Does expansion of the introduced Manila clam Ruditapes philippinarum cause competitive displacement of the European native clam Ruditapes decussatus?

• Competition between R. decussatus and the introduced R. philippinarum was examined.
• Density of clams and exposure to predators were manipulated in a field experiment.
• An expansion scenario of the introduced clam does not affect the native clam.
• Predation plays an important role regulating R. philippinarum clam expansion.

In several estuaries or lagoons of Europe the introduced Manila clam Ruditapes philippinarum has supplanted the native grooved carpet shell clam Ruditapes decussatus by occupying almost entirely its ecological niche and relegating it to restricted areas. However, it is not clear if the nonindigenous clam is the one directly responsible for these predominance patterns. Within this context, the main goal of the present study was to analyze the competitive interaction between the nonindigenous Manila clam and the native carpet shell clam to determine whether this interaction could impact directly growth and mortality of the native clam populations. The effect of exposure to predators on both species was also examined. For this purpose, between May 2010 and May 2011 a field experiment was conducted on an intertidal area in the Bay of Santander (N Spain) where both species coexist without an extreme predominance of the introduced species. Relative density of clam species was manipulated in a randomized block experimental design. The results obtained show that (i) the increased density of Manila clam simulating species expansion scenario does not affect growth or mortality of the native clam; and (ii) for densities of Manila clam substantially higher than observed in the field, predation plays an essential role in regulating both species populations, limiting their density increase. Moreover, Manila clam appears to be more negatively impacted by predation compared to the native clam. Overall, our results indicate that predation has a more significant effect on both populations when compared to competitive interaction.