Limited effects of non-native Japanese eelgrass on Manila clams in Puget Sound, WA

Non-native species have wide-ranging impacts on local ecosystems, but their effects may be particularly strong when they are ecosystem engineers. Japanese eelgrass Zostera japonica is a non-native marine plant in Puget Sound (Washington, USA), where it coexists with native eelgrass Z. marina but has replaced mudflats. The interaction of Z. japonica with local soft sediment communities and its effects on local bivalves are not well understood. Eelgrass (native or non-native) may have positive, negative, or neutral effects on Manila clams Ruditapes philippinarum , a commercially important species. Previous studies have generally focused on clam responses to Z. japonica removal, rather than quantifying differences in how clams perform between different habitats. Here, we used a 3 mo long outplanting experiment to determine the effect of habitat (native Z. marina , non-native Z. japonica , and mudflat) and site on Manila clam survival, growth, reproductive status and condition, and recruitment of other bivalves. Though habitat did not influence clam growth, reproductive status, or condition, survival was lower in Z. marina than in mudflats and Z. japonica . We observed site-specific effects that were likely related to water temperature, as clams fared best at the site where water temperature was highest. Bivalve recruitment differed between Z. japonica and mudflats, but both had similarities with Z. marina , which had lower species richness and diversity. Our results suggest that site effects mediated by temperature may be more important than habitat for clam populations.