The complex distribution patterns of organisms pose one of the most interesting puzzles in biogeographic studies. Multiple pieces of evidence suggest that these distribution patterns are the outcome of the complex interplay between environmental factors and the dispersal ability and evolutionary history of organisms. Abiotic factors are considered to be an important driver of this pattern, as they exert physiological constraints on the organism’s biology. A species can persist in a region when the environmental conditions fall within its tolerance range, which suggests that physiological tolerance is a potential determinant of species distribution. Yet, it remains unclear which physiological axis sets the limit, especially in multi-stressor systems.
Intertidal gastropods of the genus Littoraria and Echinolittorina along the Indian coast present an interesting distributional pattern. Despite having high dispersal ability through pelagic larval dispersal along the coast, they exhibit significant variation in range size. Within genera, some species show a broad range distribution, occurring along the entire coast, while others are restricted to a small coastal region. Studies indicate that abiotic factors such as sea surface salinity and tidal range may explain this pattern, but they remain correlational and do not provide a mechanistic explanation.
This thesis will experimentally test whether physiological tolerance plays a role in shaping range size at different spatial scales. First, I conducted a pilot study to examine whether variation in habitat structure at a fine scale is sufficient to detect differences in thermal tolerance in two freshwater gastropods in Bengaluru. I then scale up this question to a broad level to examine if environmental variation along latitude affects physiological tolerance and whether broad-range species show higher tolerance than range-restricted species along the east and west coasts of India. Finally, I will examine how habitat filtering shapes physiological tolerance both within- and between-species in intertidal gastropods residing on rocky shores and in mangrove-associated habitats in the same regions. Altogether, this thesis aims to build a comprehensive experimental framework that links physiology to species distribution from small local scales to broad geographic regions using both freshwater and intertidal gastropods as a model system. This multi-species, multi-stressor approach aims to provide key insights into the role of physiology in species distribution patterns.