Colonization of eelgrass (Zostera marina L.) by tunicates can lead to reduced plant growth and survival. Several of the tunicate species that are found on eelgrass in the northwest Atlantic are highly aggressive colonizers, and range expansions are predicted in association with climate-change induced increases in seawater temperature. In 2017, we surveyed tunicates within eelgrass meadows at 33 sites from New Jersey to Newfoundland. Eight tunicate species were identified colonizing eelgrass, of which four were non-native and one was cryptogenic. The most common species (Botrylloides violaceus and Botryllus schlosseri) occurred from New York to Atlantic Canada. Tunicate faunas attached to eelgrass were less diverse north of Cape Cod, Massachusetts. Artificial substrates in the vicinity of the eelgrass meadows generally supported more tunicate species than did the eelgrass, but fewer species co-occurred in northern sites than southern sites. The latitudinal gradient in tunicate diversity corresponded to gradients of summertime sea surface temperature and traditional biogeographical zones in the northwest Atlantic, where Cape Cod represents a transition between cold-water and warm-water invertebrate faunas. Tunicate density in the eelgrass meadows was low, ranging generally from 1–25% cover of eelgrass shoots, suggesting that space availability does not currently limit tunicate colonization of eelgrass. This survey, along with our 2013 survey, provide a baseline for identifying future changes in tunicate distribution and abundance in northwest Atlantic eelgrass meadows.

Several invasive species of tunicates (Ascidiacea) have become cosmopolitan and widely distributed in coastal areas worldwide over the past few decades. These non-indigenous tunicates have consequently caused fouling problems in aquaculture and marine harbors. The goal of our project was to enrich the understanding of how invasive tunicates interact with other organisms in the ecosystem. Two species of invasive tunicates (Didemnum vexillum and Botrylloides violaceus) and one species of native tunicate (Aplidium glabrum) were evaluated for their metabolic rates. The filtration rates for native blue mussels (Mytilus edulis) and invasive tunicates (Diplosoma listerianum) were determined. D. vexillum regenerated NH4+ at a faster rate than A. glabrum and B. violaceus. Both tunicates and blue mussels were feeding on phytoplankton as their major food source, although the size of particles utilized by different organisms was not examined in this study. Invasive tunicates were strongly competing with mussels to filter feed, but were not inhibiting mussel’s filtration rate.

Eelgrass (Zostera marina) is an ecologically valuable seagrass which is exposed to a wide range of stressors and has declined worldwide. The proliferation of epifaunal fouling organisms such as the sponge Halichondria panicea, colonial tunicate Botryllus schlosseri, and solitary tunicate Molgula manhattensis represents additional stress for eelgrass. Predation of this epifauna that would otherwise cause harm to eelgrass, will likely reduce their impact. On Martha’s Vineyard, an island in the Northwest Atlantic off southern Massachusetts, USA, green crabs Carcinus maenas and spider crabs Libinia dubia were examined as potential predators of sponges and tunicates attached to eelgrass. Crabs were somewhat starved for one week and then, in the lab, put in enclosures with three eelgrass shoots and tunicate/sponge epifauna. Consumption of prey items and crab survival were measured at one and 24 hours. After one hour, C. meanas did not consume any prey, while predation by L. dubia did occur. After 24 hours, C. meanas had still not consumed any tunicates or sponges, while L. dubia consumed eelgrass with H. panicea (100%); B. schlosseri (40% completely, 40% partially, and 20% unconsumed); and M. manhattensis (60% completely, 20% partially, and 20% unconsumed). High densities of M. manhattensis often occurs on eelgrass at Martha’s Vineyard (up to 6,700 per m²), thus we sought to determine a M. manhattensis consumption rate for L. dubia in the lab. A single L. dubia can consume at least 30 M. manhattensis in a 24-hour period. Because of the recent declines in eelgrass acreage, understanding the ecological mechanisms that minimize impacts to eelgrass can be advantageous to both the individual pant and the meadow. Natural predation by L. dubia in eelgrass meadows heavily fouled by tunicates and sponges is an important ecosystem function that may contribute to controlling the epifauna, and thus maintaining healthy eelgrass and eelgrass habitat. However, the extent to which crabs control eelgrass epifauna in the field is unknown. Our results were determined in a laboratory setting and further work should be done to confirm that similar results will be found in the field.

Invasive species dominate and often outcompete native species in marine harbors. The abundance of introductions due to shipping activity combined with artificial substrates and poor water quality helps to facilitate invasion success in these areas. Colonial ascidians, in particular, have broad environmental tolerances that allow them to invade novel ecosystems including urban harbors. While the effects of invasive species and poor water quality on native species have been explored, the relative influence and how these pressures may interact to degrade biodiversity is not well characterized. The purpose of this study was therefore to understand the role of interspecific competition and pollution on the native Aplidium glabrum and the invasive Botrylloides violaceus. Colony surface area has been previously identified as a sensitive toxicity endpoint and was measured weekly to assess impacts both separately and together at 0, 1, 10, 20, and 40 μg/L copper concentrations. A. glabrum was significantly impacted at 1 μg/L in the presence of interspecific competition and at 10 μg/L with no competition; however, the copper EC50 revealed increased sensitivity in the presence of B. violaceus (11.3 μg/L versus 6.9 μg/L while in competition). Conversely, significant impacts to B. violaceus growth did not occur until much higher copper concentrations with an EC50 of 37.1 μg/L while alone and 31.7 μg/L when in competition. Interspecific competition was found to significantly impact A. glabrum surface area growth at 1, 10, and 40 μg/L, while there was no significant effect on B. violaceus at any of the concentrations tested. Finally, an interaction effect was found between copper pollution and competition status only for A. glabrum. The results of this study support the hypothesis that invasive species are more tolerant of pollution while also revealing the interactive effects of pollution exposure and interspecific competition.

High-throughput sequencing (HTS) technologies offer new promise to support surveillance programs targeting marine non-indigenous species (NIS). Metabarcoding might surpass traditional monitoring methods, for example through its ability to detect rare species, a key feature in early detection of NIS. Another interest of this approach is the identification of organisms difficult to identify based on morphology only (e.g., early developmental stages), making it relevant in the context of management programs. Because many marine benthic NIS have a biphasic benthopelagic life cycle, targeting their pelagic larval stages in zooplankton may allow early detection and assessment of their establishment and potential spread. We illustrate this approach with an analysis of bulk-DNA retrieved from a time-series of zooplankton samples collected over 22 months in one bay in Brittany (France). Using HTS of amplicons obtained with two markers (COI and 18S) and a metabarcoding approach, 12 NIS were identified and their temporal larval dynamics were monitored. Importantly, we chose to focus on a closed list of species, from four metazoan classes encompassing 52 NIS reported within the study area or nearby seas, with molecular references available or obtained locally for 42 of them. The use of a custom-designed database allowed the detection of three NIS otherwise not detected when using public databases. Interestingly, NIS known to have a short-lived larval stage were detected (e.g., the bryozoan Bugula neritina or the tunicate Corella eumyota). For two molluscs Ruditapes philippinarum and Crepidula fornicata, metabarcoding results were compared to those obtained using traditional methods (i.e., barcoding of individual larvae and morphology, respectively) to show the reliability of the approach in detecting and assessing the extent of their reproductive periods. Our results also revealed that the Pacific oyster Crassostrea gigas, a notorious invasive species, failed to reproduce in the study bay, showing that metabarcoding on larval stages also provides information regarding the establishment success (or failure) of NIS. While metabarcoding has its limitations and biases, this study demonstrates its effectiveness for surveillance of targeted NIS, notably to support management strategies like the European Marine Strategy Framework Directive (MSFD).

Multiple human-mediated pathways in the marine environment provide ample opportunities for new and potentially harmful species to spread into high-value natural areas. Often, these areas are remote and reactive measures to an invasion prove to be difficult, therefore a more precautionary and proactive approach is necessary. The Fiordland Marine Area (FMA) is largely unmodified and has a unique and productive underwater environment. Following an amendment in 2012 to the Biosecurity Act 1993 the Fiordland Marine Guardians and an inter-agency government group worked to develop and implement the Fiordland Marine Regional Pathway Management Plan (FMPP), whose overarching objective is to prevent the introduction and spread of invasive marine species to the FMA. The plan involves three key elements to manage invasive species vectors: (1) that vessel owners and operators hold a current Clean Vessel Pass for their vessel; (2) that the vessel meets clean vessel standards including hull biofouling, gear biofouling and residual seawater requirements; and (3) that owners and operators maintain and can present records on the steps taken to meet the clean vessel standards. The plan was made operative in 2017, and to date, uptake has generally been positive with only a small number of compliance issues. The adoption of this plan is largely due to an integrated iwi, community and agency management approach which has encouraged engagement and participation from the Fiordland stakeholders. The ongoing success of this programme will rely on the commitment from the partnering central and local government agencies to improve and refine the plan, and on a general elevation in the importance of marine biosecurity management nationwide. Ideally, the plan will significantly reduce the risk of further marine bioinvasions within the FMA.