FIRST CHARACTERIZATION OF DRILLING PREDATION ON EASTER ISLAND - RAPA NUI

Biotic interactions such as predation can shape communities. In marine environments, drilling by gastropods is a pervasive type of predation, and one with an extensive fossil record. It has been suggested that predation rates are higher in tropical environments, driven by higher metabolic rates at high temperatures. Here, we set out to characterize drilling predation in the unique setting of Easter Island - Rapa Nui. Several features make the molluscan fauna of the island very interesting, in particular the high isolation that has led to high endemism (35-40% for mollusks), and the Polynesian affinities of the mollusk fauna that allow establishing comparisons with other islands. In order to determine predation intensity and preference for certain taxa or life habits, bivalves and gastropods (n=6122) were collected from dead shell assemblages at six sites in Rapa Nui. Mean site predation frequency was 5%, two sites had significantly higher frequencies (10% and 8%), while one was significantly lower (2%). Sites with higher frequencies were dominated by infaunal taxa, in particular the lucinid bivalve Ctena bella. This taxon was further studied to determine whether predators had a preference for smaller sizes, thinner shells or a location in the shell (n=477). Size and thickness were not significant explanatory variables, yet there was a preference for drill holes to be located on the umbo and center of the shell. Incomplete and multiple drill holes were frequent. These findings suggest that predation in Rapa Nui is lower than in other islands in Polynesia, and contribute to understand predation in remote locations, where organisms have evolved in relative isolation. Moreover, the anthropogenic pressure on Rapa Nui is increasing, making efforts to characterize and understand its endangered endemic fauna fundamental. Funded by FONDECYT grants # 3160342 and 1140841. We thank CONAF Isla de Pascua and CODEIPA for sampling permit and logistics.