Kerry slug

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.[7]

Authors have differed in their depictions of the Kerry slug's shell plate but they are consistent in showing it as a solid plate.

The internal shell as drawn by Taylor in 1907

The internal shell as drawn by Godwin-Austen in 1882 Photograph of internal shell of G. maculosus (dorsal view).

Anatomy viewed from below with frontal edge of mantle on the top. Heart (red) is surrounded by renal organ (yellow). Retractor muscles (blue) include retractor muscle of left eye tentacle (depicted on the right), retractor muscle of right eye tentacle and retractor muscle of odontophore (both on the left).

The cerebral ganglia, a central processing area which is equivalent to the brain within the nervous system of the slug

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.[7]

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.[7]

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.[7]

The Kerry slug is a hermaphrodite, as are all pulmonates. Various authors have depicted its reproductive system: Godwin-Austen (1882),[14] Sharff (1891),[13] Simroth (1891, 1894),[4][15] Taylor (1907),[7] Germain (1930),[16] Quick (1960)[17] and Platts & Speight (1988).[18] Platts & Speight [18] considered the depiction by Godwin-Austen (1882)[14] 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.[7] 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.[7]

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.[18] 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.[19] It is presumed that the bursa retractor muscle retracts the atrial diverticulum.[19]

In Geomalacus maculosus, the atrial diverticulum is longer than the bursa duct; this situation is reversed in Geomalacus anguiformis.[18]

Godwin-Austen[20] 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 reproductive system illustrated by Godwin-Austen (1882).[14] The large mass on the right is albumen gland, the mass on the lower part right is the ovotestis, the oval shape at the left is the bursa copulatrix. Mantle edge and atrium is at the top.

Drawing of "male part" of the reproductive system. From lower left to upper left: genital pore, atrium, atrial diverticulum, bursa duct, bursa retractor muscle, bursa copulatrix. On the top right: epiphallus. Center: vas deferens. Lower right: free oviduct, spermoviduct.

The radula, which is located inside the mouth, is a feeding structure that is unique to molluscs. Typically, it is a small, strong, ribbon-like structure that bears numerous complex rows of tiny teeth across it.

In the Kerry slug, the radula is 8 mm (5/16 in) long and 2 mm (1/16 in) wide, and has 240 slightly curved, transverse rows of denticles; tiny teeth. Each row of teeth is composed of one median tooth and 10 lateral and marginal teeth on each side. The median teeth are small, have one cusp and are slightly shouldered. The lateral teeth have two cusps. The admedian (next to the middle) teeth are larger than the median row and the mesocone—an extra protrusion in the middle of the tooth—is well developed. The only difference between the lateral and marginal series is that the ectocone (extra little side protrusion) present on the admedian teeth recedes in position and slightly diminishes in size in the succeeding teeth up to about the 20th row on the radula. In the marginal series, however, the ectocone gradually grows in size and importance as the margin is approached while the mesocone becomes almost correspondingly diminished. The outermost teeth show a more embryonic character.[7]

One complete row of teeth in the radula

The jaw, which has broad ribs.

The jaw of the Kerry slug is about 1 mm (1/32 in) from side to side and is distinctly arched from front to rear, crescent-shaped and very wide with broad and slightly rounded ends. The jaw is solid, dark-brown and has about 10 broad flat ribs in the middle part of the jaw. These ribs are absent or scarcely discernible on the side areas. Where the ribs meet the upper edge, they sometimes form crenulations ( a scalloped effect) and may also produce the same effect on the lower edge of the jaw. In other individuals, the ribs extend across the jaw, making both the upper and the cutting edges of the jaw clearly toothed in outline.[7]

In the Kerry slug, as in all species within the family Arionidae, the alimentary canal of the digestive system forms two loops.[21]

Within Ireland, the Kerry slug is known to occur in areas with sandstone geology in West Cork and County Kerry,[24] an area of around 5,800 km² (2,200 sq mi).[10] In 2010, a previously unknown population was recorded further north in County Galway.[22]

A significant proportion of the Kerry slug's range in Ireland is protected by being included in Special Areas of Conservation (SACs). In response to European environmental legislation, Ireland has designated seven SACs with the slug named as a "selection feature": Glengarriff Harbour and Woodland; Caha Mountains; Sheep's Head; Killarney National Park, MacGillycuddy's Reeks and Caragh River Catchment; Lough Yganavan and Lough Nambrackdarrig; Cloonee and Inchiquin Loughs, Uragh Wood and Blackwater River (Kerry).[28] In addition, St. Gobnet's Wood SAC (which was designated in relation to other selection criteria) was expanded in 2008 to protect Cascade Wood, a small area of woodland which is inhabited by the slug.[29] The species has also been recorded at other SACs where it is not a selection feature, for example in Derryclogher Bog, County Cork.[30]

Despite its first discovery at Caragh Lake and its English common name of Kerry slug, Ireland is at the periphery of this slug species' distribution; in terms of genetic diversity the distribution is centred on the north-western parts of the Iberian peninsula.[27][31] The Kerry slug has been known in northern Spain since 1868 and in northern Portugal since 1873.[24]

The southernmost locality where this species is found is the mountain range Serra da Estrela in Portugal. It is also found in the provinces Beira Alta, Douro Litoral, Minho, Trás-os-Montes e Alto Douro and in the Peneda-Gerês National Park.[23][32]

In Spain, the distribution of this species includes coastal locations in Galicia and extends through the Cantabrian Mountains as far east as Mount Ganekogorta in the Basque Country. These localities fall within the boundaries of various autonomous communities: Galicia, Asturias, Cantabria, Castile and León (provinces of León, Palencia and Zamora), and the Basque Country provinces of Biscay and Álava.[23][32] There have been unconfirmed findings of this slug from Navarra.[32]

Natura 2000 sites for this species in Spain include 48 localities (listed below, grouped by region). As at 2017, some of these sites have yet to be designated as Special Areas of Conservation:[28]

• Asturias

Muniellos; Ponga—Amieva; Redes;

• Cantabria

Camesa river; Liebana (Special Area of Conservation; Liébana (Special Protection Area); "Upper valleys of the Nansa and Saja and Alto Campoo");

• Castile and León

Hoces de Vegacervera; Lake Sanabria and its vicinities; Montes Aquilanos (Site of Community Importance); Montes Aquilanos y Sierra de Teleno (SPA);

Natural Park of Fuentes Carrionas and Fuente Cobre-Montaña Palentina (SAC); Sierra de la Cabrera (SCI partially overlapping with a SPA of the same name).

• Galicia

A Marronda; Anllóns river; Baixa Limia; Baixa Limia - Serra do Xurés; Baixo Miño; Bidueiral de Montederramo; Carballido, a yew wood in A Fonsagrada; Carnota – Monte Pindo; Cíes Islands; Costa Ártabra; Costa da Morte—Costa da Morte and Costa da Morte (Northern); Cruzul-Agüeira; Encoro de Abegondo-Cecebre; Eo river is included among the Galician sites although the estuary forms the boundary with Asturias; Costa de Ferrolterra-Valdoviño; Fragas do Eume; Macizo Central, Ourense (province); Monte Aloia; Monte Maior; Negueira; Pena Trevinca; Pena Veidosa; Serra do Candán; Serra do Cando; Serra do Xistral; Sil river canyon; Sobreirais do Arnego; Tambre - two areas, the river and its estuary; Támega river; Ulla-Deza river system

• More than one region

Ancares - This district is divided between Galicia and Castile and León. Sierra de los Ancares is a mountain range that forms the boundary between the two autonomous communities and gives its name to a Natura 2000 site in the province of León.[33] On the Galician side of the sierra are two relevant sites—Ancares (protected under the Birds Directive) and Ancares-Courel (protected under the Habitats Directive).[34][35]

Picos de Europa - This mountain range is divided between three autonomous communities. The three sites listed are Picos de Europa, Picos de Europa (Asturias), and Picos de Europa en Castilla y León, all of which include protected areas in the Picos de Europa National Park and in a regional park in Castile and Leon that is also called Picos de Europa.

It was once thought that Geomalacus maculosus lives only in wild habitats.[24] 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.[37] In Ireland, it also occurs in upland conifer plantations and areas of clear-fell.[38] The Kerry slug is not considered an agricultural pest,[24] 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.[24][38] Although there was a geographical association with sandstone areas, the new locality in Galway is on granite.[38] In Iberia it usually occurs in granite mountains,[36] and on slates, quartzite, schists, gneiss and serpentine.[39] The best predictor of its occurrence is high rainfall and high summer temperatures.[37]

The food of Geomalacus maculosus includes lichens, liverworts, mosses, fungi (Fistulina hepatica)[18] and bacteria that grow on boulders and on tree trunks.[23][24][40] In captivity, the Kerry slug has been fed on porridge, bread, dandelion leaves, lichen Cladonia fimbriata, carrot, cabbage, cucumber and lettuce.[10][7] It can be carnivorous in captivity; there are records of it consuming the snail Vitrina pellucida.[7]

A cluster of eggs deposited in captivity

The Kerry slug mates in head-to-head position with partners' genital openings facing each other.[18] The sexual organs, called atria—singular:atrium—are funnel-shaped with fluted edges after mating.[18] As in Arion, sperm is transferred in a spermatophore.[41] In the wild, eggs are laid between July and October,[10] and from February to October in captivity.[40] Self-fertilisation is also possible in this species.[10] Eggs are laid in clusters of 18 to 30,[10] 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).[40]

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,[42] 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[7] or black.[40]

The young appear to hatch in six[40] 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.[7] 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).[10] Juveniles reach maturity in two years, at a length about 2.6 cm (1.0 in).[10][40] In the wild, the Kerry slug can live for up to seven years[10] but in captivity, the lifespan rarely exceeds three years.[40] In numerous localities in Spain, very few individuals of the species were observed at any one time.[36]

Until 2014, the natural enemies of Geomalacus maculosus were not known.[43] The Kerry slug's predators include larvae of the third instar of the fly Tetanocera elata.[43]

Because of its perceived rarity and its restricted distribution, the Kerry slug is protected under the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention), EIS Bern Invertebrates Project. This decision was backed by studies of its distribution and ecology in Ireland,[18] which concluded that evidence of a decline in Iberia and uncertainty over its status in Ireland tended to support its inclusion in the Convention. Since 2006, Geomalacus maculosus has been considered a least concern species in the IUCN Red List;[1] between 1994 and 2006, however, the slug was rated as vulnerable.[24]

Geomalacus maculosus is also protected by the European Union's Habitats Directive and has been listed as an Annex II and Annex IV species since 1992.[45] The principal mechanisms used by the Directive to protect habitats and species are the creation of Special Areas of Conservation (SACs) and the protection of species independently of their habitats by other means. Seven SACs have been designated for this species in Ireland and 49 SCIs in Spain.[28] Threats to the Kerry slug will probably be greatest in areas not specifically protected as SACs. The Habitats Directive protects the Kerry slug outside the SACs by Article 12 (1), which obliges European Union member states to:

• establish ‘a system of strict protection’ for listed species
• prohibit deliberate capture or killing
• prohibit ‘deliberate disturbance … particularly during the period of breeding, rearing, hibernation and migration’
• prohibit ‘deliberate destruction or taking of eggs from the wild’
• prohibit the deliberate or non-deliberate ‘deterioration or destruction of breeding sites or resting places’.

Conservation status reports from Portugal and Spain were not yet available in August 2009.[46] Its conservation status in Spain for the IUCN criteria is vulnerable.[32]

In 1988, Platts and Speight noted that only three of the Irish sites where the slug occurred were protected; Glengariff Forest, West Cork; Uragh Wood Nature Reserve, South Kerry; and Killarney National Park, North Kerry. They concluded that the species could not be adequately safeguarded with only three sites and supported its inclusion in the Bern list, to which the Irish government is a signatory.[18] The Habitats Directive was transposed into Irish law by:

• The EC (Natural Habitats) Regulations 1997.[47] This was the principal legislation transposing the Habitats Directive and upgraded the protection of the Kerry slug's habitat by the designation of Special Areas of Conservation.
• Adapting existing legislation. The Kerry slug has been protected since 1990 under the Irish Wildlife Act of 1976; it was added to the list of protected species by Statutory Instrument 112/1990, and was the only gastropod so protected.[45][48] The Wildlife Act does not protect the slug from indirect damage but only from wilful direct damage such as collecting.

The Irish National Parks and Wildlife Service published a Species Action Plan for the Kerry slug in January 2008.[24] Efforts were made to protect the slug from indirect damage, for instance from commercial forestry.[49] Following a legal challenge to Ireland's transposition and implementation of the Habitats Directive, however, the Action Plan was superseded in May 2010 by a Threat Response Plan that addressed problems that arose when the European Court of Justice held that Ireland was not protecting the Kerry slug with the strictness the directive required for a species listed in annex 4.[30][50]

In a report to the European Commission covering 1988–2007, the conservation status of the species in Ireland was declared "favourable (FV)" in all evaluated criteria; range, population, habitat and future prospects.[10][44] The validity of this assessment, however, was put into question by the European Court of Justice ruling that held that Ireland was not monitoring the slug properly.[25]

The need to improve monitoring was discussed by the NPWS Threat Response Plan of 2010, which recognised that population statistics were still deficient, particularly outside the SACs. As the Threat Response Plan noted, species monitoring is a process in which distribution and status of the subject are evaluated systematically over time. Under this definition, no monitoring of the Kerry Slug had been undertaken in Ireland as of May 2010.[30] The Kerry Slug Survey of Ireland, a collaboration between the National Parks and Wildlife Service and the Applied Ecology Unit at the National University of Ireland, Galway, researched a "suitable monitoring protocol" for the species.[29][51] The Kerry Slug Survey's investigations resulted in the publication of a guide to the population dynamics of the Kerry slug, which was published as part of the Irish Wildlife Manual series in 2011.[52]

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.[40] 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.[40]