Invading bivalves replaced the native Levantine populations with negligible local effects on the benthic community

Abstract Constructing the Suez Canal connected the Red Sea and the Mediterranean Sea, allowing rapid marine bio-invasion. Over the last century, several bivalve species have invaded the Levantine basin, yet their distribution and impact on the benthic community have not been thoroughly studied. Large-scale benthic surveys along the rocky substrate of the Israeli Mediterranean coastline indicate that invading bivalves now dominate the rocky environment, reaching densities of tens to hundreds of individuals per m2. No native bivalve specimens were found in any of the transects surveyed. The small-scale ecological effects of the established invading populations on the benthic community were examined using in-situ exclusion experiment where all invading bivalves were either physically removed or poisoned and kept in place to maintain the physical effect of the shells. Surprisingly, the experimental exclusion showed little measurable effect of bivalve presence on the invertebrate community in close vicinity (~1 m). Bivalves presence had a small, but statistically significant, effect only on the community composition of macroalgae, increasing the abundance of some filamentous macroalgae, and reducing the cover of turf. The generally low impact of invading bivalves could be due to (1) wave activity and local currents dispersing the bivalve excreta, (2) high grazing pressure, possibly by invading herbivorous fish, reducing the bottom-up effect of increased nutrient input by the bivalves, or (3) the natural complexity of the rocky habitat masking the contribution of the increased complexity associated with the bivalve’s shell. We found that established invading bivalves have replaced native bivalve species, yet their small-scale effects on the benthic community seem to be negligible.