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email-charukasadanruwan@gmail.comAbstractSeagrass leaves provide microhabitat for epiphytic organisms but knowledge on epiphytic fauna and associated bionts in seagrass are limited. In addition, seasonal changes in population structure of epiphytic communities have not been studied widely under Sri Lankan context. This abstract presents the diversity and abundance of filamentous epiphytic algal communities on the leaves of Oceana serrulata in Kayankerni Marine Sanctuary in Eastern coast of Sri Lanka. Leaves of Oceana serrulata were collected once a month from August 2024 to January 2025 and preserved with 5% formaldehyde. Leaves were measured and divided into three similar sections and labelled as tip, middle, basal parts. Ten randomly selected leaves per month were subjected to identify the filamentous epiphytes and their abundance. Epiphyte species were identified up to genus level using guide, key and published literature. Percentage epiphytic cover of each section of leaves was estimated using the microscopic field as the sample unit under 10*10 magnification. Shanon-Wiener diversity index, Pielou’s evenness index and Dominance index for each month calculated. Arcsine converted data of epiphytic cover on entire leaf-blades among different months were compared using ANOVA to identify the temporal variations. In addition, the percentage epiphytic cover of each species among sampling occasions were compared using ANOVA. Six genera of filamentous epiphytes were reported from leaf blades and were Ulva, Gayliella, Hydrolithon, Myrionema, Herposiphonia and Calaconema. Genus Ulva reported three (03) distinct species while others reporting single species each accounting the species richness of filamentous algae up to eight (08). Percentage epiphytic cover on the leaf blades was ranged from 11.25% to 1.18% reporting the highest epiphytic cover in August and lowest in January. Contribution of different genera to total epiphytic cover was reported as follows: Ulva spp. (27.87%), Gayliella sp. (26.63%), Hydrolithon sp. (22.04%), Myrionema sp. (12.03%), Herposiphonia sp. (11.04%), and Calaconema sp. (0.38%). The abundance of Gayliella sp., Herposiphonia sp., Ulva sp.1, and Ulva sp.2 have been reducing gradually from August to January, while Myrionema sp. and Ulva sp.3 were reported throughout the sampling period in low abundance. Calaconema sp.was reported varying levels in low abundance during the sampling period. Abundance of Gayliella sp. and Herposiphonia sp. was significantly higher (p