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Freshwater mussels (Unionida) are globally endangered but perform crucial ecosystem services in temperate ecosystems. Their status and role in tropical regions are poorly understood, resulting in a nearly complete lack of effort toward their conservation. Understanding mussel functions in developing countries is particularly important because costly interventions to restore habitat functionality are often infeasible. We investigated the role of mussels in the nearshore zone of Dhanmondi Lake, a narrow, polluted lake in central Dhaka, Bangladesh, during the dry season. Sampling at 50 sites 1 to 3 m from the shore revealed dense mussel populations, averaging 218 individuals (ind)/m2 from 2 species (Lamellidens marginalis and Parreysia caerulea). Based on laboratory filtration rates and in-situ size-frequency distribution, we calculated that mussels filter the equivalent of the volume of the lake margins in 21 h. This filtration capacity appears to explain the lake's high water clarity (>2 m visibility during the dry season), despite copious nutrient availability for algal growth. Although it is heavily polluted, the near-shore zone of the lake has a diverse macroinvertebrate fauna, featuring up to 16 invertebrate families/0.25 m2. Strong positive correlations among mussel density, macroinvertebrate richness, and diversity were revealed through 2-min kick-samples, and indicate that mussels act as microhabitat engineers that promote localized biodiversity. Loss of mussel communities could result in further loss of ecosystem services, which would be costly to replace through other means. Moreover, our data suggest that mussel abundance may be used to identify sites of high macroinvertebrate biodiversity in tropical freshwaters and, thus, help to focus management efforts in the most cost-effective way. [ABSTRACT FROM AUTHOR]