Variable responses of a key salt marsh facilitator to multiple invasive predators

Since its discovery on the east coast of North America less than two decades ago, the Asian shore crab Hemigrapsus sanguineus has increased tremendously in abundance and interacts strongly with many native species. Although generally thought to be limited to rocky intertidal habitats, this invasive crab has very recently become established in several mid-Atlantic marshes. We are investigating how Hemigrapsus and another invasive predator, the green crab Carcinus maenas, affect the survival, growth and behavior of the ribbed mussel Geukensia demissa, a key salt marsh species. We used complementary field and laboratory experiments to evaluate the responses of individual mussels to the presence of each predator. Both species of crab readily ate smaller mussels, but while Geukensia thickened their shells in the presence of green crabs they did not appear to recognize Hemigrapsus as a threat. Presumably this is due to differences in the invasion histories of the two crabs – Carcinus has been in the region for over 100 years. Ribbed mussels facilitate plant growth by limiting sediment erosion and increasing nutrient availability through fertilization. As a consequence, if the mussels’ failure to respond to waterborne cues from Hemigrapsus leads to increased predation risk, negative effects of crabs on mussel populations may also alter local productivity and nutrient dynamics.