Kerry spotted slug
The Kerry slug or Kerry spotted slug (Geomalacus maculosus ) is a species of terrestrial, pulmonate, gastropod mollusc. It is a medium-to-large sized, air-breathing land slug in the family of roundback slugs, Arionidae.
Adult Kerry slugs generally measure 7–8 cm (2.8–3.1 in) in length; they are dark-grey or brown with yellowish spots. The internal anatomy of the slug has some unusual features and some characteristic differences from the genus Arion , also part of Arionidae. The Kerry slug was described in 1843—later than many other relatively large land gastropods present in Ireland and Great Britain—an indication of its restricted distribution and secretive habits.
Although the distribution of this slug species includes south-western Ireland—including County Kerry—the species is more widespread in north-western Spain and central-to-northern Portugal. Given that the slug has thus far been recorded exclusively at locations in Ireland and north-western Iberia, it can be said to tentatively possess a Lusitanian distribution. The species appears to require environments that have high humidity, warm summer temperatures and acidic soils with no calcium carbonate. The slug is mostly nocturnal or crepuscular but in Ireland it is active on overcast days. It feeds on lichens, liverworts, mosses and fungi, which grow on boulders and tree trunks.
The Kerry slug is protected by conservation laws in the three countries in which it occurs. It is now known to be less dependent on sensitive, wild habitats than when these laws were introduced. Attempts have been made to establish breeding populations in captivity to ensure the survival of this slug species but these have been only partly successful.
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starts withThe body length of adult Kerry slugs is 7–8 cm (2.8–3.1 in). These slugs are difficult to measure accurately because of their unusual startle response. Kerry slugs can also elongate themselves within crevices up to 12 cm (4.7 in). Official measurements of this species vary; Kerney et al. (1983) give a range of measurements of 6–9 cm (2.4–3.5 in). The body of a fixed (preserved) adult specimen was 7 cm (2.8 in) long with a mantle length of 3 cm (1.2 in).
The body of the Kerry slug is glossy and is covered on both sides with about 25 longitudinal rows of polygonal granulations. The slugs have two colour morphs, brown and black. In Ireland the black morph occurs in open habitats and the brown morph occurs in woodland; this correlates with the colours of the surroundings, suggesting camouflage. Experiments indicate the dark colouration is induced by exposure to light as the slug develops. There is also variation in banding; on each side of the body there can be two bands: one band just below the summit of the back and the other band further down the side of the body. When these bands are present they usually extend the whole length of the body and are overspread by numerous, ovoid yellow spots that are distributed approximately in five longitudinal zones.
Behind the animal's head is the shield-shaped outer surface of the mantle, which is about a third of the length of the body when the slug is actively crawling and thus extended; when the slug is stationary and contracted, the shield is about half the length of the body. The front of the shield is rounded and its rear is bluntly pointed. The surface texture of this area resembles the underside of undyed leather; it is spotted with pale, buff or light-coloured spots that are similar to those on the body but are more uniformly distributed.
The foot fringe, a band of tissue around the edge of the foot, is not distinctly separated; it is very pale and somewhat expanded and has indistinct lines on it. The sole of the foot is pale grey-yellow and is divided into three indistinct bands; the mid-area is somewhat darker and more transparent than the side bands. There is a caudal mucous pit situated between the foot and the body on the upper surface of the tip of the tail. The pit, which collects extra mucus, is not conspicuous, triangular and opens transversely. The mucous pit often carries a transparent, yellowish ball of mucus.
The Kerry slug's upper tentacles are smoky-black or grey, short and thick with oval ends, and have eye spots at their tips. The genital pore or opening lies behind and below the right eye tentacle. The lower tentacles are pale-grey and translucent. The skin mucus is usually pale yellow and varies in viscosity. The locomotory mucus is tenacious and usually colourless but is sometimes yellow because of mixing with body slime.
The Kerry slug has a discontinuous or disjunct distribution; it is found only in Ireland—mostly the south-western corner— in north-western Spain, and central-to-northern Portugal. It was once reported as occurring in France but this has not been confirmed and that record is considered suspect. Similar distribution patterns have been observed in other species of animals and plants. This particular disjunct distribution in Iberia and Ireland with no intermediate localities is known as a "Lusitanian distribution".
There has been speculation that G. maculosus was introduced to Ireland from Iberia by prehistoric humans; a similar introduction appears to have happened with the Eurasian pygmy shrew. In support of such an origin or of a more recent human-mediated introduction, the genetic diversity of the Kerry slug in Ireland was found to be greatly reduced compared with that of the Iberian populations.
It was once thought that Geomalacus maculosus lives only in wild habitats. In the Iberian Peninsula, it occurs on tree trunks in oak (Quercus ) and chestnut (Castanea ) forest but it is easiest to find in synanthropic habitats such as rocky walls in oak or chestnut orchards, in ruins, near houses, churches and cemeteries. In Ireland, it also occurs in upland conifer plantations and areas of clear-fell. The Kerry slug is not considered an agricultural pest, unlike some other slugs in the family Arionidae.
In Ireland, the Kerry slug occurs in woodland with oak trees, oligotrophic open moorland, blanket bogs and lake shores, especially if boulders covered with lichens and mosses are present in these habitats. Although there was a geographical association with sandstone areas, the new locality in Galway is on granite. In Iberia it usually occurs in granite mountains, and on slates, quartzite, schists,gneiss and serpentine. The best predictor of its occurrence is high rainfall and high summer temperatures.
The Kerry slug is primarily nocturnal. During daylight hours, the slug usually hides in crevices of rocks and under loose bark on trees. In Iberia, juvenile Kerry slugs become active during twilight and adults become active at night, especially on rainy or very humid nights. Because Ireland is much further north and has a considerably cooler, wetter and more humid climate, the Kerry slug is sometimes active there in the daytime if the weather is humid and overcast.
The species has in unusual defensive behaviour; whereas most land slugs retract the head and contract the body but stay firmly attached to the substrate when they are attacked or threatened, the Kerry slug retracts its head, lets go of the substrate and rolls itself into a ball-like shape. This is behaviour is unique among species in Arionidae and among slugs in Ireland.
The food of Geomalacus maculosus includes lichens, liverworts, mosses, fungi (Fistulina hepatica ) and bacteria that grow on boulders and on tree trunks. In captivity, the Kerry slug has been fed on porridge, bread, dandelion leaves, lichen Cladonia fimbriata, carrot, cabbage, cucumber and lettuce. It can be carnivorous in captivity; there are records of it consuming the snail Vitrina pellucida.
The Kerry slug is a hermaphrodite, as are all pulmonates. Various authors have depicted its reproductive system: Godwin-Austen (1882), Sharff (1891), Simroth (1891, 1894), Taylor (1907), Germain (1930), Quick (1960) and Platts & Speight (1988). Platts & Speight considered the depiction by Godwin-Austen (1882) to be the most accurate of those by earlier authors; others depicted the atrium too short.
The ovotestis—a combination of ovary and testis—is small, compact and darkly pigmented. The hermaphroditic duct, where sperm is stored, is long and convoluted, and ends in a small, spherical, seminal vesicle. The albumen gland, which produces albumen for the eggs, is elongated and shaped like a tongue. The ovispermatoduct, along which both eggs and sperm pass, is greatly twisted. This turns into the free oviduct after the vas deferens carrying the sperm branches off. The free oviduct is long and consistently thin. It opens into the atrium near the genital pore, where the muscular atrium is greatly but irregularly enlarged and connected by muscle fibres to the oviduct.
The vas deferens is long, complexly twisted, and rolled in a bundle. The bursa copulatrix for digesting spermatophore and sperm—earlier literature refers to this as the spermatheca—is globular and has a short bursa duct. There is a long retractor muscle from the bursa duct, its other end is anchored near the tail of the slug at the midline. The vas deferens and the bursa duct open nearly together into the far extremity of the atrium, the duct into which both the male and the female systems open and which connects to the outside via the genital pore. A special feature of the genus Geomalacus, is the extremely elongated atrium. The elongated portion of the atrium further from the genital pore than the insertion of the oviduct is termed the atrial diverticulum. In Geomalacus, the penis and its penial retractor muscle have been lost. The atrial diverticulum has been proposed to be the functional equivalent, homoplasy) of a penis, acting as a copulatory organ. It is presumed that the bursa retractor muscle retracts the atrial diverticulum.
In Geomalacus maculosus, the atrial diverticulum is longer than the bursa duct; this situation is reversed in Geomalacus anguiformis.
Godwin-Austen noted that the part of the atrium just inside the genital pore—he called this region the "vagina"—has "a curious arrangement" of flattened folds. The central part, situated close to the genital pore, has a pointed end. He compared this to the calcareous darts in other genera; on the preceding pages he had described such structures in the Asian slug genus Anadenus).
The Kerry slug mates in head-to-head position with partners' genital openings facing each other. The sexual organs, called atria—singular:atrium—are funnel-shaped with fluted edges after mating. As in Arion, sperm is transferred in a spermatophore. In the wild, eggs are laid between July and October, and from February to October in captivity. Self-fertilisation is also possible in this species. Eggs are laid in clusters of 18 to 30, and are held together by a film of mucus. The egg masses are about 3.5 cm × 2 cm (1.38 by 0.79 inches).
The eggs are very large compared with the size of the animal. The largest eggs are more elongate, being 8.5 mm × 4.25 mm (0.335 by 0.167 inches); the smallest are more ovoid and are 6 mm × 3 mm (0.24 in × 0.12 in). All are semi-translucent, milky-white or opalescent when fresh, although some of the larger and more elongate eggs have a semi-transparent area at the smaller end. The opalescent lustre disappears in a few days and the eggs turn yellowish and later brown or black.
The young appear to hatch in six to eight weeks, at this stage the spots on the body are barely present. The lateral bands are distinct and black, and are more conspicuous than they are in mature slugs of this species. In juveniles the shield shows lyre-shaped markings, as is the case in slugs of the genus Arion. These lyre-shaped markings become indistinct as the slugs grow larger. The Kerry slug probably overwinters in the sexually immature stage. The bodies of preserved juvenile specimens are up to 3 cm (1.2 in) long with a mantle length of 10 mm (0.39 in). Juveniles reach maturity in two years, at a length about 2.6 cm (1.0 in). In the wild, the Kerry slug can live for up to seven years but in captivity, the lifespan rarely exceeds three years. In numerous localities in Spain, very few individuals of the species were observed at any one time.
Until 2014, the natural enemies of Geomalacus maculosus were not known. The Kerry slug's predators include larvae of the third instar of the fly Tetanocera elata.
Since 1990, the Kerry slug has been successfully bred in captivity. The Wildfowl & Wetlands Trust, a British conservation organisation, operates a captive breeding programme in terraria at its "Endangered Species Breeding Unit". The project is located at the Martin Mere Wetland Centre in Lancashire, England. During the 1990s, slugs from the breeding programme were given out to a number of zoos and individuals to set up their own breeding programmes but very few of those breeding groups survived.
The most serious threat to the Kerry slug is probably the modification of habitat, which reduces its lichen and moss food sources. This can lead to the local disappearance of the species, which was documented in Spain. Other threats include intensification of land use, land reclamation, use of pesticides, overgrazing by sheep, removal of shrubs, tourism, general development pressures, planting of conifer plantations, the spread of invasive plants such as Rhododendron ponticum and habitat fragmentation (see also Moorkens 2006).
Other potential dangers to the species are climate change and air pollution, which negatively affect the lichens eaten by the Kerry slug. Climate change will probably affect the Iberian populations more acutely because the climate there is already hot and dry relative to that of Ireland, which is generally cool and damp.
