Reproductive Dynamics of the Seagrass Thalassia hemprichii (Ehrenberg) Ascherson in Iligan Bay Roseller G. Sabilla and Wilfredo H. Uy Mindanao State University at Naawan, 9023 Naawan, Misamis Oriental, Philippines totosabilla@gmail.com ABSTRACT A study on the reproductive dynamics of the dugong grass, Thalassia hemprichii (Ehrenberg) Ascherson was carried out on the seagrass meadows of Tubajon, Laguindingan and Poblacion, Naawan, Misamis Oriental, and Danlugan, Lopez Jaena, Misamis Occidental from February to May, 2005. Age reconstruction technique through the analysis of leaf scars along the vertical rhizomes indicated that the flowering frequency of T. hemprichii is once every two years. The first flowering occurs between 1.5 to 2 years based on the presence of flower scars on vertical rhizomes. The average leaf plastochrone interval (LPI) on the shoots of T. hemprichii across sites was 6.4 days. The estimated biomass allocated to flowering ranged from 0.5 to 0.72 g DW m -2 . Spatial differences in the morphology of flowers and fruits of T. hemprichii were observed across stations in the three study sites. The average number of fruits during peak reproductive season ranged from 5.5 to 6.2 m -2 , with an average number of 2.0 to 2.6 seeds fruit - 1. These results indicate that the sexual reproduction of T. hemprichii is active and its capacity to provide new recruits from seedlings is relatively high. Key words: Age reconstruction, flowering frequency, leaf plastochrone interval, recruits. INTRODUCTION Seagrasses are submerged marine angiosperms found mostly in all coastal waters of the world except in the Antarctic (Den Hartog, 1970). About 60 species of seagrasses have been recorded worldwide (Kuo and McComb, 1989; Den Hartog, 1970), 16 species of which are reported to occur in the Philippines. Seagrass beds are recognized world-wide for their economic and ecological importance. They help reduce current and wave energy, filter suspended sediments from water, and stabilize bottom sediments (Fonseca and Cahalan, 1992; Fonseca, et al. 1982) and serve as primary producers, habitats, shelters and food for fishes and marine invertebrates, many of which are economically important. Seagrass ecosystems in the ASEAN region, however, are threatened by both natural and human-induced disturbances and their physical loss is at ever increasing rates (Short, et al., 1988; Short and Wyllie-Echeverria, 1996; Rollon, 1998). Along Philippine coasts seagrass losses are largely due to destructive fishing methods and increasing pollution and siltation (Fortes, 1990). Seagrasses have been studied quite extensively in several bio-geographical regions, particularly in temperate zones (Den Hartog, 1970; Hemminga and Duarte, 2000). Reproductive doi: 10.48031/msunjear.2015.03.03