Importance of shingle beaches as habitat for Manila clam Ruditapes philippinarum

We carried out surveys to clarify how the density (no. of individuals/m(2)) and condition factor (CF) of Manila clam Ruditapes philippinarum differ between estuarine tidal flats and shingle beaches in the Bisan-Seto Strait, Seto Inland Sea, Japan. The density of Manila clams in estuarine tidal flats has been decreasing since around 2008. In contrast, Manila clams are found at considerably higher densities in some shingle beaches. Moreover, the CFs for clams inhabiting the shingle beaches are higher than those for clams inhabiting the tidal flats, indicating that shingle beaches provide a more suitable environment for Manila clams than tidal flats. Chlorophyll a concentrations in the bottom layer of seawater, used as an indicator of the amount of food available to clams, were similar in both habitats. The seawater temperature, and mud temperature, at low tide in summer were 4-6 degrees C higher for the tidal flat than for the shingle beach. The maximum seawater and mud temperatures for the tidal flats reached 34.1 degrees C and 35.4 degrees C, respectively, showing that Manila clams inhabiting these areas are exposed to critically high temperatures. These results indicate that shingle beaches are an important habitat for Manila clams.