Rapid increase in knowledge about the distribution of introduced predatory Testacella species (Gastropoda: Stylommatophora) in North America by community scientists

Testacellidae are a group of carnivorous semislugs with a vestigial ear-shaped shell near the posterior end of their elongate bodies. The single genus Testacella is native to the western Mediterranean and western Europe, but some species have been introduced into temperate countries worldwide. The species are subterranean and feed on earthworms. Due to their predominantly subterranean lifestyle, Testacella species are difficult to observe. Here, I review of the distribution of introduced Testacella species in North America and compare the knowledge based on literature references and museum specimens collected over more than a century with that observed by community scientists on the web platform iNaturalist in less than 10 years. Literature references and museum specimens indicate scattered occurrences of the introduced Testacella haliotidea in the eastern USA and a few more occurrences in the Pacific states of the USA and in British Columbia. Community scientists have nearly replicated a century of knowledge about the distribution of Testacella in North America. Their records even show a more continuous distribution of T. haliotidea in the Pacific states and confirm its presence in Tennessee, but not yet in Pennsylvania. They also provided the first records of T. haliotidea in Georgia and Mexico, and discovered a second introduced Testacella species, Testacella maugei, in California. The most distant occurrences of T. maugei in California are about 100 km apart, indicating that the species was probably introduced several years ago.


Introduction
Observations by citizen scientists have always contributed significantly to our knowledge of the diversity of organisms and their distribution. However, only a small proportion of naturalists' observations have been documented in the form of museum specimens or published reports. Many naturalists are not interested in collecting and preserving material or writing reports about their findings, and individual observations may not be newsworthy, but could contribute to statistical analyses. But even important new observations have been lost because most naturalists can only identify and assess the significance of a fraction of their observations. Tools that allow naturalists to easily make their observations available to the public could solve such problems and make a major contribution to biodiversity research. Such tools now exist in the form of online platforms where naturalists can easily upload observations in the form of images, videos, or audio files (Chandler et al. 2017;Johnson et al. 2020;Bonney 2021), such as iNaturalist (https:// www. inatu ralist. org/). These platforms also allow interactions between naturalists and between naturalists and specialists so that observations of sufficient quality can be identified. There are already many studies analysing data quality and biases on these platforms (Kosmala et al. 2016;Hochmair et al. 2020;Callaghan et al. 2021).
For land snails, studies have analysed observations reported on iNaturalist from specific regions (Barbato et al. 2021;Rosa et al. 2022), reported discoveries of new alien species (Vendetti et al. , 2019Hausdorf and Solvery 2021), and even described species new to science that were first observed on iNaturalist (Hausdorf and Kalaentzis 2021). In this paper, I summarise the knowledge of North American populations of Testacella, a group of subterranean semislugs introduced from the Western Palaearctic region, and compare what has been documented in the literature and museum collections for over a century with the observations of community scientists on the platform iNaturalist in less than 10 years, including new distribution records and even an introduced species new to North America.
Testacellidae are a group of carnivorous semislugs with a vestigial ear-shaped shell near the posterior end of their elongate bodies. It comprises a single genus Testacella Draparnaud, 1801. The Testacella species are subterranean and feed on earthworms (Oldham 1915). They have a long odontophore that moves outside the mouth to grasp a worm, pulling the first part of the body into the mouth and then ingesting the rest of the worm (Crampton 1975). For this purpose, the v-shaped radula is equipped with sharply pointed teeth with apical barbs (Crampton 1975). Testacella comprises at least six regionally restricted species plus some taxonomically questionable forms from the western Mediterranean region (Wagner 1915;Giusti et al 1995;Nardi and Bodon 2011;Quintana Cardona and Bros 2022) and three additional species, T. haliotidea Draparnaud, 1801, T. maugei Férussac, 1819, and T. scutulum Sowerby, 1821, which are more widespread in the western Mediterranean region and in western Europe (Rowson et al. 2014). In addition, the latter three species have been spread by humans to temperate countries around the world. T. scutulum has been introduced to the Canary Islands (Bank et al. 2002), T. haliotidea to Germany (Gerber and Heins 1991), Switzerland (Turner et al. 1998), the Netherlands (de Winter and van Nieulande 2011), Madeira (Bank et al. 2002), Bulgaria (Mitov and Dedov 2014), Czech Republic (Podroužková 2022), New Zealand (Barker 1999), Australia (Stanisic et al. 2010(Stanisic et al. , 2018, and North America (Pilsbry 1946), and T. maugei to Madeira (Bank et al. 2002), Canary Islands (Bank et al. 2002), Azores (Bank et al. 2002), South Africa (Herbert 2010), and New Zealand (Barker 1999). A record of T. haliotidea from Havana (Gassies and Fischer 1856) was probably not naturalized, as no Testacella was later reported from Cuba (Espinosa and Ortea 1999). Testacella scutulum has been reported from Ontario (Grimm et al. 2009). However, it is not known where it was found and whether there was an established outdoor population or just an introduced specimen in a flowerpot. Confirmation of this record is necessary before the species can be considered established in North America. Testacella maugei has been reported from greenhouses in Philadelphia (Johnson 1891;Schick 1895), but Pilsbry (1946 attributed these records to misidentifications of T. haliotidea. Orcutt (1915) reported "Testacella maugerae" from San Francisco, but Hanna (1966) also attributed this record to a misidentification of T. haliotidea. The occurrences of subterranean Testacella species in North America, which are not only difficult to detect in the field, but were even overlooked in a recent compilation of non-native terrestrial gastropods in the contiguous United States (Gladstone et al. 2020), are summarized in this paper.

Materials and methods
Records for Testacella were downloaded from GBIF. org (2022). The databases of relevant museum collections (see abbreviations below) and the platform iNaturalist (https:// www. inatu ralist. org) were checked for additional records. The literature was searched for reports of Testacella occurrences in North America. Identifications of all North American Testacella observations on iNaturalist (including those not listed in GBIF.org 2022) were checked against the characters specified in Rowson et al. (2014; see Table 1 and Fig. 1).
Abbreviations used for museum collections and internet platforms ANSP Academy of Natural Sciences of Philadelphia, Philadelphia, USA. Shell small (7-8 × 5-6 mm), slightly convex, oval and tapering to the apex with a strongly descending upper margin of the peristome large (12-16 × 6-7 mm), strongly convex, elongated with almost straight and parallel columellar and palatal margins and an only slightly descending upper margin of the peristome Body color grey-white or creamy-white to pale yellow and unspeckled, or only lightly speckled with brown greyish-pink or grey-brown and densely speckled with dark pigment Sole color whitish or pale yellowish salmon-pink to orange Dorsal grooves close together but still separate at junction with shell far apart at junction with shell Body sides usually with clearly visible parallel oblique grooves relatively smooth with hardly discernible oblique grooves Fig. 1

Distribution of Testacella haliotidea in North America
Canada, British Columbia, Capital

Comparison of information sources on the distribution of Testacella species in North America
Knowledge of the distribution of Testacella species in North America, derived from museum collections and literature references on the one hand, and from observations on iNaturalist on the other, is compared in Fig. 2. The museums listed in Material and Methods hold 42 lots collected since c. 1890 from about 32 localities in North America (some differing only in accuracy). Literature references mention a further ten localities of T. haliotidea, for which no vouchers were available in the major museum collections. As of April 2023, 80 observations of Testacella species have been published on iNaturalist since 2014 (73 since 2019) from approximately 48 locations in North America, 18 of which have obscured geographic data (represented as 0.2 × 0.2 degree cells). Most of the observations with obscured geographic data are from regions where georeferenced observations are also available (and are not listed above). Eight observations on iNaturalist clearly show T. maugei.

Discussion
Due to their predominantly subterranean way of life, Testacella species are difficult to observe. Literature references and museum specimens collected over more than a century indicate scattered occurrences of the introduced T. haliotidea in the eastern USA and a few more occurrences in the Pacific States of the USA and British Columbia (Fig. 2a). In less than 10 years, community scientists have nearly replicated a century of accumulated knowledge about the distribution of Testacella in North America on iNaturalist. Their records even show a more continuous distribution of T. haliotidea in the Pacific States and confirmed its presence in Tennessee but not yet in Pennsylvania (Fig. 2b). In addition, they provided first records of T. haliotidea in Georgia and Mexico and discovered a second introduced Testacella species, T. maugei (Fig. 1a-c), in California. As Testacella species can be identified by external morphology and most images uploaded to iNaturalist are of good quality, unequivocal identifications were possible. Obviously, the reliability of identifications increases with photos from additional perspectives, e.g. showing the characteristic foot sole of T. maugei (Fig. 1b). Similar studies of introduced limacoid or arionid slug groups are hampered by the need for anatomical characters to distinguish the species of these groups (but see Vendetti et al. 2019).
The records of T. maugei from several localities in Sonoma County, Marin County, Contra Costa County, San Francisco County and San Mateo County in California indicate that these findings represent established populations and not just individual displaced specimens. The oldest observations are from December 2018, but the distance of about 100 km between the most distant sites indicates that the species is established for several years and expanding its range. As usual for introduced Testacella populations, the colonized sites are anthropogenic habitats, like gardens, parks, or a cemetery.
In view of the new finds of T. maugei in California, one might wonder whether the record of "Testacella maugerae" from San Francisco by Orcutt (1915) Fig. 2 Distribution of Testacella species in North America based on a museum collections and literature references, and b observations on iNaturalist. Inserts show the occurrences on the west coast on a larger scale definite clarification is hardly possible without vouchers, but the fact that T. maugei has not been recorded from California for more than a century while T. haliotidea has been found there repeatedly, as well as frequent misidentifications of Testacella species a century ago, indicate that Hanna (1966) was probably right.
In contrast to T. maugei, T. haliotidea is known for more than a century from North America. Whereas it was first found in greenhouses in Nova Scotia (Binney and Bland 1869), Philadelphia (Johnson 1891;Schick 1895) and Chicago (Baker 1901), outdoor populations are known since 1915 in California (Orcutt 1915), 1928in Pennsylvania (ANSP 147813), 1947 in Oregon (Lee 1950), before 1950 in Tennessee (Lee 1950), 1994in British Columbia, before 2010 in Kentucky (Dourson 2010), before 2013 in Washington (Burke 2013), 2019 in Mexico (iN 34878679), and 2022 in Georgia (iN 109323187). The new records from Georgia and Mexico by community scientists show that T. haliotidea is still expanding its geographical range. It has to be checked whether the observed specimens represent already self-sustaining outdoor populations or whether they are only occasional escapes, e.g. from plant nurseries, which are important transport hubs for non-native species (Bergey et al. 2014).
The Testacella species are presumably transported in soil of potted plants. They are preadapted to longer transports, as they can encyst themselves in a firm, cocoon-like capsule made of soil cemented together by their mucus when conditions are dry or cold (Taylor 1902).
Introduced carnivorous snails can have devastating effects on native fauna, as in the case of Euglandina 'rosea'. Euglandina 'rosea' is a species complex (Meyer III et al. 2017), which is native to the southeastern United States. It has been introduced to many islands of the Pacific and Indian Oceans as a biological control agent for Lissachatina fulica (Griffiths et al. 1993;Civeyrel and Simberloff 1996;Lowe et al. 2000;Gerlach et al. 2021). However, E. 'rosea' prefered to prey on smaller native species and caused extinctions of at least 134 endemic land snail species, mostly from Hawaii, French Polynesia and the Mascarene Islands (Régnier et al. 2009).
In contrast, there are no reports of negative impacts of the predatory Testacella species on native fauna. This may be because Testacella species are usually restricted to anthropogenic habitats in countries where they have been introduced by humans. However, it is also possible that negative impacts were overlooked due to the snails' subterranean lifestyle and insufficient knowledge of the native soil fauna. The American earthworm fauna is being severely altered by invasive species (Hendrix and Bohlen 2002). The invasive earthworm species strongly change the soil ecosystem, especially in sites that were previously free of earthworms (Ferlian et al. 2018). The invasive earthworm species may facilitate the establishment of Testacella species. Obviously, they allow the colonization of sites that were previously free of earthworms. The further development of the introduced Testacella populations should be monitored, especially with regard to the question whether the Testacella populations remain restricted to anthropogenic habitats or whether they will also invade more natural habitats. Given the demonstrated power of community scientists to document occurrences of Testacella, they will certainly play a leading role in this endeavor.
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