Arion transsylvanus

Arion transsylvanus View in CoL

On the first collecting trip in November, we collected a single individual, a juvenile, which only later was identified as A. transsylvanus on the basis of its 16S mtDNA sequence.

The 455-bp sequence was a close match (0.2–1.7%) to the 14 A. transsylvanus 16S haplotypes generated by Jordaens et al. (2010). In particular the Hungarian sequence differed by a substitution of just 1 bp from the 402-bp haplotype AY860806.1, from an animal collected in eastern Poland. In comparison, Jordaens et al. (2010) reported a difference of 15% to the other species in the A. subfuscus species complex.

The juvenile Hungarian specimen was 45 mm when fully extended. The colouration was a particularly vivid orange, especially on the back ( Fig. 7 View Figure 7 ). This is not diagnostic since Jordaens et al. (2010) documented wide and overlapping ranges of colouration in this species and its close relatives. One difference from their description of A. transsylvanus is that the foot fringe of our slug was light without dark stripes; this might reflect its juvenile state. The mucus was sticky, but not so much as that of Arion vulgaris , for instance. After preservation our specimen was approximately half as long as when living ( 25 mm after preservation). The body had become light brown with a pinkish tinge, and the dark markings showed up more conspicuously ( Fig. 8 View Figure 8 ).

The single specimen proved to be too juvenile to conclude anything about species identity from the genital anatomy.

The site at which we found A. transsylvanus in Hungary is on the plateau of the Bükk Mountains ( Fig. 5B View Figure 5 , Table 1) at a relatively high altitude (above 800 m a.s.l. compared to the highest point of the country: 1014 m, Kékestető). It was found in a natural habitat in a beech forest. Although forestry activity takes place in the area, the locality is fairly undisturbed. The specimen was found under deep leaf litter in the stump of a beech tree ( Fig. 6B View Figure 6 ) .

After the molecular data revealed the presence of A. transsylvanus , we ( RF & BPG) returned to the original site in May 2024 to collect more specimens. All four found were juveniles and the single example sequenced had the COI mtDNA sequence of A. fuscus .

Discussion

The small size of Arion intermedius makes it is easy to overlook or to casually dismiss as a juvenile of another species. Nevertheless it is a species that can often be reliably identified on the basis of external characters, particularly the prickly appearance and the tiny dots along the foot fringe. However, these characters are not always apparent, and in particular preserved slugs often look smooth; in those cases, dissection or sequencing is needed for reliable identification.

Within Hungary, the possibility of misidentification is perhaps greatest with Arion obesoductus Reischütz, 1973 formerly referred to as Arion alpinus Pollonera, 1887 . Many populations from Central Europe and the Eastern Alps turned out to be A. obesoductus , and A. alpinus was synonymised with A. intermedius ( Manganelli et al. 2010) . Although A. obesoductus has not been recorded from Hungary, it is not improbable that it does or will occur. Arion obesoductus is known from Czechia ( Dvořák et al. 2006), eastern Germany ( Schniebs & Reise 1997) and Austria ( Duda et al. 2019). More brownish colour morphs of A. intermedius resemble A. obesoductus , although the latter lacks the prickly appearance and dots along the foot sole. Internally, A. intermedius has a short, thin and tubular oviduct, whilst that of A. obesoductus is longer and wider. Also the bursa copulatrix differs: large and spherical with a short and wide peduncle in A. intermedius and elongated and thin without a prominent transition into its peduncle in A. obesoductus .

Certain species of the subgenus Kobeltia ( Arion hortensis and Arion distinctus ) might also be confused with a dark-grey A. intermedius . If such slugs lack the diagnostic external characters of A. intermedius , Kobeltia species can be ruled out by their longer and thicker oviduct, which consists of more parts ( A. distinctus 2 or 3 parts; A. hortensis always 3 parts) ( Backeljau & De Bruyn 1990).

Surprisingly, one of the biggest species of Arion , Arion ater can also superficially resemble A. intermedius in external appearance. Hatchlings of A. ater overlap A. intermedius in size and may also have straw-coloured bodies with dark tentacles. The proportions and, later in development, the prominence of the foot fringe give a different general impression, but those without experience of both species should not identify an A. intermedius lacking the prickly tubercles or the small dots without checking for maturing genitalia. However, A. ater is rare or extinct in Hungary (Turóci & Páll-Gergely in press).

In other genera, juveniles of Krynickillus melanocephalus and certain colour morphs of some Deroceras species might look superficially similar to A. intermedius but these can be immediately ruled out by the position of the pneumostome relative to the mantle (posterior in Agriolimacidae , anterior in Arionidae ) and the shape of the tail (pointed in Agriolimacidae , rounded in Arionidae ).

The life cycle of A. intermedius in the wild is one year. In southern England, individuals are highly synchronised in when they mature, in September ( Hutchinson et al. 2017). The species was believed to reproduce only by selfing, based on genetic analyses ( Foltz et al. 1982) and because of the lack of observations of spermatophores and mating ( Davies 1977; Backeljau & De Bruyn 1990). Garrido et al. (1995) reported the presence of spermatophores for the first time, and the later discovery of some heterozygosity of allozymes implies that occasional outcrossing does occur ( Reise et al. 2001; Jordaens et al. 2013).

A revisit to the collecting site of A. intermedius (by RV) in December 2024 resulted in the rediscovery of one living specimen (not collected). The occurrence in winter corresponds with studies elsewhere in its range ( Hutchinson et al. 2017), but also December 2024 was mild even though the area lies in the vicinity of the eastern slopes of the Alps. That month in Szombathely, on 20 days the minimum temperature fell below freezing but none were colder than −5 °C (AccuWeather). So possibly a population could persist over winter in that area .

Arion intermedius View in CoL is presumed native to Western Europe, over much of which it occurs frequently ( Rowson et al. 2014). Although it has recently appeared in Austria, which neighbours Hungary ( Duda et al. 2019), whether the species is native to Central Europe is questionable. The closest location where the species shows a continuous distribution is in Czechia ( Fig. 5A View Figure 5 ), more than 400 km from the Hungarian locality ( Hlaváč & Horsák 2000). The absolute closest localities to Hungary are in Slovakia and Austria but these spots seem to be part of a sporadic rather than a continuous distribution. That suggests that A. intermedius View in CoL is most likely an introduction to Hungary. In Kámoni Arborétum more than 3000 plant taxa are found, implying extensive exchange of horticultural specimens. Arion intermedius View in CoL has several characteristics that enable it to exploit such interchange so as to colonise new sites. These include a broad ecological tolerance even of disturbed habitats, a tendency to ride on horticultural and agricultural products together with an ability to survive such journeys, and uniparental reproduction.

The situation with A. transsylvanus View in CoL is rather different. In Hungary A. transsylvanus View in CoL occurs in close proximity to A. fuscus View in CoL , from which it is not distinguishable on external characters. Jordaens et al. (2010) provide anatomical characters to distinguish A. transsylvanus View in CoL from A. subfuscus View in CoL and A. fuscus View in CoL . We cannot add to this since our specimen was juvenile.

Arion transsylvanus might also be confused with species besides those in the same complex. Certain colour morphs of Arion vulgaris juveniles can be similar to this species. Generally, A. vulgaris juveniles are more reddish than A. transsylvanus , which is usually yellowish with some orange tinge. The lineolation on the foot fringe is more prominent in A. vulgaris juveniles. It can be useful to consider the size of the tubercles; they are larger and the grooves between them are deeper in A. vulgaris , which results in a coarser skin surface compared with A. transsylvanus . The lateral bands are typically also stronger and wider in A. vulgaris .

According to current knowledge, A. transsylvanus is predominantly a Carpathian species, mostly restricted to the Transylvanian region in Romania (eastern and southern Carpathian Mountains and the Apuseni Mountains). The species is the only known representative of the Arion subfuscus species complex in Transylvania ( Jordaens et al. 2010).

Besides the Romanian records, Jordaens et al. (2010) reported this species only from one site in eastern Poland. We are not aware of published records of the species since then, which may partly reflect a lack of confidence amongst malacologists in determining it anatomically and a lack of resources for DNA sequencing. However, Gural-Sverlova & Gural (2015) found no A. transsylvanus , only A. fuscus , in Ukraine, even though Ukraine bridges the gap between the Romanian and Polish records. Nevertheless, the absence of records between our finding in northern Hungary and the nearest locality in Romania (c. 200 km away) does not dissuade us from proposing that the species might be native in Hungary. One reason is that the Bükk Mountains are geographically part of the Western Carpathians. Furthermore, we found this species at a relatively high altitude (c. 800 m) in little-disturbed beech forest, which is consistent with occurrence data in Romania ( Grossu 1970; Jordaens et al. 2010). Our collecting site was remote from settlements (along a hiking track c. 3 km from the village Répáshuta), which would be the most likely point of introduction. If it is indeed native in Hungary, A. transsylvanus would be the only such one of the nine slug species that we have either discovered as new to Hungary in the last six years or that were considered doubtful records until we confirmed them; the other eight species are very likely introductions. Besides A. intermedius and A. transsylvanus , these “new” species are: Ambigolimax parvipenis Hutchinson, Reise & Schlitt, 2022 ; Ambigolimax valentianus (A. Férussac, 1821) ; Deroceras invadens Reise, Hutchinson, Schunack & Schlitt, 2011 ; Krynickillus melanocephalus Kaleniczenko, 1851 ; Limacus maculatus (Kaleniczenko, 1851) ; Milax nigricans (Philippi, 1836) ; and Tandonia kusceri (H. Wagner, 1931) . On the other hand, in Romania A. transsylvanus occurs also in synanthropic and other degraded habitats ( Jordaens et al. 2010), so there is the potential that it has or will spread to such environments in countries adjacent to Romania, including Hungary.

The discovery of these two species of Arion may provide a guide to predicting what sort of further slug species might turn up new to the Hungarian fauna. Most likely are synanthropic species that spread as a result of trade and usually turn up in habitats like gardens, perhaps facilitated by the milder winters. A prediction for such a species is Deroceras panormitanum (Lessona & Pollonera, 1882) s.s., which has recently been reported in France (Ventura et al. in press). But additionally perhaps further native species have been overlooked in natural habitats in Hungary; these species may already be known from neighbouring countries. For instance, the genus Lehmannia has a centre of diversity in Romania, but the species are particularly hard to find during daylight and their identification requires dissection.

Acknowledgements

Thanks to Fruzsina Matolcsi for helping to sequence our A. intermedius specimens, to Heike Reise for invaluable discussions and help, and to the referees Ben Rowson and Ivaylo Dedov for their most useful suggestions to improve the manuscript.

Funding Declaration

This study was supported by the Scientific Patronage Grant (Tudományos Mecenatúra Pályázat, MEC_K 140853) funded by the National Research, Development and Innovation Office and the Ministry of Innovation and Technology to BPG. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The Senckenberg Museum of Natural History Görlitz paid for some of the sequencing.

Authors’ Contribution

ÁT & BPG initiated the concept. RV, ÁT, RF and BPG collected the specimens. BS, MZN & KB were responsible for the sequencing. RV constructed the maps. ÁT wrote the manuscript, and all authors contributed to the final version, especially JMCH who corrected and edited the drafts.

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