European Journal of Ecology, 6.1, 2020, pp. 121-136 CHOOSING EXOTIC OVER THE FAMILIAR TASTE: HABITAT-SPECIFIC PREFERENCES OF A MALACOPHAGOUS LEECH FOR FRESHWATER SNAILS AS PREY Pranesh Paul, Rupsha Karmakar, Gautam Aditya* Department of Zoology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata – 700019 Abstract. The spread of freshwater invasive species through aquarium trade poses a threat to the ecosystem, economy and human health. The availability of the exotic freshwater gastropod mollusc, Planorbarius corneus (Linnaeus, 1758) (Gastropoda: Planorbidae), in pet shops in Kolkata, West Bengal, India, raises concern for its potential establishment as an invasive species. Assuming the role of the native predators as a potential biotic barrier for invasion, observations on the predation pattern of the malacophagous leech, Glossiphonia weberi (Blanchard, 1897) (Clitellata: Glossiphoniidae) against P. corneus was made in single as well as in combination with the native snail Indoplanorbis exustus (Deshayes, 1834) (Gastropoda: Planorbidae). Using varying size classes and densities of P. corneus, the predation potential of G. weberi was estimated after a 24 hour period. In another set of experiments, the predation of G. weberi against the snail P. corneus was observed in the presence of I. exustus under four different habitat conditions. The purpose was to justify - (1) the effect of conspecific and het- erospecific conditions of the prey availability and (2) the effect of habitat complexity on the predatory efficacy of G. weberi. The results indicated that G. weberi consumed varied numbers of P. corneus, depending on the size and the densities of the prey and predator. Although the prey heterogeneity and complex habitats caused significant reduction of predation on both of the prey species, G. weberi significantly preferred P. corneus over I. exustus in open (P < 0.0001), macrophyte (P = 0.002) and pebbles and macrophyte containing habitats (P < 0.0001). Apparently, G. weberi preferred the exotic snail P. corneus in presence of alternative prey snail I. ex- ustus, under different habitat conditions. Therefore,G. weberi may act as a significant biotic resistance against the colonization and establishment of P. corneus in the Indian context. However, further studies including the multiple prey and predators are required to ascertain the food web level impact of the exotic snail P. corneus in invaded freshwater ecosystems. Key words: Invasive species; Planorbarius corneus; Indoplanorbis exustus; habitat complexity; Glossiphonia weberi; prey preference. Introduction observation, it is evident that in the freshwater lakes In course of the dispersal beyond the native where the invasive predatory crayfish Orconectes range, the invasive species can cause several direct rustics increased in number, vast decline occurred and indirect effects on human health, local econo- in the snail abundance (Kreps et al. 2012). Invasive my and the ecosystem by competing with (Human animals do not only disrupt the faunal biodiversity, & Gordon 1996; Dobson 2004) or predating upon but they can also execute serious harm to native veg- the native species (Doherty et al. 2016), altering the etations as observed for Pomacea canaliculata in the community structure (Sanders et al. 2003; Gillis & freshwater ecosystem (Carlsson & Lacoursière 2005; Chalifour 2010) and increasing the disease or pest Carlsson & Brönmark 2006) and Achatina fulica in load (Crowl et al. 2008; Hosokawa et al. 2014; Paini the terrestrial ecosystem (Raut & Barker 2002). et al. 2016). Therefore, the invasive species cause Apart from natural reasons, human-mediated ac- significant damage in ecological communities (Gro- tivities are often responsible for the invasion of var- sholz 2002; Carlton 2003) and contemplated to be a ious non-native freshwater species by breaking the major threat of biodiversity loss (Wilcove et al. 1998; geographical barriers of species distribution (Su et al. Mack et al. 2000). For example, the invasive Argen- 2016). The increasing popularity of ornamental pet tine ant Linepithema humile successfully displaced trade and aquaria are suspected to introduce non-na- a native ant species by interspecific competition in tive plants, gastropods, and fishes in different parts of a biodiversity reserve of California (Human & Gor- the world (Rixon et al. 2005; Chang et al. 2009; Ng don 1996). Besides competition, from the long-term et al. 2016). The availability of diverse freshwater or- 121 Pranesh Paul et al. – Predation of G. weberi on P. corneus namental gastropods and bivalves in the pet shops as several biotic and physical factors. For instance, the well as non-ornamental species present as fish food or chances of the establishment of invading species in hitchhikers, indicate a high risk of the spread, coloni- a newer habitat can be reduced considerably by the zation and establishment of invasive species through native predator assemblages (Baltz & Moyle 1993). aquarium trade (Ng et al. 2016). Once established, the In the Indian context, the malacophagous leeches and invasive species may displace the native snails and hemipteran insects are considered as effective pred- subsequently become abundant in the benthic com- ators of different native and exotic snails (Aditya & munities, as observed for the snail Tarebia granifera, Raut 2001, 2002a, b, c, 2004, 2005). Thus, in the a freshwater snail introduced in South Africa (Miran- present study, we employed the leech, Glossiphonia da et al. 2011; Miranda & Perissinotto 2014a, b). Al- weberi (Blanchard, 1897) (Clitellata: Glossiphonii- though many aquatic snails feed primarily on detritus, dae) as a potential native predator against the exotic algae and microscopic organisms, some groups of snail P. corneus. The diet of glossiphoniid leeches snails consumed fresh hydrophytes which can cause largely consists of chironomid larvae, oligochaetes damages to the agricultural or economically beneficial and snails (Young & Ironmonger 1980; Raut & plants (Estebenet 1995). For instance, Pomacea spp. Nandi 1984; Wilken & Appleton 1991; Davies et al. were introduced from South America to the Southeast 1997; Kutschera 2003; Krupski et al. 2018). In labo- Asian countries as a potential protein source that es- ratory observations, G. weberi exhibited a preference caped from aquaculture, and due to their high fecun- for the non-operculate snails against the operculate dity and quick growth these species established in nat- snails found in the freshwater communities of West ural habitats and turned into agricultural pests of rice Bengal, India (Aditya & Raut 2005). A non-opercu- fields and wetland floral vegetations (Cowie 2002; late snail Indoplanorbis exustus (Deshayes, 1834) Carlsson & Lacoursiere 2005; Kwong et al. 2008). (Gastropoda: Planorbidae), which is passably mor- The freshwater gastropod Planorbarius corne- phologically similar to the non-native snail P. corne- us (Linnaeus, 1758) (Gastropoda: Planorbidae) is us, was considered as an alternative prey. The native a much-admired ornamental snail (Seddon & Van planorbid snail, I. exustus is common in different Damme 2011) and is commonly known as “the great parts of India, including West Bengal, bearing sig- ramshorn snail” among the aquarium enthusiasts. One nificance in helminth disease transmission (Malek of the interesting features that the planorbid snails & Cheng 1974; Biswas 1991; Sangwan et al. 2016) possess is the presence of hemoglobin molecules in as well as multifunctional roles in the maintenance their hemolymph (Lieb et al. 2006). Because of the of the community structure functions (Raut 1981). high hemoglobin content in the hemolymph of the Therefore, it would be appealing to study how native snail P. corneus, they appear reddish to brown which predatory G. weberi interact with non-native P. cor- makes this snail as an interesting aquarium pet. The neus prey when presented with native I. exustus prey snail P. corneus is generally found all over the Eu- in the same habitat. ropean countries (Wesselingh et al. 1999; Seddon & Along with the biotic factors, physical factors Van Damme 2011) and is sold as an ornamental snail also influence the prey-predator interaction. Among in different parts of the world (Duggan 2010; Zhao the physical factors, the physical arrangement of dif- 2013). In Kolkata, India, the gastropod P. corneus is ferent habitat elements may vary between different sold in the aquarium pet shops and markets without habitats and create many microhabitats, which imply any legal restriction or precautions (Parveen et al. a significant influence on faunal composition and in- 2019). Previous studies indicated that high fecundi- teractions between the prey and predators (Beukers ty (Costil & Daguzan 1995a, b), rapid growth (Zotin & Jones 1997; Grabowski 2004). Both the predator 2018) and long life span (Berrie 1963) can facilitate and the prey can benefit from the complexity of the the establishment of the exotic snail P. corneus in habitat condition. The aquatic vegetation, pebbles, the freshwater habitats if released from such aquaria. sediments or detritus can help the prey to refuge from Despite the small body size, P. corneus is a matter of the attack of predators and thus result in a decrease of great concern to the native ecosystem health, similar predation rate. For example, the leaves of sea-grass to other non-native gastropods as introduced before. protect the small fishes and shellfishes from the at- The establishment of the non-native