Exploring Biotic Impacts fromCarcinus maenasPredation andDidemnum vexillumEpibiosis onMytilus edulisin the Gulf of Maine

Abstract

Mytilus edulis (Blue Mussel) is an ecologically important species in the Gulf of Maine. However, many introduced species that have a direct negative impact on the Blue Mussel have entered this system, some as predators (e.g., Carcinus maenas [Green Crab]) and others as aggressive epibionts (e.g., Didemnum vexillum [Carpet Sea Squirt]). Didemnum vexillum has been increasing in abundance throughout the Gulf for the past 10 years and form large mat-like growths on mussel beds, covering individual mussels completely. The first part of our study used a predator-exclusion experiment to determine the impact of predators on the plantigrade stage of the Blue Mussel life cycle. During this stage, no epibiosis occurs due to a protective periostracum layer on the mussel shell. The second part of our study used laboratory trials to assess how overgrowth by D.vexillum impacts predator choice, handling time, and consumption of mussels. There were a significantly greater number of Blue Mussel plantigrades on exclusion panels than on the exposed-cage control panels. Green Crab and Nucella lapillus (Dog Whelk) predators were present on our non-exclusion panels. In laboratory trials, Green Crab handling time of Blue Mussels was not significantly different between mussels that were clean and mussels that were overgrown, but crab behavior and overall consumption showed a greater selection for clean mussels. This selection indicates an associational predator-resistance effect of D.vexillumt epibiont on Blue Mussels. The results of our study, while focused on one specific predator species, suggest that while young Blue Mussels with no epibionts are preyed upon heavily, D.vexillum likely deters predators from older mussels. Because D.vexillum form large mat-like colonies that can cover a large area, their presence may have a significant impact on community structure in the Gulf of Maine.