Predator-prey interactions in a bioinvasion scenario: differential predation by native predators on two exotic rocky intertidal bivalves

The mechanisms determining the strength of interactions between non-indigenous and native species in the invaded environment are of great interest to both ecologists and managers. On a Brazilian rocky shore, we experimentally measured predation intensity and prey preference of native predators on 2 exotic bivalves, Perna perna (which has been present for centuries) and Isognomon bicolor (introduced ca. 20 yr ago). Overall, predation was more intense on P perna than on I. bicolor. Furthermore, P. perna was preyed upon more intensively by benthic crawling predators (whelks and, possibly, crabs), while larger, more mobile predators (fish and birds) were less selective. In addition, the larger, more abundant whelk Stramonita haemastoma selectively preyed on P. perna (for which handling time was shorter), while another whelk (Trachypollia nodulosa) preferred I. bicolor, although handling time was longer. Different shell morphologies of the 2 exotic prey and resource partitioning between the whelks may explain S. haemastoma and T. nodulosa feeding preferences, respectively. The thicker valves of I. bicolor compared to those of P. perna reduced the drilling or chipping efficiency by whelks. Although these prey species belong to the same functional group, differences in their shell characteristics could entail different mechanical constraints to predators. Therefore, native predators in the study system may prefer P. perna over I. bicolor because they are still adapting their foraging skills to handle the more recent invader, I. bicolor.