columbia NewfoundlandChance Cove Provincial Park, Newfoundland Quebec (specifically on Mont Bellevue in Sherbrooke) United States [invasive]: Within its native habitats, this species functions as a decomposer/consumer and often becomes an agricultural pest. This species is well-established in Washington and Oregon, and it is considered to represent a potentially serious threat as a pest, an invasive species which could negatively affect agriculture, natural ecosystems, human health or commerce. Therefore, it has been suggested that this species be given top national quarantine significance in the USA.Cowie R. H., Dillon R. T., Robinson D. G. & Smith J. W. (2009). "Alien non-marine snails and slugs of priority quarantine importance in the United States: A preliminary risk assessment". American Malacological Bulletin 27: 113–132. PDF . Australia [invasive]: The Museum Victoria reports the black slug to have been first documented in Australia in 2001 with multiple reports of these slugs in cultivated gardens and farms since, but as of 2009, the species was still not considered to be an established species in Australia."Black Slugs." Museum Victoria. 05 Apr. 2009. Web.
Ecology
The black slug is mainly nocturnal and avoids exposure to sunlight, although in introduced areas, it has been observed to be active all parts of the day and night. It is omnivorous,Svart skogssnigel – Arion ater at Natural history museum of Gothenburg eating carrion, fungi, animal feces, algae, lichen, and vegetation (living or decaying). These slugs prefer to eat on cloudy days or at night when temperatures are above 10 degrees Celsius. Mucus-production necessitates moist habitats, such as among leaf-litter, mosses, or fallen trees. Consequently, the black slug becomes most active after rainfall. A 1976 study compared black slug assimilation rates to that of similar slugs in the UK. The study determined the black slug feeds at similar rates to other European slugs (in terms of biomass), suggesting the black slug is an equally important decomposer/consumer in forest systems. The study theorized these high assimilation rates might result from abundant enzymes in slug digestive systems: amylase, invertase, cellulase, xylanase, and chitinase. The last might be especially indicative of how these slugs can consume large quantities of fungi such as Armillaria mellea.Jennings, T. J., and J. P. Barkham. "Quantitative Study of Feeding in Woodland by the Slug Arion Ater." Oikos 27.1 (1976): 168. Web. Microbiota within black slug digestive systems produce these enzymes—such as the bacteria that produce cellulase—enabling the slugs to break down lignocellulose. A recent study further explored Arion ater’s gut microbiome and discovered the black slug to have far more bacteria within its gut than did neighboring, northern European insects, including members of the orders Coleoptera, Isoptera, Orthoptera, and Diptera. This study learned the black slug’s gut microbiome to be functional at a wide range of temperatures and pH levels, and its results suggest that the mycobacterial culture may be uniform throughout the black slug’s gut and that Gammaproteobacteria composes most of this culture. This study contrasted its results to studies of North American black slugs that presented higher levels of cellulolytic activity at a narrower pH range. In summary, Arion ater can digest diverse food material such as cellulose because gut bacteria produce digestive enzymes.Joynson, Ryan, Arvind Swamy, Paz Aranega Bou, Ambre Chapuis, and Natalie Ferry. "Characterization of Cellulolytic Activity in the Gut of the Terrestrial Land Slug Arion Ater: Biochemical Identification of Targets for Intensive Study." Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 177–178 (2014): 29–35. Web.
As decomposers/consumers, slugs engage an important niche, contributing to ecosystem health. A 2005 study looked at another Arion slug species and concluded slugs may promote plant biodiversity. This study suggested that slugs promote plant—especially forb—species diversity and that slugs support annual plant species’ fitness, reducing rates of successional change.Buschmann, H., M. Keller, N. Porret, H. Dietz, and P. J. Edwards. "The Effect of Slug Grazing on Vegetation Development and Plant Species Diversity in an Experimental Grassland." Functional Ecology Functional Ecology 19.2 (2005): 291–98. Web. Additionally, slugs help disperse seeds and spores through their waste, and slugs facilitate nutrient cycling by being an omnivorous consumer (especially a decomposer) and by leaving behind their mucus, which also facilitates decomposition.
Slugs are common agricultural and horticultural pests. They consume abundant vegetative matter, especially seedlings.
Most animals prefer not to prey upon the black slugs because of the taste of its mucus and because this mucus can make them slippery and consequently difficult to capture; however, this slug does have some natural predators, including the hedgehog, badger, shrew, mole, mouse, frog, toad, snake, carnivorous beetle, and some birds.Nickel, June 1998. The Slimy, Yet Special Slug. Natural History, 107:18. When picked up or touched, the black slug will contract to a hemispherical shape and begin to rock from side to side. This defensive behavior confuses predators, and is unique in the family Arionidae.The large black slug
Invasive species
Arion ater has been introduced to southeastern Australia and to North America, where it occurs in Newfoundland, British Columbia, the Pacific Northwest of the United States and some parts of Alaska. In the past two decades, its mating with the non-native (at northern latitudes) pest species Arion vulgaris (or Spanish slug) has resulted in a more resilient hybrid exhibiting increased tolerance to cold.