Freshwater molluscs are an important component of macro-invertebrate communities in aquatic ecosystems. However, they face severe threats from anthropogenic activities, leading to a significant decline in biodiversity. Freshwater ecosystems have seen an 80 percent decline in species populations, in contrast to 36 percent in marine and 38 percent in terrestrial habitats, emphasising the urgent need for studying freshwater biodiversity and implementing conservation measures. This thesis addresses the systematics, ecology, and livelihoods associated with freshwater molluscs in Northeast India, providing an updated checklist of species, analysing phylogenetic relationships of two genera, documenting their bioresource potential and understanding determinants of their distribution.

Northeast India is a part of the Himalaya and Indo-Burma hotspot exhibiting high levels of diversity and endemism. Despite the attention given to vertebrates and angiosperms, lower organisms like molluscs are often neglected. This chapter aims to fill this gap by providing a detailed checklist of freshwater molluscs in Northeast India. A total of 90 species across 38 genera and 14 families were recorded, with 50 percent being endemic to the region. The genera Paludomus and Brotia are notably diverse. The checklist revealed taxonomic discrepancies and many unidentified species, suggesting potential new species. Assam, Manipur, and Arunachal Pradesh exhibited the highest species richness. Significant global taxonomic changes are not fully reflected in Indian checklists, highlighting the need for thorough taxonomic revision using integrative taxonomic approaches.

India's unique geological history, shaped by continental drift and tectonic activities, has had a significantly influenced its current biodiversity. The systematics of the genus Brotia, a widely distributed freshwater mollusc genus in Northeast India and Southeast Asia, was a focal point of this study. Using mitochondrial markers (COI and 16S rRNA), phylogenetic analyses were conducted with Maximum Likelihood and Bayesian Inference methods. Species delimitation was performed using multiple methods, and evolutionary timelines were estimated with BEAST. The phylogenetic analyses showed high support for the relationships within Brotia, consistently recovering it as a monophyletic clade. Indian Brotia species formed a weakly supported clade nested within Southeast Asian clades, indicating patterns of "out-of-Asia" and "into-India" dispersal during Oligocene. This study presents the most comprehensive phylogenetic analysis of Brotia to date, covering 24 localities and identifying up to nine putative species. Integrating molecular data with the morphological characters is crucial for accurate species delimitation.

The genus Paludomus, another significant but understudied group within the family Paludomidae, restricted to lotic systems, was also studied. Field sampling in Northeastern India and phylogenetic analyses using mitochondrial (COI) and nuclear (28S) genes revealed that the genus is monophyletic, with three distinct clades: the Northeast-Southeast Asian clade, the Arunachal Pradesh clade, and the Western Ghats clade. Species delimitation analyses suggested between 19 and 27 distinct molecular taxonomic units, indicating a higher diversity than currently recognised. This study is the first global phylogeny of Paludomus, highlighting the necessity for integrative taxonomic methods to delineate species accurately and call for serious taxonomic revision.

Molluscs have long been valued for their bioresource potential, serving as food and medicine. In Northeast India, freshwater molluscs are an important source of protein for tribal communities. This study documents the diversity of harvested freshwater molluscs, their collection sites, vendor demographics, and traditional knowledge regarding their use. DNA barcoding revealed that several species are mixed and sold as the same species. Freshwater molluscs are collected from various habitats and sold in markets, with a significant gender bias in vendor demographics. This comprehensive documentation of the diversity and ethnic uses of edible freshwater molluscs in Northeast India emphasises their significance for the economic benefits and nutritional security of the dependent communities.

Freshwater habitats, despite their limited global coverage, are vital biodiversity reservoirs supporting over 140,000 known species. These habitats exhibit higher species richness per unit area compared to marine and terrestrial ecosystems but are experiencing rapid biodiversity declines due to anthropogenic pressures. The study explores the factors determining the distribution of freshwater molluscs in Northeast India, analysing environmental parameters across various habitats. Significant differences in species richness and abundance were observed between lentic and lotic systems, with specific environmental factors influencing mollusc distribution. The findings provide critical insights into the determinants of freshwater mollusc distribution, emphasising the need for comprehensive surveys and integrative approaches to inform conservation strategies.

In conclusion, freshwater molluscs in Northeast India are diverse and play significant ecological and socio-economic roles. Their habitats are under threat, necessitating urgent conservation efforts. This thesis offers a comprehensive understanding of the systematics, ecology, and livelihoods associated with these molluscs, highlighting the importance of integrative taxonomic approaches and documenting their bioresource potential. These findings are vital for developing effective conservation strategies and ensuring the long-term survival of freshwater ecosystems and their biodiversity.