BookPDF Available

Slugs of Britain and Ireland: identification, understanding and control.

January 2014

Edition: 1
Publisher: Field Studies Council, Shropshire, UK.
ISBN: 978 1 908819 13 0

Authors:

Ben Rowson

National Museum Wales

James A Turner

National Museum Wales

Roy Anderson

Queen's University Belfast

Larger Arionidae (Arion subgenus Arion). Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support ($75), those below are Bayesian posterior probabilities, expressed as % ($80). Grey bars indicate clades. Species new to the fauna of Britain and/or Ireland are indicated by *. doi:10.1371/journal.pone.0091907.g001

…

. Datasets and species delimitation by ABGD.

…

Arionidae: Arion subgenus Mesarion. Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support (NJ), those below are Bayesian posterior probabilities, expressed as %. Grey bars indicate clades. Species new to the fauna of Britain and/or Ireland are indicated by *. doi:10.1371/journal.pone.0091907.g002

…

. Cont.

…

Other Arionidae. Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support (NJ), those below are Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *. doi:10.1371/journal.pone.0091907.g003

…

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The Slugs of Britain and Ireland: Undetected and

Undescribed Species Increase a Well-Studied,

Economically Important Fauna by More Than 20%

Ben Rowson

*, Roy Anderson

, James A. Turner

, William O. C. Symondson

1National Museum of Wales, Cardiff, Wales, United Kingdom, 2Conchological Society of Great Britain & Ireland, Belfast, Northern Ireland, United Kingdom, 3Cardiff

School of Biosciences, Cardiff University, Cardiff, Wales, United Kingdom

Abstract

The slugs of Britain and Ireland form a well-studied fauna of economic importance. They include many widespread

European species that are introduced elsewhere (at least half of the 36 currently recorded British species are established in

North America, for example). To test the contention that the British and Irish fauna consists of 36 species, and to verify the

identity of each, a species delimitation study was conducted based on a geographically wide survey. Comparisons between

mitochondrial DNA (COI, 16S), nuclear DNA (ITS-1) and morphology were investigated with reference to interspecific

hybridisation. Species delimitation of the fauna produced a primary species hypothesis of 47 putative species. This was

refined to a secondary species hypothesis of 44 species by integration with morphological and other data. Thirty six of these

correspond to the known fauna (two species in Arion subgenus Carinarion were scarcely distinct and Arion (Mesarion)

subfuscus consisted of two near-cryptic species). However, by the same criteria a further eight previously undetected

species (22% of the fauna) are established in Britain and/or Ireland. Although overlooked, none are strictly morphologically

cryptic, and some appear previously undescribed. Most of the additional species are probably accidentally introduced, and

several are already widespread in Britain and Ireland (and thus perhaps elsewhere). At least three may be plant pests. Some

evidence was found for interspecific hybridisation among the large Arion species (although not involving A. flagellus) and

more unexpectedly in species pairs in Deroceras (Agriolimacidae) and Limacus (Limacidae). In the latter groups,

introgression appears to have occurred in one direction only, with recently-invading lineages becoming common at the

expense of long-established or native ones. The results show how even a well-studied, macroscopic fauna can be vulnerable

to cryptic and undetected invasions and changes.

Citation: Rowson B, Anderson R, Turner JA, Symondson WOC (2014) The Slugs of Britain and Ireland: Undetected and Undescribed Species Increase a Well-

Studied, Economically Important Fauna by More Than 20%. PLoS ONE 9(3): e91907. doi:10.1371/journal.pone.0091907

Editor: Donald James Colgan, Australian Museum, Australia

Received August 1, 2013; Accepted February 17, 2014; Published March 19, 2014

Copyright: ß2014 Rowson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits

unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This study was funded by a Research Grant from the Leverhulme Trust to the authors (RPG-068). The funders had no role in study design, data

collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

* E-mail: ben.rowson@museumwales.ac.uk

Introduction

Slugs are among the invertebrates most readily encountered by

people in north-west Europe, with many species occurring in

gardens and in or around buildings. The British and Irish slug

fauna of 36 species [1] includes important pests, post-glacial relicts,

indicators of ancient woodland and even putative endemics [2].

However, most are widespread European species, many of them

introduced in other parts of the world. Almost all the introduced

slugs of other temperate regions also occur in Britain, including

those of South Africa (11 species [3]) and New Zealand (14 species

[4]). At least 18 species (half the currently recognised British fauna)

are established in the USA and/or Canada [5,6,7]. Some of these

species have a long history of study. The early depictions of British

slugs in Lister (1685) [8] were a source for the descriptions by

Linnaeus (1758) [9] and other early European workers. After

Scharff’s (1891) Irish monograph [10], the British fauna was

monographed twice in the 20

century [11,12], establishing a

benchmark for identification guides [13,14], population genetic

studies (e.g. [15,16,17]) and applied works (e.g. [18,19]). In Britain

and Ireland, slugs have been included in a pioneering mollusc

distribution mapping scheme since the 1880s [2] so have been

subject to careful public-participatory recording and study,

resulting in updated and comprehensive checklists (most recently

in 2008 [1]). As a result, the British and Irish slug fauna must rank

among the world’s best studied.

However, additional species in the fauna have nevertheless been

recognised relatively recently either by examining ‘‘aggregates’’ of

superficially similar species [20,21] or direct detection [22,23,24].

Wide-ranging phylogeographic work has also demonstrated the

presence of additional taxa in Britain [25]. In order to detect such

taxa they must be distinguished from those already known to be

present, which must themselves be adequately characterised. Slugs

present particular problems in identification due to overlapping

external morphology and the need to examine internal characters,

so despite their importance and conspicuousness they are often

neglected during biodiversity assessments and also by amateur

malacologists. In 2011 we began producing a new comprehensive

identification guide to the British and Irish slugs, aimed at non-

specialists (the most recent being [14]). Such guides depend upon

correctly identified reference specimens, ideally vouchered in an

accessible museum collection. In the case of slugs these should

PLOS ONE | www.plosone.org 1 March 2014 | Volume 9 | Issue 3 | e91907

preferably be photographed alive since some diagnostic features

are lost or obscured on preservation. We took this opportunity to

sequence mitochondrial DNA (mtDNA) from these and other

specimens. This provided an independent corroborator of

identifications where conspecific reference sequences already

existed, or a potential future reference where they did not.

The survey also allowed us to use species delimitation

techniques to test the contention that there are currently 36 slug

species established in Britain and Ireland [1]. ‘‘Established’’ refers

to reproducing populations (whether native or introduced) as

opposed to ‘‘adventive’’ populations that do not sustain themselves

by reproduction. Invertebrate ‘‘species’’ in a checklist such as [1]

have usually been delimited, studied and recorded by non-

molecular techniques (Arion (Mesarion) fuscus (Mu¨ller) is an

exception [26]). In general, they have been considered species

under a biological species concept invoking reproductive isolation.

This species concept is sometimes difficult to apply in European

slugs, which are typically simultaneous hermaphrodites with a

varying tendency to self-fertilise [16]. In the family Arionidae in

particular, a flexible species concept may have to be adopted as a

consequence of mixed breeding systems and variable evolutionary

rates [25,26,27,28,29]. The phylogenetic species concept, where

species are delimited as genetically distinct monophyletic lineages,

and evolutionary species concept, where a species is a lineage with

a separate evolutionary fate, have been discussed or invoked in

these studies as being more appropriate. This approach is followed

here. We stress that our study is an exercise in the species

delimitation of the fauna of Britain and Ireland and not a

phylogenetic or phylogeographic study of wide-ranging European

species. However, this does not preclude species being distin-

guished first in an island fauna that has, through natural

establishment and introduction, sampled that of the continent

(e.g. [20,23,24]).

Sequence data from mtDNA have proven effective in the

diagnosis and delimitation of European slugs as phylogenetic

species, showing at least some concordance with the morphology

and behaviour of species recognised by non-molecular methods

and sometimes supporting their recognition as biological species

also (e.g. [30,31,32,33] in addition to those cited above). However,

sequence data alone do not always provide sufficient objective

criteria for species delimitation [34], and mtDNA has its own

limitations as an evolutionary marker [35]. It is also evident that

intraspecific mtDNA divergences vary considerably across terres-

trial mollusc families [36]. A number of methods have recently

been developed to make species delimitation from sequence data

more consistent, and more independent of delimitation based on

non-molecular data to which it can then be compared. Pre´vot et

al. [37] recently applied and compared several of these in a

European terrestrial mollusc with a mixed breeding system. Each

delimited slightly different numbers of species, all of which were

readily recognised as monophyletic on phylogenetic trees, leading

them to question how a particular delimitation method might

justifiably be favoured a priori. One method tested, Automatic

Barcode Gap Discovery (ABGD) [38] has several advantages. It

requires no prior knowledge of the likely intraspecific distances

and is robust to their variation within a dataset. Being distance-

based it does not use monophyly (allowing it to be investigated

independently from distinctness) and is computationally very

efficient [38]. In gastropod studies [37,39] it delimited as few or

fewer species than the coalescent-based General Mixed Yule

Coalescent (GMYC) delimitation method [40], most ABGD

species being a subset of those delimited by GMYC. This contrast

was more marked where some species were represented by many

fewer sequences than others [39]. This is a sampling problem

considered an inescapable consequence of biological rarity in

delimitation studies [41]. As our aims were to test a hypothesis of

36 species and to attribute specimens to these whenever justified,

we used the distance-based and conservative ABGD method in

our study.

To best aid future identification by confirming whether external

morphological variation was intraspecific, our sampling paid more

attention to the more conspicuously variable slug species,

especially the ‘‘larger Arionidae’’ (Arion subgenus Arion). This

group includes A. (A.) ater (Linnaeus), A. (A.) rufus (Linnaeus), and

the notorious pest A. (A.) vulgaris Moquin-Tandon (called A.

lusitanicus auct. non Mabille in some publications). These are

morphologically similar species that either hybridise (e.g.

[42,17,43,44,45]) or overlap morphologically to give the impres-

sion of having done so [46]. It has been suggested, at least in the

British media, that they may also hybridise with Arion (A.) flagellus

Collinge, a common species in Ireland and Britain but not

elsewhere outside Iberia. The potential for hybridisation further

complicates the application of species concepts in the larger

Arionidae, so we sought evidence of hybridisation between these

species as well as aiming to clarify their genetic identity. In the

event, the data suggested that hybridisation occurs in other

families.

Materials and Methods

Ethics statement

All necessary permits were obtained for the described study,

which complied with all relevant regulations. The only protected

species in this study is Geomalacus maculosus Allman, which occurs in

the Republic of Ireland but not the UK. For this species, the

necessary Licence under the Wildlife Act 1976-2010 and

Derogation Licence under the European Communities (Natural

Habitats) Regulations 1997 were obtained from the Department of

Arts, Heritage and the Gaeltacht.

Samples

Slugs were collected by hand, often at night and/or in wet

weather, from over 200 sites across Britain (England, Scotland and

Wales) and Ireland (Northern Ireland and the Republic of Ireland)

between 2010 and 2012. Attempts were made to maximise the

range of latitude, longitude, altitude, habitat and soil types

covered. As native species were the initial focus, most collecting

was done in less disturbed areas including National Nature

Reserves and National Parks but we also included agricultural,

urban and brownfield sites. Some known sites for the rarer species

were visited repeatedly. Additional specimens were collected by

members of the Conchological Society of Great Britain and

Ireland and members of the public, or taken from the collections of

the National Museum of Wales, Cardiff, UK, where all specimens

are now deposited. The single exception was a Testacella specimen

from the Natural History Museum, London, UK (Table S1).

Specimens for sequencing were selected to cover the widest

possible range of morphology, geography and habitat. This meant

one or at most a few individuals from each population were

selected, increasing the number of populations sampled. Adult

specimens were dissected where necessary and identified with

reference to the taxonomic literature and museum collections.

Inevitably, a small number of juvenile slugs could only be

identified tentatively by morphology and were identified a posteriori

from the DNA results.

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Sequencing

In Arionidae, the modified universal primers 16SAR and

16SBR ([7], modified from [47]) were used to amplify an approx.

440 bp region of the 16S large subunit mitochondrial ribosomal

DNA. This region has been established as the mitochondrial

marker of choice in arionid studies (e.g. [27,28,30,48]). For other

families, the universal primers LCO1490 and HCO2198 [49]

were used to amplify an approx. 650 bp region of the COI gene.

Approximately 2 mm

of tail or other tissue from each specimen

was incubated in 1 ml 0.16Tris EDTA (‘‘low TE’’) at 20uC for

30 min to replace preservatives. DNA was extracted with the

Qiagen DNEasy kit with a single elution with 200 ml Buffer AE.

Two ml of this extract was used as template in PCRs using GE

Healthcare illustra PuReTaq PCR beads with 0.25 ml of each

primer (10 mM) and ultra-pure water to a volume of 25 ml. Cycling

conditions (Eppendorf Mastercycler) for COI and 16S were: 94uC

for 2 min 30 s, (94uC for 30 s, 47uC for 45 s, 72uC for 1 min

15 s640 cycles), 72uC for 10 min. Conditions for ITS-1 were:

94uC for 3 min, (95uC for 1 min, 55uC for 1 min, 72uC for

2 min635 cycles), 72uC for 5 min. After visualisation and

quantification on agarose gels, products were cleaned with GE

Healthcare illustra ExoStar 1-Step and sequenced by Euro-

fins|MWG Operon (www.eurofinsgenomics.eu). Sequences were

edited and compiled in BioEdit 7.1.3 [50] and submitted to

GenBank (accession numbers KF894075-KF894388, details in

Table S1).

Assembling datasets

The data were divided into subsets for analysis because: i) two

different mitochondrial fragments were used; ii) several arionid

subgenera or species groups have been the target of previous

genetic work; iii) particular sampling attention had been paid to

the larger Arionidae; and iv) several of the relevant families are not

closely related to one another [51]. For 16S analysis the subsets

were 1) Arion subgenus Arion, including A. flagellus;2)Arion subgenus

Mesarion, in which great intraspecific variability has been found

[25,26] and 3); the remaining, smaller Arionidae. Preliminary

analysis indicated that A. flagellus could have been included in

either dataset 1 or 2; the former was chosen given the suggestion

that this species may hybridise with others in this dataset. Analyses

were repeated on a dataset including all Arionidae. For COI data

the subsets included: 4) Limax; and 5) Other Limacidae. Analyses

were repeated on a dataset including all Limacidae. The

remaining subsets were: 6) Agriolimacidae; 7) Milacidae; 8)

Testacellidae; 9); Boettgerillidae; and 10) Trigonochlamydidae.

All relevant sequences available from members of each family in

GenBank were downloaded and incorporated into each subset (7

Oct 2013; details in Table S2) with the exception of native North

American and Asian arionid genera and the eastern European

limacid genus Bielzia. Some short Limax sequences showing limited

overlap with our dataset, and a possibly misidentified Deroceras

sequence (FJ917286), were excluded from further analysis.

Sequences in each subset were aligned by CLUSTALW in

MEGA 5.1 [52] using default parameters and checked by eye.

Identical sequences were collapsed to haplotypes. Sequences were

realigned for the whole family datasets for Arionidae and

Limacidae.

Analysis

Species were delimited, refined and identified in a three-stage

process. Firstly, putative species were delimited statistically using

the 16S and COI data to produce a primary species hypothesis for

each dataset (PSH). This required no a priori information on the

number of species expected, or how genetically different such

species might be. It used a consistent approach across families and

simultaneously dealt with existing GenBank data from Europe and

beyond. Next, those putative species that included individuals

from Britain and/or Ireland were assessed with other data to

refine each PSH to a secondary species hypothesis (SSH). Finally,

the species accepted in the SSH were identified by comparing

them to the currently known species of the fauna [1].

To examine whether hybridisation has occurred between

species-level lineages in Arion subgenus Arion, a rapidly evolving

nuclear marker was also sequenced from these species. The

primers ITS1 and 5.8C [25] were used to amplify an approx-

imately 570 bp region of the internal transcribed spacer (ITS-1).

These authors considered topological conflict (non-monophyly)

between ITS-1 and 16S data an indicator of hybridisation and

introgression between species-level lineages in subgenus Mesarion

(see also [28,29]).

Table 1. Datasets and species delimitation by ABGD.

Dataset Region Sequences Haplotypes

Max. prior intraspecific K2P

distance over which stable

Number of putative

species (total)

Number of putative species

(Britain/Ireland only)

Arion (Arion) 16S 134 60 0.050 9 6

Arion (Mesarion) 16S 190 160 0.115 15 5

Other Arionidae 16S 185 96 0.038 11 9

All Arionidae 16S 509 316 0.038 34 20

Limax COI 203 179 0.012 27 3

Other Limacidae COI 72 70 0.066 6 6

All Limacidae COI 275 249 0.028 27 9

Agriolimacidae COI 102 80 0.022 9 7

Milacidae COI 30 28 0.007 5 5

Testacellidae COI 22 17 0.200 4 4

Trigonochlamydidae COI 6 2 n/a n/a n/a

Boettgerillidae COI 4 3 n/a n/a n/a

doi:10.1371/journal.pone.0091907.t001

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Table 2. Summary of primary species hypotheses (PSH) and secondary species hypotheses (SSH). Haps., number of haplotypes; Inds., number of individuals.

PSH

PSH generated by

ABGD Haps. Inds. Mean K2P distances Monophyly Morphologically

unique? SSH Name applied to SSH

(whole family ABGD analyses) Whole family Subset

Whole family Subset Intraspecific

Interspecific

(minimum) NJ BI NJ BI External Internal

1 (1A +1B +1C

form a single

PSH)

- 23 46 0.02 0.08 66 0 0 0 X (Split PSH 1) (no SSH)

1A ‘‘ X 20 36 n/a n/a 99 100 100 99 X X 1A Arion (Arion) ater (Linnaeus, 1758)

1B ‘‘ X 2 8 n/a n/a 93 63 93 59 X 1B+1C Arion (Arion) cf. vulgaris Moquin-

Tandon, 1855

1C ‘‘ X 1 2 n/a n/a n/a n/a n/a n/a ? ? ‘‘ ‘‘

2 X X 9 22 0.00 0.07 100 100 100 100 X 2 Arion (Arion) rufus (Linnaeus, 1758)

3 X X 7 8 0.01 0.07 99 100 100 100 ‘‘ 3 Arion (Arion) cf. empiricorum A.

´russac, 1819

4 X X 14 26 0.00 0.11 100 100 100 100 X X 4 Arion (Arion) vulgaris Moquin-Tandon,

1855

5 X X 3 24 0.00 0.10 100 100 100 100 X X 5 Arion (Arion) flagellus Collinge, 1893

6 X X 51 54 0.01 0.17 100 100 100 100 ? X 6 Arion (Mesarion) fuscus (O. F. Mu

¨ller,

1774)

7 X X 21 47 0.01 0.10 96 100 95 100 X X 7 Arion (Mesarion) subfuscus

Draparnaud, 1805 (two, currently

indistinguishable evolutionary species

[25])

8 X X 44 45 0.01 0.14 100 100 100 100 ‘‘ ‘‘ 8 ‘‘

9 X X 2 2 0.02 0.13 100 100 100 100 X 9+10 Arion (Mesarion) cf. iratii Garrido,

Castillejo & Iglesias, 1995

10 X X 2 2 0.03 0.17 100 100 100 100 ‘‘ ‘‘ ‘‘

11 X X 48 76 0.04 0.20 100 100 100 90 X X 11 Arion (Carinarion) circumscriptus

Johnston, 1828 (including A. (C.)

silvaticus Lohmander, 1937)

12 X X 13 20 0.01 0.20 100 100 100 100 X X 12 Arion (Carinarion) fasciatus (Nilsson,

1823)

13 X X 8 22 0.01 0.22 100 100 100 100 X X 13 Arion (Kobeltia) distinctus J. Mabille,

1868

14 X X 4 10 0.00 0.18 100 100 100 100 X X 14 Arion (Kobeltia) hortensis A. Fe

´russac,

1819

15 X X 7 22 0.01 0.20 100 100 100 100 X X 15 Arion (Kobeltia) intermedius Normand,

1852

16 X X 1 3 0.01 0.13 100 97 100 98 ? X 16 Arion (Kobeltia) occultus Anderson,

2004

17 X X 3 12 0.00 0.13 100 100 100 100 X X 17 Arion (Kobeltia) owenii Davies, 1979

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Table 2. Cont.

PSH

PSH generated by

ABGD Haps. Inds. Mean K2P distances Monophyly Morphologically

unique? SSH Name applied to SSH

(whole family ABGD analyses) Whole family Subset

Whole family Subset Intraspecific

Interspecific

(minimum) NJ BI NJ BI External Internal

18 X X 4 4 0.00 0.14 100 100 100 100 ? X 18 Arion (Kobeltia) cf. fagophilus de

Winter, 1986

19 X - 1 4 0.00 0.07 n/a n/a n/a n/a X X 19 Geomalacus maculosus Allman, 1843

20 X X 23 25 0.01 0.06 99 100 99 100 X X 20 Limax cinereoniger Wolf, 1803

21 X X 21 24 0.00 0.06 99 100 99 100 X X 21 Limax maximus Linnaeus, 1758

22 X X 10 10 0.03 0.08 99 100 99 100 X X 22 Limax cf. dacampi Menegazzi, 1854

23 X X 9 9 0.00 0.11 100 100 100 100 X X 23 Limacus flavus (Linnaeus, 1758)

24 X X 14 14 0.01 0.11 100 100 100 100 X 24 Limacus maculatus (Kaleniczenko,

1851)

25 X X 26 26 0.01 0.14 100 100 100 100 X X 25 Lehmannia marginata (O. F. Mu

¨ller,

1774)

26 X X 8 9 0.01 0.08 100 100 100 100 X 26 Ambigolimax valentianus (A. Fe

´russac,

1822)

27 X X 8 8 0.02 0.08 83 99 97 91 X 27 Ambigolimax nyctelius (Bourguignat,

1861)

28 X X 5 5 0.01 0.13 100 100 100 100 X X 28 Malacolimax tenellus (O. F. Mu

¨ller,

1774)

29 X n/a 8 8 0.02 0.05 99 100 n/a n/a X 29 Deroceras agreste (Linnaeus, 1758)

30 X n/a 11 14 0.00 0.05 99 84 n/a n/a X 30 Deroceras reticulatum (O. F. Mu

¨ller,

1774)

31 X n/a 21 26 0.02 0.09 99 100 n/a n/a X 31 Deroceras invadens Reise et al., 2011

32 X n/a 9 11 0.01 0.08 99 100 n/a n/a X 32 Deroceras panormitanum (Lessona &

Pollonera, 1882)

33 X n/a 7 10 0.01 0.05 99 100 n/a n/a X 33+34+35+36 Deroceras laeve (O. F. Mu

¨ller, 1774)

(multiple lineages; further study

required)

34 X n/a 4 5 0.00 0.06 99 100 n/a n/a ‘‘ ‘‘ ‘‘

35 X n/a 4 4 0.01 0.05 99 100 n/a n/a ‘‘ ‘‘ ‘‘

36 X n/a 12 20 0.02 0.05 57 0 n/a n/a ‘‘ ‘‘ ‘‘

37 X n/a 4 4 0.01 0.11 100 100 n/a n/a X X 37 Milax gagates (Draparnaud, 1801)

38 X n/a 10 11 0.00 0.10 100 91 n/a n/a X 38 Tandonia budapestensis (Hazay, 1881)

39 X n/a 3 3 0.04 0.11 99 97 n/a n/a X 39 Tandonia cf. cristata (Kaleniczenko,

1851)

40 X n/a 10 10 0.03 0.10 98 100 n/a n/a X X 40 Tandonia sowerbyi (A. Fe

´russac, 1823)

(may include T. marinellii Liberto et al.,

2012)

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Primary species hypotheses

PSHs were generated with the distance-based method ABGD

[38]. Briefly, this divides a set of aligned sequences into groups,

statistically inferring the groups most likely to correspond to

species. It does so recursively, using a range of potential maximum

pairwise intraspecific distances inferred from the data. It produces

an array of PSHs with different numbers of groups delimited using

different distance values. For example, very small or large values

inevitably delimit unrealistically many or few species [38,39]. To

avoid having to discuss all PSHs produced, some criterion is

required to select which of them to investigate further. We chose

the PSH corresponding to the smallest number of groups that was

stable over three or more successive distance values. When the

initial and recursive partitions indicated different numbers of

groups at the same distance value, we chose the smaller of the two.

This criterion had the advantages of being i) independent of the

expected number of species; ii) consistent across different datasets;

and iii) conservative in terms of the numbers of putative species

recognised.

ABGD analyses were run at http://wwwabi.snv.jussieu.fr/

public/abgd/abgdweb.html on aligned haplotype sequences using

K2P distances. To ensure a wide range of distance values were

investigated, Pmin was kept at 0.001 but the maximum potential

Pmax was raised from 0.1 to 0.2. To allow comparison between a

greater number of alternative distance values, the number of steps

was raised from 10 to 20. Other settings were as default (relative

gap width, X = 1.5; number of bins = 20).

Secondary species hypotheses

The SSH for each dataset consisted of an equal or smaller

number of species, some putative species being combined in order

to remain conservative. In the sole case where ABGD delimited a

putative species in one analysis (Arionidae) but split it in the subset

analysis (Arion (Arion)), both alternatives were considered (see

Results and Discussion).

As a result of the ABGD analysis, putative species were by

definition genetically distinct from their close relatives. To be

considered a species under the SSH, each putative species had also

to show evidence of reciprocal monophyly with respect to a sister

group, so being at least a phylogenetic species. This was

investigated and visualised using neighbour-joining trees based

on the K2P distance, with support for monophyly quantified with

1000 bootstrap pseudoreplicates in MEGA. Gaps were treated by

pairwise deletion. As a comparison using a character-based

method involving a model of sequence evolution, Bayesian

Inference (BI) was also used, with support for monophyly

quantified with posterior probabilities. FindModel (www.hiv.lanl.

gov/content/sequence/findmodel/findmodel.html) was used to

select the most appropriate evolutionary model (GTR +I+Cin

each case) according to log likelihood and the Akaike information

criterion. BI was implemented in MrBayes v3.1.2 [53] with two

parallel runs of 5,000,000 generations, sampling trees every 1000

generations, with the first 25% of the trees discarded as burn-in

and other settings as default. Convergence on a stable log

likelihood before the burn-in period was evident in all analyses.

These methods were also used to analyse the ITS-1 data from the

Arion (Arion) species.

Morphological distinctness was also considered as a criterion for

the recognition of species in the SSH. The external and internal

(adult genital) morphology of each putative species was investi-

gated using standard techniques. Putative species that were closely

related (i.e. showed evidence of joint monophyly) but did not differ

in morphology were generally not recognised as separate species in

the SSH. However, it became clear that in two cases (interpreted

Table 2. Cont.

PSH

PSH generated by

ABGD Haps. Inds. Mean K2P distances Monophyly Morphologically

unique? SSH Name applied to SSH

(whole family ABGD analyses) Whole family Subset

Whole family Subset Intraspecific

Interspecific

(minimum) NJ BI NJ BI External Internal

41 X n/a 1 2 0.00 0.19 n/a n/a n/a n/a X X 41 Tandonia rustica (Millet, 1843)

42 X n/a 5 7 0.01 0.22 100 100 n/a n/a X X 42 Testacella haliotidea Draparnaud, 1801

43 X n/a 5 8 0.00 0.22 100 100 n/a n/a X X 43 Testacella maugei (A. Fe

´russac, 1819)

44 X n/a 2 2 0.00 0.20 100 100 n/a n/a X X 44 Testacella scutulum G. B. Sowerby I,

1821

45 X n/a 5 5 0.00 0.20 100 100 n/a n/a X X 45 Testacella cf. scutulum G. B. Sowerby I,

1821

n/a n/a n/a 2 4 0.00 n/a n/a n/a n/a n/a X X (46) Boettgerilla pallens Simroth, 1912

n/a n/a n/a 2 6 0.00 n/a n/a n/a n/a n/a X X (47) Selenochlamys ysbryda Rowson &

Symondson, 2008

doi:10.1371/journal.pone.0091907.t002

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as evidence of introgression, see below) there was conflict between

putative species assignment and morphology. Thus the criterion of

morphological distinctness, though useful, could not be consis-

tently applied. To do so would also preclude the recognition of

genuinely cryptic species ([54]) as SSH species, should either

occur.

Refinement of PSHs to SSHs by this method is an example of

integration of molecular with other data, here specifically of

‘‘integration by congruence’’ in the sense of Padial et al. [55]. This

approach requires congruence of (for example) mtDNA and

morphology for species recognition. According to Padial et al.

[55], integration by congruence is less likely to overestimate species

numbers and better promotes future taxonomic stability than the

alternative, integration by cumulation, which requires no initial

congruence between datasets.

Comparison of the SSH to the known fauna

The species in the SSH were then compared to the species

making up the known fauna of Britain and Ireland [1]. For most

SSH species this was straightforward from morphological identi-

fications already made, and/or the inclusion of GenBank

sequences in each PSH. Others required recourse to museum

collections and the wider taxonomic literature.

Results and Discussion

450 sequences were obtained from 388 individuals (Table S1)

and compared with 659 sequences from GenBank (Table S2),

representing 695 haplotypes in all. For all three gene regions, some

haplotypes were found at more than one site. The three most

Figure 1. Larger Arionidae (

Arion

subgenus

Arion

). Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support

($75), those below are Bayesian posterior probabilities, expressed as % ($80). Grey bars indicate clades. Species new to the fauna of Britain and/or

Ireland are indicated by *.

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Figure 2. Arionidae:

Arion

subgenus

Mesarion

.Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support (NJ),

those below are Bayesian posterior probabilities, expressed as %. Grey bars indicate clades. Species new to the fauna of Britain and/or Ireland are

indicated by *.

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Figure 3. Other Arionidae. Midpoint-rooted NJ tree based on 16S data; values above branches are % bootstrap support (NJ), those below are

Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *.

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Figure 4. Limacidae (genus

Limax

). Midpoint-rooted NJ tree based on COI data; values above branches are % bootstrap support (NJ), those below

are Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *.

doi:10.1371/journal.pone.0091907.g004

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extreme examples were 16S haplotypes attributed to A. (A.)

flagellus,A. (A.) ater, and A. (A.) vulgaris (22, 11 and 9 individuals

respectively) that were found throughout Britain and Ireland.

Primary species hypotheses

The PSHs generated by ABGD analyses and selected

according to our criteria are summarised in Table 1. Histograms

Figure 5. Other Limacidae. Midpoint-rooted NJ tree based on COI data; values above branches are % bootstrap support (NJ), those below are

Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *.

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and graphs illustrating the full range of PSHs generated

are given in Figs. S1–S3. Variation was too limited in

Boettgerillidae and Trigonochlamydidae to conduct the

analysis (mean intraspecific K2P distance 0.001 and 0.000

respectively) so each was considered to comprise a single

species.

Figure 6. Agriolimacidae. Midpoint-rooted NJ tree based on COI data; values above branches are % bootstrap support (NJ), those below are

Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *.

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The maximum prior intraspecific K2P distance over which the

number of putative species was stable varied considerably between

taxa (0.038–0.115 in Arionidae, 0.012–0.200 in other families)

(Table 1). Above this value, the number of putative species

delimited fell rapidly (Figs. S1–S3). The exception was Milacidae

where it rose from 5 to 8 and was again stable up to K2P 0.038,

although our conservative criteria selected the former value.

Compared to the whole family analysis, subset analyses generated

more putative species in total in Arionidae (34 versus 35), and

fewer putative species in total in Limacidae (27 versus 33).

However, these differences were due to the differential splitting of

continental putative species (e.g. several provisionally recognised

Limax species). In both families, the total number of putative

species occurring in Britain and Ireland was identical in the whole

family and subset analyses (Table 1). The overall PSH, comprising

the selected PSH from each of the subset analyses, delimited 45

putative species for Britain and/or Ireland, plus one species each

from Boettgerillidae and Trigonochlamydidae.

Secondary species hypotheses

Each of the 45 putative species was represented by at least two

sequences and most comprised several haplotypes and/or

sequences from more than one locality (Table 2). Almost all were

identically delimited, genetically distinct, and monophyletic

(Figs. 1–8; whole family trees available on request). All but two

putative species were identically delimited in the whole family and

subset analyses. One was PSH 19, G. maculosus, in which the Irish

and Spanish haplotypes formed a single putative species in the

whole family analysis, and two in the subset. The other was in the

Arion (Arion) dataset, where PSH 1 was delimited in the ABGD

analysis of all Arionidae, but split into PSH 1A, PSH 1B, and PSH

1C in the subset.

Figure 7. Milacidae & Testacellidae. Midpoint-rooted NJ trees based on COI data; values above branches are % bootstrap support (NJ), those

below are Bayesian posterior probabilities, expressed as %. Species new to the fauna of Britain and/or Ireland are indicated by *.

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Figure 8. Arionidae. Midpoint-rooted BI tree based on ITS-1 data; values above branches are % bootstrap support ($75), those below are Bayesian

posterior probabilities, expressed as % ($85).

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All putative species were genetically highly distinct. Where more

than one haplotype was found the minimum mean interspecific

distance was always 2.5 or more times greater, often tens of times

greater, than the mean intraspecific distance. The intraspecific

distance was lower than the prior value given by ABGD in all cases

except two putative species in the Milacidae (PSH 39 and PSH 40)

in which it slightly exceeded it (Table 2). This may be a result of

the limited data available on this family, whose ABGD analysis

was also unique in offering an alternative, yet larger, stable

number of putative species. Under this alternative PSH, for

example, Italian sequences including Tandonia marinellii Liberto et

al. [56] would be separated from T. sowerbyi (Fe´russac). However,

our criterion favours more the conservative PSH.

Almost all putative species with more than one haplotype were

strongly supported as monophyletic in all NJ and BI analyses.

Again the exception was PSH 1: there was very weak support for

the monophyly of PSH 1 as a whole, yet support for the

monophyly of PSH 1A and a clade comprising PSH 1B +PSH

1C.

Most (35) of the putative species were morphologically unique,

externally and/or (in adult or near-adult) specimens, internally.

Most of the remainder were in Arionidae: PSH 1, whose

constituents PSH 1A and PSH 1B were each unique; PSH 1C,

represented by a single GenBank haplotype from the USA (and

discussed because of its delimitation as part of PSH 1); PSH 3,

represented in Britain only by a juvenile; PSH 7 and PSH 8, which

both corresponded to A. (M.) subfuscus (Draparnaud) but could not

be satisfactorily distinguished from one another even using the

genital features of [28]; and PSH 9 and PSH 10 which both

corresponded to A. (M.) iratii Garrido et al. but could not be

satisfactorily distinguished from one another. In Agriolimacidae,

the four PSHs into which Deroceras laeve (Mu¨ller) was split could not

be satisfactorily distinguished, partly because most specimens were

aphallic. More detailed discussion of morphological features,

geographical distribution, and identification of certain putative

species is given below.

In consequence, 36 of the 45 putative species in the PSH were

readily accepted as species in the SSH we propose. One of the

remainder, PSH 1, was split into two, SSH 1A and SSH 1B +1C,

because of its lack of monophyly and morphological heterogeneity.

PSH 7 and PSH 8, although morphologically indistinguishable,

were maintained as separate because together they were not

monophyletic in any analysis. In contrast, and to remain

conservative, two sets of other morphologically indistinguishable

putative species were combined (PSH 9 +PSH 10, and PSH 33 +

PSH 34 +PSH 35 +PSH 36). The SSH we propose thus

recognises a total of 42 species, or 44 including Boettgerillidae and

Trigonochlamydidae.

Comparison of the SSH to the known fauna

Of the 44 SSH species, 32 included one or more GenBank

sequence. The remaining 12 had presumably not previously been

sequenced for the gene region in question. As expected, many

species in each category (24 in the former and 9 in the latter,

totalling 33) could readily be considered equivalent to known

species in the British and Irish fauna [1] (Table 2, and names on

Figs. 1–8). This was consistent with the morphological features we

used to identify specimens and, in general, the names of previously

identified GenBank sequences. For example, PSH 4 consists of

sequences of A. (A.) vulgaris (or ‘‘A. lusitanicus’’ non Mabille) from six

continental countries, and from specimens from around Britain

and Ireland whose morphology conforms to that species (e.g.

[21,42]). This confirms it is widespread in Britain and Ireland,

including in SW England where it has been recorded since at least

the 1960s [2,21]). Most of the remaining 32 British and Irish SSH

species correspond to other widespread and relatively well-

characterised species. Two species described from Ireland, G.

maculosus and A. (A.) flagellus, are each moderately closely related to

Spanish haplotypes (Fig. 1). We note however that unless the Irish

haplotypes are detected in Spain, neither native status or ancient

introduction can be ruled out (e.g. see [57,58]). For brevity, we do

not discuss the currently known SSH species further, except for

those which appear to show evidence of hybridisation (see below).

This leaves 11 SSH species needing further discussion. In one

case, a single SSH species corresponded to more than one known

species: A. (Carinarion) circumscriptus Johnston and A. (C.) silvaticus

Lohmander. Together these formed a single putative species, PSH

11 which was monophyletic (Fig. 3). Both names were thus

associated with the single species SSH 11, although within it, the

British and Irish haplotypes identified as A. (C.) circumscriptus and A.

(C.) silvaticus clustered in separate monophyletic groups. This is

consistent with the findings of Geenen et al. [27] who suggested

the three widespread Carinarion taxa be considered a single

biological species. However, like them we found that the third of

these, A. (C.) fasciatus (Nilsson) (SSH 12) was considerably more

distinct than the others and more reliably identifiable morpho-

logically. Given this and the evidence of habitat separation

between A. (C.) circumscriptus and A. (C.) silvaticus in Britain and

Ireland [2] we suggest these two be treated as a single species,

perhaps with two recognised subspecies. As A. (C.) fasciatus is more

distinct and formed a separate PSH we retain it as a species.

In another case, two morphologically indistinguishable SSH

species corresponded to a single known species. SSH 7 and SSH 8

each corresponded to specimens identified as A. (M.) subfuscus,it

being uncertain to which (if either) the name should be

preferentially applied [26]. Indeed Pinceel and others [25,26,28]

found this taxon to consist of multiple deeply divergent 16S

lineages which they attributed to allopatric divergence and an

accelerated rate of mutation. This included two present in Britain

and Ireland (their S1 and S2 [25]) that correspond to our SSH 7

and SSH 8. As there was some evidence of interbreeding between

these they treated them as evolutionary species requiring further

revision before being named, as was possible with A. (M.)

transsylvanus Simroth, endemic to Romania and Poland [28]. We

found limited agreement between SSH 7 and SSH 8 and the

subtle morphological characters of S1 and S2 offered by [28]. All

S2 specimens had genitalia corresponding to these in that the

epiphallus joined the atrium between the bursa and oviduct, but

this pattern was also seen in some S1 specimens. S2 was found

only in northern, western and upland areas (Table S1) although

has also been found sympatric with S1 in central and southeast

England by [25]. Until further data are available we treat both as

part of the still-enigmatic taxon A. (M.) subfuscus.

This left eight SSH species that did not correspond to any of the

36 species in [1], 35 of which were themselves successfully

delimited using the same criteria. This represents an increase of

22% on the known fauna, a striking and unexpected finding that

led to further investigation of these ‘‘additional’’ species.

Additional species

Of the eight additional SSH species, four were in Arionidae and

one each in Limacidae, Agriolimacidae, Milacidae and Testacelli-

dae. Up to four of the species appear previously undescribed, but

we refrain from formal description here until further data are

available. We provide brief diagnoses and use the term ‘‘cf.’’

(confer) to indicate a nominal species to which the species should

be compared.

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Figure 9. Morphology of four potentially new species. External appearance and salient parts of genitalia shown alongside those from

sequenced similar species for comparison. Abbreviations: at, atrium; bc, bursa copulatrix; ep, epiphallus; ov, oviduct; pe, penis; pr, penial retractor

muscle.

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We note that none of the eight additional species are

morphologically cryptic, at least not by the strictest of definitions,

i.e., they are not impossible to reliably identify based on

morphology alone [54]. They could therefore have been

recognised without sequence data and so can be considered

previously overlooked. We also note that six of them were found at

more than one widely separated site, with two occurring in both

Britain and Ireland. The introduced or native status of each

remains open to question. The remaining two, as yet known only

from single established populations, are almost certainly accidental

introductions.

SSH 1B

1C: Arion (Arion) cf. vulgaris Moquin-Tandon,

1855. Potentially a new species. Each of the SSH species in Arion

subgenus (Arion) (with the exception of A. (A.) flagellus) was

approximately equally genetically distant from one another. SSH

1B was found at four sites in the east of England, from Yorkshire to

Kent, with another haplotype near London (a museum specimen

collected in 2004). They were mainly from disturbed habitats.

Externally the animal is variable and readily confused with either

A. (A.) vulgaris (SSH 4) or A .(A.) rufus (SSH 2). It is generally greyish

brown, with faint lateral colour bands persisting in some adults,

and the sole is often paler than typical A. (A.) vulgaris. It lacks the

rocking response of A. (A.) ater (SSH 1A) and A. (A.) rufus (e.g. [12]).

Internally it is very distinct from both A. (A.) ater and A. (A.) rufus,

having an elongate corrugated ligula in the oviduct, generally like

that of A. (A.) vulgaris. However, the ligula is delicately fringed for

all or part of its length as well as being corrugated (Fig. 9). These

delicate fringes were not found in the sequenced A. (A.) vulgaris

specimens. The correct name for this species is difficult to establish

at present (see also SSH 3, and the section on hybridisation below)

and more data on whether other populations can be consistently

distinguished from A. (A.) vulgaris is desirable. It appears not to

correspond with any of the four species discussed in the arionid

review by Jordaens et al. [29] whose ‘‘non-Iberian A. lusitanicus’’

presumably corresponds to A.(A.) vulgaris. Neither ITS-1 nor 16S

data suggest a close relationship between our species and A. (A.)

flagellus, or to the true Portuguese A. lusitanicus Mabille (Fig. 8). It is

possible that some of the British ‘‘A. lusitanicus’’ populations studied

by Davies [21] in her initial discrimination of these from A. (A.)

flagellus may belong to this species, especially given inconsistencies

in the mating behaviour recently highlighted by Dreijers et al.

[45]. However, our data suggest that A. (A.) vulgaris itself (SSH 4),

being much more widespread in Britain and Ireland, is also likely

to have been present in Surrey and the other areas discussed by

Davies [21]. It is unknown where else in Europe our species occurs

and whether it is native to Britain, there being a lack of data from

central and southern France in particular. Noble & Jones [17]

discussed a ‘‘non-pest’’ form of A. (A.) vulgaris (again as ‘‘A.

lusitanicus’’) from southern France while Noble [42] discussed

alternative Pyrenean forms that might correspond to this species.

The most similar available 16S sequences are those recognised in

some analyses as PSH 1C (Fig. 1). These are from ‘‘Arion rufus’’

from the west coast of the USA (Washington State: [30]), whose

morphology may deserve further investigation.

SSH 3: Arion (Arion) cf. empiricorum A. Fe

´russac

1819. Sequences of this SSH species have been identified as

‘‘Arion rufus’’ from Belgium, Denmark, and (as introduced) Canada

and the USA. In Britain it was represented by a single juvenile

from a cemetery in central London identified as (and sympatric

with) A.(A.) vulgaris, whose juveniles can look very similar. It has

been proposed [59] that the name A. (A.) rufus Linnaeus be applied

to the species occurring in Britain (presumably the widespread and

common SSH 2 in our study). Alternative names available for

similar continental species include Arion empiricorum Fe´russac, [1],

but this cannot be resolved here. The London population is likely

to be an introduction and as with SSH1B +1C, should be

considered a potential plant pest.

SSH 9

10: Arion (Mesarion) cf. iratii Garrido, Castillejo

& Iglesias, 1995. This SSH combines PSH 9 and PSH 10, both

from upland forestry areas in the Brecon Beacons National Park

and South Wales valleys. They could not be distinguished

morphologically but both differ from all sampled British and Irish

A. (M.) subfuscus and A. (M.) fuscus in having dark spots or speckles

on the back in at least some individuals. This feature is shared with

three species described by Garrido et al., 1995 [60] from the

Pyrenees which differ from each other only very subtly: A. (M.)

iratii,A. (M.) lizarrusti, and A. (M.) molinae. Given the accelerated

rate of 16S evolution posited for Mesarion species [25] it is not clear

whether the constituent PSHs should be combined in one SSH or

not, but analyses suggest both are related to A. (M.) iratii and A.

(M.) lizarrusti rather than to A. (M.) subfuscus, A. (M.) fuscus,A. (M.)

transsylvanus etc. (Fig. 2). Neither Wales nor the Pyrenees were

represented in the analysis of Pinceel et al. [28,29]; indeed they

noted that sampling in Iberia had yielded none of the A. (M.)

subfuscus lineages S1–S5. This SSH is therefore provisionally

associated with A. (M.) iratii. Its introduction to Britain with

forestry or industry cannot be ruled out, but neither can a native

distribution that includes the highest uplands in southern Britain.

SSH 18: Arion (Kobeltia) cf. fagophilus (de Winter,

1986). Potentially a new species. Genetically it is distinct from

all British and Irish Kobeltia and from the central European A. (K.)

obesoductus Reischutz (formerly A. (K.) alpinus Pollonera) (Table 2,

Fig. 3). Morphologically it is most similar to A. (K.) occultus

Anderson from Northern Ireland and A. (K.) fagophilus de Winter

from the western Pyrenees. Externally, the coarser tubercles

distinguish the new species from all British or Irish Kobeltia except

A. (K.) occultus. The latter is yellower and flatter, with even coarser

tubercles. Both resemble A. (K.) fagophilus externally except in

tentacle colour, which is bright red in A. (K.) fagophilus [61] versus

cold blue-black in these species. Davies [20] considered tentacle

colour one of the most reliable external features distinguishing

Kobeltia species. Internally, this species lacks an epiphallus process

and differs from A. (K.) fagophilus and A. (K.) occultus in having a

donut-like swelling at the entrance of the bursa copulatrix to the

atrium, rather than a thickening as in the other species (Fig. 9)

[23,61]. It is common at several sites in lowland river valleys in

South Wales, in wet Alnus woodland. It is probably an introduction

from the Pyrenees, although this small and indistinct species could

be an overlooked native restricted to this relatively natural habitat

type.

SSH 22: Limax cf. dacampi Menegazzi, 1854. Potentially

a new species. Genetically it is clearly not part of the two known

British and Irish species which form separate PSHs (Table 2,

Fig. 4). It forms a clade (Fig. 4) with sequences from the Apennines

of central Italy to which no species name is applied, and another

group comprising sequences from Italy, Switzerland and Croatia

that depositors identified as Limax cf. dacampi Menegazzi [32]. In

the whole family ABGD analysis, both clades form a single PSH

but form two PSHs in the analysis of Limax only. In Britain it was

found at a single site in North-east Yorkshire, in an ornamental

mixed woodland in the grounds of a school. Adults and juveniles

have been found there on several occasions over the past two years

(A. Norris & T. Crawford, pers. comm.). The species is externally

and internally distinct from the other British species (e.g. as

monographed by Quick [12]), notably in having a much longer

penis than either (both in absolute terms and in being longer than

the body even when retracted) (Fig. 9). It is almost certainly an

introduction from central Italy.

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SSH 32: Deroceras panormitanum (Lessona & Pollonera,

1882). An established population in a public city garden in

South Wales has the internal anatomy of this Sicilian/Maltese

species (one blunt penial appendage, one tapering) and an almost

identical COI haplotype (Fig. 6). This species and its relatives were

recently thoroughly revised based on molecular, morphological

and behavioural data [33]. Its external morphological variation

overlaps with that of the widely introduced D. invadens Reise et al.,

so it may have been overlooked elsewhere. In Britain, it is

probably a recent introduction.

SSH 39: Tandonia cf. cristata (Kaleniczenko,

1851). Found in South Wales and western Ireland in allotments,

adjacent woodlands and a churchyard. Museum collections

suggest that it is more widespread than our data indicate (although

we could not amplify DNA from these). It has evidently been

misidentified as juvenile T. budapestensis (Hazay), T. sowerbyi or

Milax gagates (Draparnaud) in the past but is genetically distinct

from these (Fig. 7). It differs from these in its smaller size, generally

paler colour, plain sole, and in having dark pigment confined to

the spaces between tubercles. Using Wiktor’s key [62] it keys to

Tandonia cristata (Kaleniczenko), a species of countries bordering

the Black Sea. There is some discrepancy between accounts of the

internal anatomy of this species [62,63], and there are many

similar species. Until these can be better studied we consider the

British and Irish species likely to represent T. cristata, which would

be introduced. It is potentially a root crop pest like other Tandonia

species.

SSH 45: Testacella cf. scutulum G. B. Sowerby I,

1821. Potentially a new species. Genetically it is as distinct from

T. scutulum Sowerby as the other two Testacella species are from one

another (Fig. 7), and forms a PSH separate to them in delimitation

analysis (Table 2). Morphologically, it is close only to T. scutulum

with which it keys out in the synoptic key of Nardi & Bodon [64].

However, the two differ in the width of the penis (apparently

independently of the size of the animal) and in whether or not the

penis tapers (Fig. 9). Externally, they differ in body colour and in

the shape formed by the meeting of the two dorsal grooves,

although these features might not be consistent. The subterranean

Testacellidae are rarely collected so only limited material is

available and intraspecific morphological variation remains poorly

understood [65] (although there is abundant evidence that T.

haliotidea Draparnaud and T. scutulum are distinct, contrary to [2]).

It also remains to be established to which of the two similar species

Sowerby’s name ought to be restricted. Early descriptions of the

anatomy of T. scutulum from near the type locality in London

[11,66] show a broad, tapering penis. Both species are probably

introductions from the Mediterranean [2]; recent figures suggest

both may occur in Italy [67].

Hybridisation

Here, intraspecific hybridisation is considered the exchange of

genes between PSHs that would otherwise be considered

phylogenetic species, and would otherwise correspond precisely

to morphologically distinct and widely-recognised species (e.g.

‘‘A.(A.) vulgaris’’). Although limited, our data suggest that hybrid-

isation may have occurred in the larger Arionidae. More

unexpectedly, contrasts between mtDNA and morphology also

suggest hybridisation in Agriolimacidae and Limacidae.

Interspecific hybridisation has long been suspected, if not fully

proven, among the large Arionidae, in particular between A. (A.)

ater and A. (A.) rufus and between one or both of these and A. (A.)

vulgaris (e.g. [15,42,43,44,68,69]). Recently, Dreijers et al. [45]

showed experimentally that German A. (A.) vulgaris (as A. lusitanicus)

and German A. (A.) rufus can reciprocally exchange sperm. ITS-1

sequences showed far less variability than mtDNA sequences. NJ

and BI analyses show that all five SSHs in the larger Arionidae

(SSHs 1A, 1B, 2, 3 and 4) form a single clade without internal

structure, to the exclusion of A. (A.) flagellus (SSH 5) and all other

Arionidae (Fig. 8). Similar conflicts between mtDNA and ITS-1

data have been interpreted as evidence of hybridisation or a

shared nuclear gene pool in arionid studies [25,27]. By these

criteria, hybridisation might well have occurred between any

combination of the five SSHs in the larger clade, either before or

since their arrival in Britain or Ireland. In contrast, A. (A.) flagellus

is genetically distinct from any of them in both mtDNA and ITS-1.

We conclude that evidence for its hybridisation with other British

or Irish species is lacking.

Morphologically, we found that all adult specimens we

sequenced could be referred to one of the known British species

in the large Arionidae, or the newly-discovered SSH 1B, and were

unconvinced that any individual was indisputably hybrid. We

found no specimens whose species assignment by morphology was

incompatible with their assignment by mtDNA (unlike the non-

arionid examples below). It has been suggested that hybrids are

morphologically recognisable [17,42,68] but this was not clarified

by our data. It remains possible that hybrids in this group

(especially after the F1 generation) could resemble either parent

species and be effectively impossible to distinguish from them

morphologically [46]. Elsewhere in Europe, such hybrids might

account for the inclusion of GenBank haplotypes of Belgian,

Polish, Spanish and Czech ‘‘A. rufus’’, and Polish ‘‘A. lusitanicus’’ in

our PSH 1A. We nonetheless remain confident that this SSH be

called A. (A.) ater, it being the only SSH that included jet-black

individuals from remote western and upland regions, considered

typical of A. (A.) ater [11,17,42,68] (Table S1). The identities of the

other SSHs in the group are discussed above. While the taxonomy

may be revised in future, we provisionally recognise five species in

this group in the British fauna, three of which occur in Ireland (not

including A. (A.) flagellus).

In Agriolimacidae, in ABGD, NJ and BI analyses (Fig. 6)

Deroceras reticulatum (Mu¨ller) and D. agreste (Linnaeus) formed two

PSHs/clades. Using the internal morphological criteria of Wiktor

[70] however, neither morphospecies was monophyletic and so

they cannot be accepted as separate SSHs here unless some

individuals are considered hybrids. All individuals with D. agreste

anatomy had a D. agreste mtDNA sequence, but some individuals

with D. reticulatum anatomy had a D. agreste mtDNA sequence.

These were largely from the same sites as the D. agreste individuals,

or nearby. The pattern could be explained by one-way introgres-

sion of D. reticulatum genes into D. agreste populations. As mtDNA is

maternally inherited in pulmonates, the D. reticulatum individuals in

the D. agreste clade would be introgressed hybrids descended from a

D. agreste mother whose eggs were fertilised by D. reticulatum sperm.

That no D. agreste individuals with D. reticulatum mtDNA were

found may indicate that fertilisation in the opposite direction has

not occurred, is rarer, or results in sterile offspring. One-way

interspecific sperm transfer is known in other Deroceras species [33].

In Britain and Ireland D. agreste is a rare, northerly distributed

post-glacial relict species characteristic of uplands and other wild

habitats, while D. reticulatum is a common, widespread and often

synanthropic species [2,13]. If the speculation is correct, this

interaction between the species could partly explain their habitat

and range discrepancies, with the post-glacial D. agreste in long-

term retreat exacerbated by hybridisation with D. reticulatum.

Similarly in Limacidae, in ABGD, NJ and BI analyses (Fig. 5)

Limacus flavus (Linnaeus) and L. maculatus (Kaleniczenko) formed

two PSHs/clades. Again, using internal morphological criteria

[71] neither morphospecies was monophyletic.The only morpho-

Increase in the Slug Fauna of Britain and Ireland

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logically typical adult L. flavus (i.e. one with the bursa copulatrix

duct originating high up on the vagina; [71]) was in PSH 23 in

which all other specimens either had immature genitalia or were

typical of L. maculatus (i.e. with the bursa duct obtaining from the

base of the vagina, atrium, or base of the penis). Many of them

resembled L. maculatus externally [13,14]. All specimens in PSH 24

resembled L. maculatus internally and externally, including one

from near the type locality in Crimea, Ukraine. GenBank

sequences attributed to L. flavus were split between the two clades,

with continental European sequences [31] in the L. flavus clade and

British sequences in the L. maculatus clade. The pattern can be

interpreted in a similar way to D. agreste/reticulatum. Neither Limacus

species is considered native although L. flavus has been known in

Britain since at least 1685 [2,8] and was the only Limacus known in

Ireland in 1891 [10]. Anecdotal evidence and the increasing

frequency of L. maculatus being recorded in Britain and Ireland

(e.g. [72], also personal observations) suggest it is becoming

common and widespread while L. flavus becomes rarer. One-way

introgression from the more recent invader could explain this

trend.

Conclusion

This study demonstrates how a simple, geographically extensive

yet morphologically informed approach to DNA sampling can be

effective in screening a fauna for additional taxa and other changes

resulting from past invasions. The detection of so many additional

species, and the evidence for hybridisation in Agriolimacidae and

Limacidae, were unexpected. The proportion (22%) of additional

species revealed is remarkably high compared to other recent

studies and given the long history of study of this fauna. For

example, no additional taxa were found in DNA barcoding

surveys of the complete Irish solitary bee fauna of 55 species [73]

or the Welsh native and archaeophyte angiosperm and conifer

flora of over 1000 species [74]. Our estimates also exceed the

proportion of cases of deep intraspecific divergence (i.e. potential

additional species) found and discussed among Bavarian myria-

pods (3% of 122 species [75]), Romanian butterflies (4% of 180

species [76]) or Bavarian geometrid moths (5% of 400 species

[77]). With the exception of [75] these studies were largely based

on existing museum collections so may have overlooked additional

taxa in the field, but as we found such taxa may also be present in

existing collections as in the case of Tandonia cf. cristata. Only

among British and Irish earthworms [78] and Scottish tardigrades

[79] have similar studies reported as high a proportion of

additional putative taxa. These are groups with a much less rich

history of taxonomic description, participatory study and record-

ing than slugs, and do not include as many potentially serious

pests. Indeed, further undetected slug species may already be

lurking, particularly in large urban areas which our survey could

not thoroughly cover. We conclude that although the British and

Irish slug fauna is well-studied, it was far from fully-known, even as

recently as 2008 [1]. If further invasions are to be detected or

controlled, this and other slug faunas worldwide will need to be

watched more closely in future.

Supporting Information

Figure S1 ABGD analysis for Arionidae. Arrow indicates

selected PSH and its corresponding position on the distribution of

pairwise K2P distances.

(TIF)

Figure S2 ABGD analysis for Limacidae and Agriolima-

cidae. Arrow indicates selected PSH and its corresponding

position on the distribution of pairwise K2P distances.

(TIF)

Figure S3 ABGD analysis for Milacidae and Testacelli-

dae. Arrow indicates selected PSH and its corresponding position

on the distribution of pairwise K2P distances.

(TIF)

Table S1 Specimen data, haplotypes, and GenBank

accession numbers.

(XLS)

Table S2 GenBank data and haplotypes included in

analyses.

(XLS)

References S1 Additional references cited in Table S2.

(DOC)

Acknowledgments

Among the many people who assisted with fieldwork or provided

specimens we thank in particular Chris du Feu, Adrian Norris, Simon

Allen, Adrian Sumner, Ron Carr and Christian Owen. We thank Nico

Puillandre for advice on ABGD and two reviewers for comments on the

manuscript.

Author Contributions

Conceived and designed the experiments: BR RA JAT WOCS. Performed

the experiments: BR RA JAT WOCS. Analyzed the data: BR RA JAT

WOCS. Contributed reagents/materials/analysis tools: BR RA JAT

WOCS. Wrote the paper: BR RA JAT WOCS.

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Feb 2025
J CONCHOL

Semilimax pyrenaicus, a terrestrial gastropod with a primarily Lusitanian distribution, is mainly confined to the Pyrenees and Ireland, with a few isolated populations in western France. It was first recorded in Schoten (Belgium) in February 2024. Further investigations confirmed the presence of a well-established, localized population, with different age classes, ranging from early juveniles to adults. The species identification was confirmed through genetic analysis. The occurrence of the population near two garden centers suggests a recent introduction via human-mediated pathways, such as the horticultural trade. This finding also highlights the significant role of citizen science in documenting new occurrences.

Using radio frequency identification technology to track the movement of slugs within domestic garden habitats

Article

Apr 2025

Slugs (Gastropoda: Stylommatophora) are common domestic garden inhabitants in the United Kingdom (UK) but few studies have explored the spatial behaviour of slugs within these habitats, largely due to limitations in available mark-recapture methodologies. Attempts to improve such methodologies for slugs have previously come at a high economic cost, short lifespan, and inability to detect slugs beneath the soil surface. The use of radio-frequency identification (RFID) technology has previously shown potential in overcoming these limitations, reducing the cost and impracticality of studying slug movement. However, this has yet to be applied to a domestic garden setting. This study explored the potential of RFID technology to track slug movement in a UK domestic garden. Both the common garden pests Deroceras reticulatum and Arion hortensis and the non-pestiferous Limacus maculatus were tagged with modest to no detrimental effects on survival at 62.5%, 25%, and 0% respectively. A novel method for the containment of D. reticulatum within a section of garden habitat was also assessed and found to be effective in containing 80% of slugs for a period of 67 days. RFID technology was able to identify the location of D. reticulatum amongst dense garden foliage and sub-soil for 67 days, enabling slug tracking within a domestic garden habitat. No homing instinct was identified in this study when attempting to induce a home territory over a period of 67 days for the species D. reticulatum. This study demonstrates the potential of RFID technology to track the locomotion of slugs in UK domestic garden habitats and provides an opportunity to update our knowledge on this subject by overcoming the methodological limitations associated with the high cost and impracticality of studying slugs.

Chemotactic Responses of Oscheius myriophilus to Mollusk Mucus

Article

Full-text available

Dec 2024

Terrestrial slugs and snails can significantly harm agriculture. Due to environmental concerns associated with chemical molluscicides, biological control methods are increasingly being explored. Oscheius myriophilus (Poinar, 1986), a nematode species recently discovered in association with Arion vulgaris Moquin-Tandon, 1855, holds promise as a biocontrol agent for gastropod pests. In this study, we investigated the chemotactic response and motility of O. myriophilus when exposed to the mucus of five mollusk species: Helix pomatia Linnaeus, 1758, Cernuella virgata (Da Costa, 1778), Deroceras reticulatum Müller, 1774, A. vulgaris, and Tandonia budapestensis Hazay, 1880. Our experiments were conducted at two temperatures (20 °C and 25 °C) to assess how environmental conditions influence nematode behavior. The results demonstrated that the chemoattractiveness of mollusk mucus to O. myriophilus was significantly influenced by both the species of mollusk and the temperature. Overall, nematode motility was higher at 20 °C than at 25 °C, indicating that lower temperatures may enhance the activity of O. myriophilus. Among the tested mollusk species, C. virgata mucus consistently attracted the highest number of nematodes, especially at the lower temperature. Our findings indicate that the chemotactic response of O. myriophilus to mollusk mucus may have potential for the targeted biocontrol of pest mollusks. While C. virgata demonstrated strong attractant potential at the tested temperatures, particularly under cooler conditions (20 °C), further research is needed to confirm whether this represents a consistent temperature-related effect. Future studies should aim to identify the specific chemical cues in mollusk mucus that trigger nematode attraction and examine how these signals interact with a broader range of environmental variables, including temperature, to influence nematode behavior.

Microplastic contamination is widespread across invertebrate taxa frequently consumed by terrestrial vertebrates

Article

Apr 2025
ENVIRON TOXICOL CHEM

Little is known about the contamination of terrestrial food webs by microplastics. We conducted community-level assessments of invertebrates at 51 sites in England, sampling four trophic levels and six taxonomic groups. We hypothesized that plastic contamination would be most common in herbivores and detritivores, owing to the accumulation of microplastics in soil and plant material. A total of 581 samples, comprising pooled individuals from the same site and species, were digested using hydrogen peroxide and subsequently examined by micro–Fourier Transform infrared microscopy. Plastic was found in 11.8% (95% CI 9.3%, 14.6%) of the samples, including all tested taxonomic groups and trophic levels, regardless of life history traits or dietary habits. The highest prevalence of plastic-positive samples was in Opisthopora (earthworms) (29.4%) and Stylommatophora (snails and slugs) (24.1%). Herbivores and detritivores were the trophic groups with the highest proportion of plastic-positive samples (15.4% and 10.2% respectively). Although the prevalence was lower in carnivores, these animals ingested significantly larger microplastics when compared with other trophic levels. The most abundant polymers were polyester, polyethylene, polypropylene, nylon, and polyacrylamide. Polyester was present across all taxonomic groups and trophic levels, along with varying amounts of other polymers. The lack of correspondence between soil samples and invertebrate samples highlights the requirement to survey a range of invertebrates as well as soil to gain a full insight of the level of contamination in each site to ensure that the correct conclusions are drawn.

Chemotactic Responses of Slug-Parasitic Nematodes to Potato-Tuber-Emitted Volatile Organic Compounds

Article

Full-text available

Apr 2025

Parasitic nematodes play a vital role in soil ecosystems, contributing to natural pest suppression. Among them, slug-parasitic species such as Phasmarhabditis papillosa (Schneider) Andrassy, Oscheius myriophilus (Poinar), and Oscheius onirici Torrini et al. are promising biological control agents against mollusk pests. These nematodes rely on plant-emitted volatile organic compounds (VOCs) for host location, yet their chemotactic responses to specific VOCs remain unclear. This study assessed the responses of P. papillosa, O. myriophilus, and O. onirici to VOCs emitted by potato (S. tuberosum) tubers under varying temperature (18 °C, 22 °C) and concentration conditions (pure compound, 0.03 ppm). The results indicate that octanal was the strongest attractant, particularly for O. myriophilus, while nonanal exhibited species-dependent effects. Hydrocarbons such as undecane and 1,2,4-trimethylbenzene had minimal or repellent effects, whereas 6-methyl-5-hepten-2-one showed moderate attraction. Chemotactic responses were stronger at 18 °C, and attraction increased with higher VOC concentrations, suggesting a threshold-dependent response. These findings enhance our understanding of plant–nematode interactions and suggest that octanal and 6-methyl-5-hepten-2-one could improve nematode-based slug control strategies. However, environmental factors such as soil composition and microbial activity may influence VOC diffusion and nematode recruitment. Future research should focus on optimizing VOC formulations, assessing field applicability, and integrating these findings into sustainable pest management programs.

First records of Arion intermedius and Arion transsylvanus (Stylommatophora: Arionidae) in Hungary.

Article

Full-text available

Mar 2025

Responses of Parasitic Nematodes to Volatile Organic Compounds Emitted by Brassica nigra Roots

Article

Full-text available

Mar 2025

Parasitic nematodes, particularly those in the Rhabditidae family, are vital components of belowground ecosystems, contributing to pest regulation and sustainable agriculture. This study investigated the chemotactic responses of three nematode species—Phasmarhabditis papillosa, Oscheius myriophilus, and O. onirici—to volatile organic compounds (VOCs) emitted by Brassica nigra roots under herbivory by Delia radicum larvae. Using a chemotaxis assay, the effects of five VOCs (dimethyl sulfide, dimethyl disulfide, allyl isothiocyanate, phenylethyl isothiocyanate, and benzonitrile) were tested at two concentrations (pure and 0.03 ppm) and two temperatures (18 °C and 22 °C). The results revealed that VOCs and temperature significantly influenced nematode responses, while nematode species and VOC concentration showed limited effects. Benzonitrile consistently demonstrated strong chemoattractant properties, particularly for O. myriophilus and O. onirici. Conversely, allyl isothiocyanate exhibited potent nematicidal effects, inhibiting motility and causing mortality. Dimethyl disulfide and dimethyl sulfide elicited moderate to strong attractant responses, with species- and temperature-dependent variations. Significant interactions between VOCs, temperature, and nematode species highlighted the complexity of these ecological interactions. These findings emphasize the ecological roles of VOCs in mediating nematode behavior and their potential applications in sustainable pest management. Benzonitrile emerged as a promising candidate for nematode-based biocontrol strategies, while allyl isothiocyanate showed potential as a direct nematicidal agent. The study underscores the importance of integrating chemical cues into pest management systems to enhance agricultural sustainability and reduce reliance on chemical pesticides.

Las babosas terrestres de Cataluña y Andorra

Article

Full-text available

Apr 2025

En este trabajo estudiamos las babosas de la Comunidad Autónoma de Cataluña (Nordeste de España), formada por las cuatro provincias de Barcelona, Tarragona, Lleida y Girona y del Principat d'Andorra. Se revisan los extensos antecedentes bibliográficos sobre estos moluscos en la región (es el área más estudiada del estado español) y se presenta una lista de 33 especies, muchas de ellas recientemente descubiertas y nombradas, en especial del complicado género Arion, con una breve descripción de cada una de ellas y la mención de las citas bibliográficas existentes, y otras de nuestras propias prospecciones y recolecciones. En algunos casos se agrupan las formas similares en complejos de especies, cuestión que se discute más extensamente para el género Arion. Además, se presentan mapas de distribución de las 33 especies, fotografías de las más significativas y dibujos esquemáticos de las genitalias de todas ellas. In this paper we study the slugs of Catalonia (NE Spain), formed by the four provinces of Barcelona, Tarragona, Lleida and Girona, and the Principality of Andorra. The extensive bibliographic background on these molluscs in the region (it is the most studied area of Spain) is reviewed and a list of 33 species is presented, many of them recently discovered and named, especially of the complicated genus Arion, with a brief description of each of them and the mention of the existing bibliographic citations, and also other species found during our own prospections and collections. In some cases, similar forms are grouped into species complexes, a question that is discussed more extensively for the genus Arion. Distribution maps of the 33 species, photographs of the most significant species and schematic drawings of the genitalia of all of them are also presented.

Arion occultus n.sp., a new slug of subgenus Kobeltia Seibert (Pulmonata: Arionidae) from Ireland

Article

Full-text available

Dec 2004
J CONCHOL

Roy Anderson

A new species of slug, Arion occultus, is described from Co Down in Northern Ireland. It belongs to the hortensis group in subgenus Kobeltia of Arion, and is superficially similar to Arion distinctus Mabille. Its origins are unknown and it appears to be a recent immigrant to Ireland. Distinctive features include: a rough external appearance due to strongly raised and keeled tubercles on the dorsum; a uniformly yellow and rather broad foot fringe; an epiphallus which lacks either pigmentation or a bulbous swelling at its apex and which also lacks an epiphallus process at the junction of the epiphallus with the distal oviduct, features which are otherwise general in the hortensis group; the possession of a simple spermatophore comprising an oblong, flattened sac lacking serrations or other distinctive features. Externally the integument possesses abundant yellow chromatophores lightening an otherwise predominantly dark dorsum. Size is larger than distinctus with broad, high lateral bands, below which the sides are predominantly pale or white, rather than grey. Although superficially similar to the central European Arion alpinus the latter does not belong to the hortensis group and has a very different genital anatomy.

Identifying Land Snails and Slugs in Canada: Introduced Species and Native Genera

Book

Full-text available

Apr 2010

Arion flagellus Collinge and A. lusitanicus Mabille in the British Isles: A Morphological, Biological and Taxonomic Investigation

Article

Dec 1987
J CONCHOL

Stella M. Davies

Tandonia rustica (Millet), a Slug new to the British Isles

Article

May 1987
J CONCHOL

Eric G Philip

Atlas of Land and Freshwater Molluscs of Britain and Ireland

Book

Jan 1999

Michael P. Kerney

The Arion subfuscus complex in the eastern part of the Iberian Peninsula, with redescription of Arion subfuscus (Draparnaud 1805) (Gastropoda: Pulmonata: Arionidae)

Article

Mar 1995

Segregates of the Arion hortensis complex (Pulmonata: Arionidae), with the description of a new species, Arion owenii

Article

Nov 1979
J CONCHOL

S.M. Davies

Terrestrial Slugs: Biology and Control

Article

Jan 1992

A. South

British slugs (Pulmonata; Testacellidae, Arionidae, Limacidae)

Article

Jan 1960

H E Quick

Reise, H., Hutchinson, J.M.C, Schunack, S. & Schlitt, B. (2011): Deroceras panormitanum and congeners from Malta and Sicily, with a redescription of the widespread pest slug as Deroceras invadens n.sp.

Article

Dec 2011

ABSTRACT: The name Deroceras panormitanum is generally applied to a terrestrial slug that has spread worldwide and can be a pest; earlier this tramp species had been called Deroceras caruanae. Neither name is appropriate. The taxonomic descriptions apply to a species from Sicily and Malta. This true D. panormitanum and the tramp species are distinct in morphology and mating behaviour. For instance, the penial caecum of D. panormitanum is more pointed, everting faster at copulation. The size of the penial lobe varies considerably in preserved specimens but is always prominent at copulation. D. panormitanum is distinct from the Maltese endemic Deroceras golcheri, but a phylogeny based on COI mtDNA implies that they are more closely related than is the tramp species. D. golcheri has a still closer counterpart on Sicily, but we leave the taxonomy of this “species X” unresolved. In interspecific crosses, D. panormitanum may transfer sperm to the partner’s sarcobelum whereas the partner fails to evert its penis (D. golcheri) or to transfer sperm (the tramp species). Names previously applied to the tramp species originally referred to D. panormitanum or are otherwise invalid, so it is here formally redescribed as D. invadens. Deroceras giustianum Wiktor, 1998 is synonymised with D. panormitanum. KEY WORDS: simultaneous hermaphrodites, unilateral copulation, genital morphology, mating behaviour, COI mtDNA, cryptic species, reproductive isolation, terrestrial slugs, Agriolimacidae, Mediterranean

Slugs of Britain and Ireland: identification, understanding and control.

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