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 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.
Taxonomy and etymology The Kerry slug is a gastropod, a class of molluscs that includes all snails and slugs, including terrestrial, freshwater and marine species. The Kerry slug, a member of the order Panpulmonata, is terrestrial; it breathes air with a lung. It is in the clade Stylommatophora, members of which have two sets of retractable tentacles, the upper pair of which have eyes on their tips. Its family is Arionidae, the round-backed slugs. The Kerry slug has no keel on its back, unlike the slugs in the families Limacidae and Milacidae. Many of its anatomical features are shared with species in the genus Arion, which is a more species-rich and widely distributed group of slugs within Arionidae. The Kerry slug is placed in the genus Geomalacus, which means "earth mollusc".
The Kerry slug's scientific name is Geomalacus maculosus, where maculosus means "spotted" from the Latin word macula, a spot. The English-language common name is derived from County Kerry in the south-west of Ireland, where the type specimens that were used for the formal scientific description were collected. In 1842, a Dublin-based naturalist William Andrews (1802–1880) sent specimens he had found at Caragh Lake in County Kerry to the Irish biologist George James Allman. The next year, Allman exhibited them at the Dublin Natural History Society and published a formal description of the new species and genus in the London literary magazine The Athenaeum. The full scientific name, including the taxonomic authority, is Geomalacus maculosus Allman, 1843. The synonyms are other binomial names that were given over time to this taxon by authors who were unaware that the specimens they were describing belonged to a species already described by Allman.
The species' binomial name is sometimes written as Geomalacus (Geomalacus) maculosus because the genus Geomalacus contains two subgenera; the nominate subgenus (subgenus of the same name) Geomalacus and a second subgenus Arrudia Pollonera, 1890. The subgenus Geomalacus contains only one species, the Kerry slug; three species comprise Arrudia. The Kerry slug has been included in molecular phylogenetics studies since 2001.
Description
The body length of adult Kerry slugs is . These slugs are difficult to measure accurately because of their unusual startle response. Kerry slugs can also elongate themselves within crevices up to . Official measurements of this species vary; Kerney et al. (1983) give a range of measurements of . The body of a fixed (preserved) adult specimen was long with a mantle length of .
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.Scharff, R. F. (1891). The slugs of Ireland . The Scientific Transactions of the Royal Dublin Society, volume IV., series II. Dublin, Royal Dublin Society; London, Williams & Norgate. 513–563. Cited pages: 551 –556. 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.
Internal anatomy Shell Within the mantle, most land slugs have the remnants of what was in the evolutionary past a larger, external shell. Usually this remnant is either a small, thin, shell-like plate or a collection of calcareous (chalky) granules. The Kerry slug has an internal shell or shell plate that resembles those found in land slugs of the genus Limax; it is ovoid, solid and chalky with a transparent conchiolin (horny) base. The shell plate is usually convex above and concave beneath and has some indistinct, concentric lines of growth. According to Godwin-Austen, the exterior of the shell plate is covered with a thin, transparent protein layer called the periostracum and with the nucleus—the first part to form—situated near the front. In young Kerry slugs the shell is very thin and convex, abruptly cut off behind, and with an extremely thin layer that projects in front and contains minute granules.
Authors have differed in their depictions of the Kerry slug's shell plate but they are consistent in showing it as a solid plate.
Various organ systems
The circulatory and excretory systems of the Kerry slug are closely related; the heart is surrounded by the triangular kidney, which has a lamellate (layered) structure and two ureters. In this species, the ventricle of the heart is directed towards, and is very close to, the anal and respiratory openings. The ventricle of the heart is further away and further back than it is in species of the related genus Arion, the type-genus of the family Arionidae.
The gland above the foot, the suprapedal gland, is deeply imbedded in the tissues and reaches far back. The cephalic (head) gland known as Semper's organ is well developed and shows as two strong, flattened lobes. The salivary and digestive glands are the same as those found in Arion species but the vestigial osphradium (kidney-like structure) within the mantle chamber is more distinct than it is in Arion species.
Muscles In the Kerry slug, the cephalic retractors (muscles for pulling in the head) are very similar to those in Arion species. The right and left tentacular muscles, which pull in all four of the tentacles, divide early for the upper and lower tentacles but only the muscles of the ommatophores—the two upper tentacles, which have eye spots—are darkly pigmented. The right and left muscles that pull in the eye-spot tentacles are attached at the base to the back edge of the mantle on the right and left respectively. The pharyngeal (throat) retractor muscle is furcate (split) where it attaches to the back of the buccal bulb (mouth bulb); its other end is anchored on the right side of the body, just behind the site of attachment of the right tentacular muscle.
Reproductive system
The Kerry slug is a hermaphrodite, as are all pulmonates. Various authors have depicted its reproductive system: Godwin-Austen (1882),Godwin-Austen, H. H. (1882). Land and freshwater mollusca of India, including South Arabia, Baluchistan, Afghanistan, Kashmir, Nepal, Burma, Pegu, Tenasserim, Malaya Peninsula, Ceylon and other islands of the Indian Ocean; Supplementary to Masers Theobald and Hanley's Conchologica Indica. Plates to Volume I. Taylor and Francis, London. Plate XII , figure 5. Sharff (1891), Simroth (1891, 1894), Simroth, H. (1891). "Die nacktschnecken der portugiesisch-azorischen fauna in ihrem Verhältnis zu denen der paläarktischen region überhaupt" . Nova Acta Academiae Caesareae Leopoldino-Carolinae Germanicae Naturae Curiosorum 56(1): 201–424, Tab. IX-XVIII. Halle. Geomalacus maculosus is on page 351 –355. Taylor (1907), Germain (1930), Germain L. (1930). Mollusques terrestres et fluviatiles. Première partie. Faune Fr., lechevalier, Paris, 21: 477 pp. Quick (1960)Quick H. E. (1960). "British slugs (Pulmonata; Testacellidae, Arionidae, Limacidae". Bulletin of the British Museum (Natural History). Zoology 6(3): 103 -226. 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.