Predators Associated with Marinas Consume Indigenous over Non-indigenous Ascidians

Abstract

The establishment of non-indigenous species is influenced both by species traits and the biotic interactions that create resistance to or facilitate invasion. Predation, for example, creates resistance to non-indigenous ascidians in benthic habitats. Daytime surveys of fouling communities on floating docks in the Charleston Marina, Oregon, USA in August 2014 revealed that the indigenous ascidian Distapla occidentalis was present at all sites and was being consumed by the flatworm Eurylepta leoparda and the nudibranch Hermissenda crassicornis. In contrast, the non-indigenous Botrylloides violaceus and Botryllus schlosseri were present at fewer sites and lacked signs of predation. Parallel surveys in July 2015 again revealed predators found on docks were eating the indigenous ascidian. Examination of surrounding intertidal habitats revealed the dominant predators were the hermit crab Pagurus granosimanus and shore crab Hemigrapsus oregonensis, species not found in the marina fouling community. Feeding assays with the four predators and the ascidians showed that the flatworm specialized on the indigenous D. occidentalis, eating it almost 50 times faster than the NIS, which it did not consume. The nudibranch and shore crab mainly fed on D. occidentalis as well, consuming it two to four times faster than the NIS. In contrast, the benthic hermit crab mainly fed on the non-indigenous ascidian, feeding on it almost three times faster than it ate D. occidentalis. Hence, the non-indigenous ascidian likely faced reduced predation pressure in the marina fouling communities, increasing invasion risk. The study also examined several aspects of ascidian palatability, which showed that predator feeding was affected by differences in tunic structure. The study suggests that the different predators in the fouling and benthic communities in this marina have a role in the invasion of ascidians.