Using behavioural and ecological approaches to unravel the invasiveness of the amazon sailfin catfish (pterygoplichthys spp.) in the indian context

Abstract

Exotic species have become a global threat to ecosystems (1-3). Deliberate introductions of species outside its geographic range by human assistance for the purpose of aquaculture, ornamental trade and via ballast water exchanges has resulted in tremendous loss of native species (4-5). Once the exotic species are introduced in an ecosystem they proliferate in the absence of native predators (6), establishing breeding populations and thereby altering the habitat structure and function of the ecosystem (7-8), in turn impacting the survival of the native species (7, 9). For example, zebra mussel (Dreissena polymorpha), which is native to Eastern Europe and central Asia, has been introduced in aquatic ecosystems across all of the North America. Studies have reported that zebra mussels caused a devastating impact on the native species via alteration of habitat structure, from soft, muddy substratum to hard calcified substratum (10-12). Moreover, zebra mussels are filter feeders, (13) feeding voraciously on planktons (14), and in most habitats, they outcompeted the native species in terms of food and space (15-17). Similarly, another vertebrate species, tilapia (Oreochromis spp.), which is native to Africa, has invaded multiple freshwaters across the globe (18). One of the profound effects of tilapia includes altering the productivity of the ecosystem via enrichment of nutrients (voracious feeder generating huge amount of faecal waste), thus, making the ecosystem eutrophic leading to toxic algal blooms and severe depletion of dissolved oxygen (19-20). On an average, globally, freshwater habitats are far more vulnerable (when compared to marine ecosystems) to exotic invasions due to high rate of human reliance on freshwater resources, and prevalence of lenient regulations on management of such ecosystems (21-22). This is majorly concerning as freshwaters support greater biodiversity, particularly the fish taxa, when compared to marine ecosystems and hence need to be safeguarded from invasion of exotics (23-24). The World Wide Fund for Nature’s Living Planet Index showed that the rate of decline of species in freshwaters (81% between 1970 and 2012) continues to outpace the rates of decline in other ecosystems such as marine (36%) and terrestrial (38%) (25) habitats. Thus, there is an urgent need for management and eradication of exotic species within the freshwaters, however, there is no ‘silver bullet'. Most studies on exotic invasions have shown that understanding the biology of the species can contribute towards discovering their weaknesses relating to different life history traits, which can be further targeted for developing management strategies (26-28). For example, Common carp (Cyprinus carpio) native to eastern Europe and central Asia (29) has successfully established breeding populations across the globe (30-31). Invasion of carp is associated with decline in aquatic macrophytes, degradation of water quality, concentrating the nutrients, making the habitat prone to cyanobacterial blooms (32-33). Physical removal of radio-tagged fish by netting and electrofishing proved to be an inefficient management strategy (34-35). However, investigating and understanding the feeding behaviour of the carp helped to unravel its significant preferences for corn (when compared to other algal based food items) (24, 36), and the preferred corn was used as a bait to attract the carp species, resulting in removals of such targeted aggregations (32, 37, 38). Similarly, sea lamprey (Petromyzon marinus), which is native to the Atlantic ocean, had invaded the Great Lakes causing a significant impact on the native trout populations (39-40). Detailed study on understanding the behavioural biology of the lamprey species led to identification of species-specific chemical cues, sex (41) and migratory pheromones (42), which was further used for customising management strategies to control and eradicate the lamprey species (43-44). However, with massive introductions of exotic species in multiple freshwater habitats across the globe, similar biological and ecological studies need to be conducted on a vast number of organisms, and that too in different ecological habitats for robust assessment of their impact on a given ecosystem. With this background, my dissertation focuses on understanding ecological and biological characteristics of a popular pet fish, Amazon sailfin catfish (Pterygoplichthys spp.), while using a variety of assessments, ranging from community-level studies including the catfish and different freshwater fishes native to India, to dyadic behavioural interactions in the context of feeding. Amazon sailfin catfish, native to the Amazon basin of South America (45-46), is a voracious algae eater (47), and is thus a candidate species for aquaculture contributing towards prevention of eutrophication in the ponds (48). Catfish have hard body armor with spines, a ventral mouth associated with suckers, and they exhibit a vermiculated pigmentation pattern (46). Amazon sailfin catfish has been tagged as an invasive species in several parts of the globe (45, 47, 49-57) for its impact over the ecosystems. The catfish species has also invaded several Indian freshwater ecosystems and have established breeding populations in major rivers [Gangetic river basins in Bihar and West Bengal (58-59), Gayathripuzha river in Kerala (60), Cauvery river in Tamil Nadu (61), Wainganga river (62) and Bhima river (63) in Maharashtra], lakes [Chilka lake in Bhubaneshwar (64)], and wetlands [East Kolkata wetlands in West Bengal (65-66) and wetlands of Chennai (67)]. Studies on the Amazon sailfin catfish in the Indian context have majorly focused on occurrences (68-69) and have speculated their impact on several economically important species (70-71). A study by Suresh et al., (65) showed a substantial dietary overlap, in terms of gut contents, between the Amazon sailfin catfish and several fish species that are native to the Indian freshwaters. However, to the best of my knowledge, no studies have included a thorough systematic assessment to analyse the impact of the catfish on the Indian freshwater ecosystem. To address this, I have conducted mesocosm experiments to investigate whether Amazon sailfin catfish have any impact on the health of the ecosystem, in terms of growth of the freshwater fishes native to India, plankton abundance and quality of the abiotic environment. Apart from ecological measures, behaviour of a species often portrays its sensitivity towards changes in the immediate environment, thus, behavioural studies can further facilitate analysing the impact caused by the exotic catfish on the species native to India. Thus, in my dissertation, I have focussed on understanding feeding interactions between the Amazon sailfin catfish and one of the economically important, native fish of India, the rohu (Labeo rohita). To further understand the specificity of the feeding behaviours of the catfish, I attempted to characterise the ‘releaser’ (72) chemical, triggering the feeding behaviour in the catfish species. Overall, my dissertation includes three major sections (or data chapters), detailing a community-level assessment of the Amazon sailfin catfish, understanding feeding interactions between the catfish and the rohu (Labeo rohita, one of the native species of India) and exploring potential stimulus or stimuli for feeding behaviour of the catfish.