Spatial and temporal variation in diversity and abundance of Malacostraca associated with environmental variables in a tropical River in South India

Malacostracans are significant in several environmental processes, making them a major biotic component in aquatic ecosystems. An investigation on the spatio-temporal variations in the diversity and abundance of malacostraca was carried out in the Kallada River for a period of two years from 2019 to 2021. The malacostracan community was represented by 35 species in 17 families and 4 orders during the investigation period. Decapods (24 species), amphipods (7 species), isopods, and tanaids (2 species each) were identified from the Kallada River. Seasonally, PoM (Post-monsoon) witnessed the presence of 33 species closely followed by PrM (Pre-monsoon) and MoN (Monsoon) with 32 species each. The amphipod species Melita zeylanica belonging to the family Melitidae dominated in terms of abundance throughout the study period. Diversity, richness, and dominance indices were increased from the upstream to downstream sites. Contrarily, the highest values of evenness were recorded the upstream and lowest recorded downstream. Diversity analysis values were maximum at downstream sites during the PrM season while the abundance was maximum during the PoM season. Multivariate tools such as CCA (canonical correspondence analysis) and hierarchical clustering were applied to investigate the co-existence of various malacostracan taxa and the relation between the species and water quality parameters. Similar feeding habits and water quality requirements allowed the co-occurrence of malacostracan species. CCA indicates the freshwater and estuarine influences on malacostracan diversity and abundance. The study also assessed the detrimental effects of sewage and municipal effluents on malacostraca at the midstream stations. However, the low diversity at the upstream stations was attributed to the natural environmental conditions.