The pet trade has facilitated the spread of invasive alien species (IAS) globally, with negative consequences for biodiversity. The prediction of impacts is a major goal for invasion ecologists, and is especially crucial in an industry often lacking knowledge about traded species. We focused on the predatory gastropod Anentome helena, a species originating in south-east Asia and traded around the world, but with taxonomic uncertainty. We first set out to determine where our study organism fell within the A. “helena” species complex, known to comprise at least four cryptic species, before assessing the effect of temperature on the number of prey, the pulmonate snail Physella acuta, killed per predator via functional response experiments at two temperatures. We used 22 °C as a recommended temperature for housing the species in captivity, and 18 °C as a representative summer lake temperature in temperate climates of Europe. We also assessed the role of predator group size (1×, 2×, 3×) on predation (total consumption and average per capita consumption) at the experimental temperatures with fixed densities of prey, as well as the effect of these temperatures on prey activity. Our organisms belonged to a cryptic species originating from Thailand (Anentome sp. A), matching the findings of aquarium trade samples in other continents. In the functional response experiments, we found maximum feeding rate to be significantly reduced at the lower temperature. A similar result ensued from group feeding, with total consumption significantly reduced and the reduction in average per capita consumption approaching significance at the lower temperature. There was no significant effect of group size on the average per capita consumption in the group trial, indicating neutral conspecific interactions. No significant effect of temperature on the activity of the prey species was found, suggesting decreased consumption was mainly driven by predator, rather than prey. These results suggest limited A. helena impacts in the short-term, but increasing temperatures with climate change may facilitate greater consequences from releases. We suggest future studies assess other potential predatory impacts and survival across relevant abiotic conditions, and encourage the use of similar methods to assess the impacts of other commonly traded species.

Anentome helena, ecological impact, functional response, invasive alien species, molecular identification, pet trade

The unrelenting spread of invasive alien species (IAS) is one of the major drivers of global biodiversity loss (Seebens et al. 2017; IPBES 2019). An increasingly globalised world has led to new pathways facilitating this movement of species, and the pet trade is one important driver of this development (Lockwood et al. 2019). While many species within the pet trade spend their entire lives in confinement, the risk of escape or release from producers, importers, retailers and owners is ever-present. Indeed, the pet trade has facilitated the establishment of non-native populations globally, and is deemed responsible for 53% of invasive vertebrate species (Saul et al. 2017; Gippet and Bertelsmeier 2021) and a third of all aquatic IAS (Padilla and Williams 2004), with escapes and releases by aquarists a major contributor of invasive freshwater species in Europe (Nunes et al. 2015). The industry is also known to be poorly regulated (Raghavan et al. 2013), with species often only identified to their genus names, or worse, misidentified (Shivambu et al. 2020b).

Molluscs, primarily gastropods and bivalves, are popular within the pet trade globally (Ng et al. 2016b; Banha et al. 2019; Shivambu et al. 2020a), for both ornamental appeal and algae control (Patoka et al. 2017). Releases have led to a number of high-profile mollusc IAS emerging from the pet trade with negative ecological impacts. For example, the South American golden apple snail (Pomacea canaliculata Lamarck, 1822) has established non-native populations in the United States, Japan, China, Taiwan, Thailand and Indonesia (Shivambu et al. 2020a), and is believed to have reduced native freshwater snail populations, become an agricultural pest, and acted as a host for parasites (Halwart 1994; Rawlings et al. 2007). Aquarium releases have also seen Melanoides tuberculata (O. F. Müller, 1774), a species with a broad African and Asian native range, and the south-east Asian Tarebia granifera (Lamarck, 1816) establish extensive invasion histories (Rader et al. 2003; Raw et al. 2015; Da Silva et al. 2019; Makherana et al. 2022), posing threats to native biodiversity. While the majority of gastropods in the pet trade are thought to be herbivorous or detritivorous, the “assassin snail” Anentome helena (von dem Busch, 1847; formerly Clea helena) is readily available (Patoka et al. 2017; Shivambu et al. 2020a) and in possession of carnivorous habits. Uncertainty surrounds its taxonomic identity, with A. helena thought to comprise a cryptic complex of at least four species (Anentome “helena”: Strong et al. 2017), however a sample from the US pet trade was found to be conspecific with those already established in Singapore, a global ornamental pet trading hub, which may indicate ubiquity of this species in the trade (Ng et al. 2016a; Strong et al. 2017). With concern over further non-native establishment elsewhere (Bogan and Hanneman 2013), its potential to exert impacts in ecosystems beyond its native range is in need of study.

With A. helena found to be amongst the most readily available aquatic gastropods in Germany (Dickey et al. in prep), we aimed to solve the taxonomic uncertainty by phylogenetically analysing our study organisms, and then by assessing the role of temperature on its potential predatory impact. While availability has been shown to be a suitable proxy for propagule pressure (Duggan et al. 2006; Chucholl 2013), abiotic barriers to growth and survival must be overcome (Blackburn et al. 2011), and abiotic tolerances, especially to low temperatures, have proven crucial to the establishment success of numerous pet trade species (Veselý et al. 2015; Standfuss et al. 2016). Therefore, we set out to assess how temperature affects the feeding rates of A. helena on Physella acuta (Draparnaud, 1805) prey, a wide-ranging aquatic pulmonate snail present in Europe since the 1700s (Vinarski 2017). We used two experimental temperatures: 22 °C, a recommended lower limit for keeping the species in home aquaria (e.g. Aquariumtips.org 2020; Interaquaristik.de 2022; Tierlexikon.info 2022), and 18 °C, a temperature representative of summer water temperatures in temperate climates such as in Europe. We tested per capita feeding rates under these temperature treatments using functional response experiments (i.e. measures of how prey consumption varies with prey density), which have been used effectively as a means of assessing the impacts of invasive species over the past decade due to their ability to incorporate a wealth of context dependencies (Dick et al. 2014, 2017). However, one potential limitation of this measure of per capita consumption is that it typically does not account for the role of conspecific interactions inherent in group foraging, something potentially important in the context of pet species releases, when small numbers of individuals might be released together. There are three broad categories of conspecific interactions: neutral, antagonistic (prey risk reducing: Livernois et al. 2019), and synergistic (prey risk enhancing: Livernois et al. 2019), and using different experimental predator densities (i.e. fixed prey densities with single, double and triple predators), we assessed how temperature affects group feeding rates. We also assessed the role of temperature on the activity levels of P. acuta in order to elucidate whether predator or prey were the primary drivers of any differences in consumption at the two experimental temperatures. In summary, our study objectives were to 1) determine where our A. helena study organisms fell within the Anentome “helena” species complex; and test: 2) whether temperature would affect per capita consumption of A. helena using functional response experiments, 3) whether temperature and/or group size would affect feeding rates (total consumption and average per capita consumption) using our group feeding tests, and 4) whether our experimental temperatures would affect the activity levels of the prey species, P. acuta.

Anentome helena (Figure 1) were ordered online (garnelen-direkt.de) and arrived on 15^th March 2022. The snails were held in an 18 °C lab with an 8-hour light regime for five days in a 56 L glass tank (i.e. 60 cm × 30 cm × 30 cm length, width and height) containing a 1 cm-deep layer of sand, and a heater (Eheim, Germany) and probe (Aqua-medic Titanium, Germany) to maintain the water temperature at 22 °C. Anentome helena were held under these conditions for five days before being split into two groups, i.e. a “warm” group and a “cold” group. On day six, the temperature of the “cold” group was dropped to 20 °C, and further to 18 °C on day seven. Temperature was adjusted via thermostat, so no additional disturbance of the snails in the “cold” group occurred. Snails in both groups were fed once every three days with frozen mysids (Petman Premium, Germany). The prey species, P. acuta (Figure 2), were ordered from Shrimpfarm-Frankfurt.de and arrived on 10^th March. They were held in a covered plastic aquaria (approx. 50 P. acuta per 2 L, 18 cm × 12 cm × 12 cm) with an air stone and wooden sticks for habitat complexity, and fed “Veggie Wafers” food pellets (Tetra, U.S.A.) every three days. They were held in the same 18 °C lab as A. helena.

Figure 1. 

Photographs of predator A. helena. Morphological similarity noted between our study individuals and Anentome sp. A in the A. “helena” complex (Strong et al. 2017).

Figure 2. 

Photographs of prey P. acuta specimens (a–d) with a demonstration of relative size between the predator, left, and prey (d).

The international pet trade is highly diverse, but knowledge gaps exist for many of the species sold, both in terms of taxonomic identification and the potential ecological impacts they might have if released. Temperature remains a crucial barrier to IAS establishment, proliferation and impact (Rahel and Olden 2008; Standfuss et al. 2016; Hesselschwerdt and Wantzen 2018), and with the risk of pet releases or escapes ever-present, there remains a need to establish its effect on the potential impacts of commonly traded species. Here, we show that a readily available mollusc in the German pet trade, A. helena, or Anentome sp. A (Strong et al. 2017), is capable of surviving and feeding at an ecologically relevant temperature lower than typically recommended for the species, but with reduced predatory capacity.

Functional responses, and maximum feeding rates in particular, have been used effectively as ways of quantifying the ecological impacts of invasive species across abiotic contexts (Dick et al. 2014; Dickey et al. 2021). Here, we demonstrated that at the 18 °C temperature treatment there was a significantly reduced maximum feeding rate relative to the 22 °C treatment for A. helena on P. acuta prey. The functional response method is ultimately a per capita measure of impact, however, and while a valuable and popular impact assessment method, it may give a reduced or enhanced consumption rate by often failing to account for interactions between conspecifics. Indeed, conspecific interactions, both synergistic and agonistic, have been shown to facilitate invasive species success. For example, aggression towards conspecifics is thought to facilitate spread (e.g. aggressive individuals inhabiting the range frontier: Groen et al. 2012), while a lack of aggression towards conspecifics may facilitate coexistence in high densities in the invaded range (e.g. Argentine ant, Linepithema humile Mayr, 1868: Suarez et al. 1999; Asian shore crab, Hemigrapsus sanguineus De Haan, 1835: Hobbs et al. 2017). Further, simply the presence of conspecifics in feeding trials may limit stress and give more representative feeding rates. We therefore incorporated group feeding trials into our study, with different predator densities (i.e. foraging group sizes) and fixed prey densities in order to assess the role of temperature and group size on the total feeding rates and average per capita feeding rates (defined as total consumption divided by group size). For the latter measure, we propose that consistent feeding rates across group sizes would indicate neutral interactions, whereas increasing would be indicative of synergistic interactions and decreasing indicative of agonistic interactions. In our trials, temperature and group size were shown to significantly affect total consumption, however, there was a lack of significant two-way interaction, indicating that group size played the same role on consumption rates at both temperatures. While there was an effect approaching significance of temperature on average per capita feeding in the group trial, the lack of predator density effect suggests neutral rather than synergistic (for example, coordinated group hunting) or agonistic group feeding effects (such as interference competition).

While not significant between the two temperature treatments, prey activity levels were higher at 22 °C, which could potentially have increased the encounter rate and in turn led to a higher attack rate, but this turned out not to be the case, with no significant difference in attack rates between the two treatments. However, maximum feeding rates were significantly different, and this may have been driven by the predator’s metabolism and digestion (Jeschke et al. 2002; Brown et al. 2004; but see Marshall and McQuaid 2011) or handling time. Hunting by A. helena involves subduing prey with their foot, and then consuming it via its aperture (Berkhout and Morozov 2022). It may be that hunting or digesting prey became more difficult, or more protracted processes at the colder treatment. Considering the 48-hour experiment time, digestion may have had the stronger influence on observed consumption, thus the observed differences are probably due to a longer digestion time at the colder treatment. This may have long-term implications for survival of the species at more stressful temperatures. We recommend future studies to assess the role of temperature on alternative, less energetically costly food resources, such as less motile or less protected prey, or on carrion which they are also known to feed upon (Bogan and Hanneman 2013). Indeed, a switch from hunter to scavenger at lower temperatures may facilitate survival. Studies assessing the role of various abiotic stressors on A. helena mortality would prove useful too (e.g. Paiva et al. 2020; Cuthbert et al. 2021), such as lower temperatures, differing dissolved oxygen levels and salinity levels representative of estuarine conditions or urban ponds (Van Meter et al. 2011). Further, as a dioecious gastropod (Coelho et al. 2013), unlike invasive species exhibiting parthenogenesis such as M. tuberculata (Brande et al. 1996) and Tarebia granifera (Miranda et al. 2011), the presence of both sexes is crucial to the establishment of wild populations. How such abiotic conditions affect reproduction and whether there are survival differences between sexes require further study.

In the short term, it appears likely that the areas most at risk from A. helena are thermally influenced freshwater system, of which there are many in Europe. The Rhine, the Weser and the Po watersheds have all been shown to have a large proportion of their flows affected by thermal pollution during the year (Raptis et al. 2016). The Gillbach-Erft river system in Germany, for example, with temperatures rarely falling below 19 °C, became the home of numerous tropical and subtropical fish, crustaceans and molluscs (Emde et al. 2016; Jourdan et al. 2017; Lukas et al. 2017), and thermal pools in Hungary have also seen the establishment of diverse, non-native crayfish assemblages (Weiperth et al. 2020). Further, climate change as well as expanding urban areas with heat island effects – deemed “gateways to new ranges” (Borden and Flory 2021) – are of particular note considering that this is where most pet trade releases occur. Further research is required focused on how such habitats might host populations capable of undergoing inherent or prolonged lags (Crooks 2005), or “sleeper populations” waiting for a trigger to become abundant and disruptive (Spear et al. 2021), and facilitate adaptation and spread to colder conditions. Going forward, we propose that combining the monitoring of pet stores, garden centres, websites and online marketplaces (Ng et al. 2016b; Shivambu et al. 2020a, 2020b; Olden et al. 2021), with methods to assess the potential impacts of the species sold (Dickey et al. 2022) and their “hitchhikers” (Stanicka et al. 2022), will prove increasingly crucial in helping to determine the next potential invaders.

JWED was supported by the Alexander von Humboldt Foundation.

JWED, JMJ and EB conceived the study. JWED performed the experiments, with valuable assistance from GTS, and performed statistical analysis and prepared the initial manuscript. EK conducted molecular laboratory work, while RSB processed the molecular data. All authors provided valuable input to the development of the final manuscript and have given approval for publication.

Sequence data used for species identification is available on NCBI accession number OQ075971. Accessions for all samples used in this study can be found in Suppl. material 1: table S1.

The authors would like to acknowledge the thematic editor Ian Duggan and two anonymous reviewers for their thorough and constructive comments during the review process which undoubtedly helped improve the quality of the manuscript.