Research Question: Is there predation on invasive tunicates in the San Francisco Bay?

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In this project, I studied whether the invasive tunicates (primarily Didemnum vexillum, as well as Botrylloides violaceus and Botryllus schlosseri) of the Bay area are subject to any significant predation from native or invasive predators. These species have proven very resistant to chemical and physical removal methods, and biocontrol has been suggested as an option, and the goal of my research was to investigate the viability of this method.

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This involved an experimental approach as well as a survey of the California Academy of Natural Sciences’ collection of preserved tunicate samples. For the experimental component, I collected and maintained live samples of tunicates, as well as several predator species that were selected for abundance and co-occurrence with invasive tunicates. Feeding trials were run with these specimens, and any feeding behavior or potential feeding damage to prey samples was described and recorded.

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In its native range and in a similar invasion in Alaska, D. vexillum is subject to predation from a lamellarid snail that is a tunicate specialist. No lamellarids were collected live from the wild during the course of my research, and the purpose of examining the Academy’s samples was to determine if there was any historical evidence of lamellarid predation. These snails are cryptic and can easily be preserved with specimens when collecting tunicates. Although tunicate-specialist lamellarids may not currently be present in the bay simply because they are not as tolerant of transport or conditions in the Bay and never established a population, the presence of lamellarids in historical samples could indicate that there was previously a significant population, but that it was not able to sustain itself. This could shed light on the differences between the Bay invasion and invasions in other areas where lamellarids are present, and would have implications for the possibility of using introduced lamellarids as a method of biocontrol.

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No lamellarids were found in the preserved specimens, and observed predation in the experimental tanks was negligible. Based on my results, predation on established adult colonies, either by their invasive specialists or by native predators, is not a significant threat to invasive tunicates in the Bay. Biocontrol is not likely to be any more effective than current mechanical or chemical removal methods. Efforts to reduce the impact of invasive tunicates should focus on a multi-pronged removal approach, and better enforcement of ship ballast/hull/equipment sanitation to prevent continued re-introduction.