EFFECTS OF FOREST FLOOR ENVIRONMENTAL FACTORS ON THE SPATIAL DISTRIBUTION PATTERNS OF POLYMESODA SPP. IN THE MANGROVE FOREST OF IRIOMOTE ISLAND, JAPAN

Abstract

Polymesoda spp., which represent bivalves in the mangrove ecosystem, inhabit the mangrove forests of the Indo-Pacific region. They are edible bivalves consumed by inhabitants located around the mangrove forests in the region. The bivalves also have a cultural significance because their shells are used for religious ceremonies on the Yaeyama Islands in Japan. However, detailed studies of these bivalves is scarce and their biology is not well understood. We conducted this study in the mangrove forest along Urauchi River of Iriomote Island, Japan. The spatial distribution patterns of the bivalves and the forest floor environment were surveyed. We used 75 large (10m2 quadrat) plots to survey plant species and above-ground root density, and small (1m2 quadrat) plots to count the number of bivalves and measure median particle diameter, sediment temperature and Oxygen Reduction Potential (ORP) in each large plot. In addition, the relationships between these forest floor environmental factors and the spatial distribution patterns of the bivalves were analyzed. Polymesoda spp. was present in 34 plots. Bruguiera gymnorhiza was dominant in 79.4% of plots. A correlation between the distribution and median particle diameter and ORP and sediment temperature was not found. In 94% of the plots, the above-ground root density was over 50% of the large plot. The relationships between the spatial distribution patterns of the bivalves and plant species and the above-ground root density were revealed.