Seasonal patterns and environmental drivers of gastropod distribution in southeastern Bangladesh.

Gastropods serve as important indicators of biodiversity in coastal ecosystems, fulfilling critical ecological roles. This study comprehensively assessed gastropod diversity across three southeastern coastal islands of Bangladesh (Kutubdia, Moheskhali, and Sonadia) and examined its relationship with seasonal environmental factors. We documented 144 gastropod species from 65 genera, 28 families, and 3 orders. Monthly in-situ measurements of key environmental variables-including salinity, temperature, dissolved oxygen, nutrient concentrations, and suspended/dissolved solids-revealed marked seasonal fluctuations. For example, the monsoon season featured high total suspended solids alongside low salinity and total dissolved solids, whereas the post-monsoon period exhibited peak temperature and pH levels. Gastropod species richness varied significantly across seasons, peaking in winter and pre-monsoon. Principal Component Analysis identified salinity and total dissolved solids as primary environmental drivers influencing gastropod abundance and community composition. Generalised Linear Mixed Models confirmed that elevated salinity and total dissolved solids were major determinants of species richness, particularly enhancing it during winter. Additionally, species composition displayed pronounced seasonal shifts, with distinct assemblages characterising the post-monsoon period. Indicator species analysis highlighted Oliva sp. as a key indicator of the post-monsoon season and Umbonium sp. for winter assemblages. Our findings underscore that gastropod diversity in southeastern Bangladesh is intricately shaped by dynamic seasonal environmental changes. Understanding these patterns is critical for advancing knowledge of coastal ecosystem dynamics and for guiding conservation efforts in this climate-sensitive region. Several edible gastropods were documented-e.g., Littorina undulata, Umbonium spp. (U. vestiarium), Telescopium, and Babylonia spp.-which were locally abundant at multiple study sites. These species play important functional roles and are economically significant, contributing to food security and coastal livelihoods.