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.

The species appears to require environments that have high humidity, warm summer temperatures and acidic soils with no calcium carbonate.

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.

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.

[6] 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.

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.

[7] The Kerry slug's upper tentacles are smoky-black or grey, short and thick with oval ends, and have eye spots at their tips.

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.

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.

Various authors have depicted its reproductive system: Godwin-Austen (1882),[14] Sharff (1891),[13] Simroth (1891, 1894),[3][15] Taylor (1907),[7] Germain (1930),[16] Quick (1960)[17] and Platts & Speight (1988).

[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.

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).

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.

[25][26] 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.

[22] A significant proportion of the Kerry slug's range in Ireland is protected by being included in Special Areas of Conservation (SACs).

[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.

[citation needed] 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.

[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.

Other potential dangers to the species are climate change and air pollution, which negatively affect the lichens eaten by the Kerry slug.

The Habitats Directive protects the Kerry slug outside the SACs by Article 12 (1), which obliges European Union member states to: Conservation status reports from Portugal and Spain were not yet available in August 2009.

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.

[50] 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][51] 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][45] 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.