The Green Wave: Reviewing the Environmental Impacts of the Invasive European Green Crab (Carcinus maenas) and Potential Management Approaches

The European green crab (Carcinus maenas), native to northwestern Europe and Africa, is among the top 100 most damaging invasive species globally. In some regions, including the Atlantic coast of North America, C. maenas has caused long-term degradation of eelgrass habitats and bivalve, crab, and finfish populations, while areas are near the beginning of the invasion cycle. Due to high persistence and reproductive potential of C. maenas populations, most local and regional mitigation efforts no longer strive for extirpation and instead focus on population control. Long-term monitoring and rapid response protocols can facilitate early detection of introductions that is critical to inform management decisions related to green crab control or extirpation. Once C. maenas are detected, local area managers will need to decide on management actions, including whether and what green crab control measures will be implemented, if local invasion might be prevented or extirpated, and if population reduction to achieve functional eradication is achievable. Due to the immense operational demands likely required to extirpate C. maenas populations, combined with limited resources for monitoring and removal, it is unlikely that any single government, conservation and/or academic organization would be positioned to adequately control or extirpate populations in local areas, highlighting the importance of collaborative efforts. Community-based monitoring, and emerging methods such as environmental DNA (eDNA), may help expand the spatial and temporal extent of monitoring, facilitating early detection and removal of C. maenas. While several C. maenas removal programs have succeeded in reducing their populations, to our knowledge, no program has yet successfully extirpated the invader; and the cost of any such program would likely be immense and unsustainable over the long-term. An alternative approach is functional eradication, whereby C. maenas populations are reduced below threshold levels such that ecosystem impacts are minimized. Less funding and effort would likely be required to achieve and maintain functional eradication compared to extirpation. In either case, continual control efforts will be required as C. maenas populations can quickly increase from low densities and larval re-introductions.

Challenges in eDNA Detection of the Invasive European Green Crab, Carcinus Maenas

The early detection of invasive species is essential to cease the spread of the species before it can cause irreversible damage to the environment. The analysis of environmental DNA (eDNA) has emerged as a non-harmful method to detect the presence of a species before visual detection and is a promising approach to monitor invasive species. Few studies have investigated the use of eDNA for arthropods, as their exoskeleton is expected to limit the release of eDNA into the environment. We tested published primers for the invasive European green crab, Carcinus maenas, in the Gulf of Maine and found them not species-specific enough for reliable use outside of the area for which they were designed for. We then designed new primers, tested them against a broad range of local faunal species, and validated these primers in a field study. We demonstrate that eDNA analyses can be used for crustaceans with an exoskeleton and suggest that primers and probe sequences must be tested on local fauna at each location of use to ensure no positive amplification of these other species.

Can The Timing And Duration of Planktonic Larval Development Determine Invasion Success? A Case Study Comparing Range Expansion In The European Green Crab, Carcinus Maenas, And The Native Lined Shore Crab, Pachygrapsus Crassipes, In The North-Eastern Pacific.

Major El Niño events and oceanic heat waves are linked to the range expansion of many marine species. For the shores of the northeast Pacific, we compared range expansion in the European green crab, Carcinus maenas, which was introduced to San Francisco Bay prior to 1990, to that of the native lined-shore crab, Pachygrapsus crassipes, which has existed on the coast since at least the end of the last Ice Age (>10,000 years ago). The initial northern range limit of these species was central California and central Oregon, respectively. Both species increased their northern range along the open coast to northern Oregon, Washington and Vancouver Island after strong El Niño events. C. maenas, however, in just a matter of decades, successfully established populations in inlets on the west coast of Vancouver Island, and possibly also in the Salish Sea, while P. crassipes, in thousands of years, never has. We hypothesize that this difference in invasion success is due to the shorter larval duration of C. maenas, < 2 months, compared to that of P. crassipes, 3-4 months and timing of larval release, winter for both species. Because the residency times of water in the inlets of the west coast of Vancouver Island are ~1-2 months, they can act as an incubator for the larvae of C. maenas, while those of P. crassipes are likely flushed out to the open sea before they can complete their development.

Targeted Next Generation Sequencing of environmental DNA improves detection and quantification of invasive European green crab (Carcinus maenas)

In the northeast Pacific Ocean there is high interest in developing eDNA-based survey methods to aid management of invasive populations of European green crab (Carcinus maenas). Expected benefits are improved sensitivity for early detection of secondary spread and quantification of abundances to assess the outcome of eradication efforts. A new eDNA-based approach we term ‘Targeted Next Generation Sequencing (tNGS)’ is introduced here and shown to improve detection relative to qPCR at low eDNA concentrations, as is characteristic of founding or spreading populations. tNGS is based on the premise that the number of NGS reads from non-normalized (i.e. equal volumes) targeted PCR amplicons will approximate the starting DNA amount. Standard DNA concentrations that were 10-to 100-times lower than the qPCR limit of detection returned significant numbers of sequencing reads, which in our field assessments translated to a 7% - 10% increase in crab detection probability from tNGS relative to qPCR at low abundances. We also found that eDNA concentration was highly correlated with crab abundance, as measured from traditional trapping methods, for both assays; however, tNGS data had greater precision and less error than qPCR. When partitioning the sources of variation in each assay we identified greater between-site variability for tNGS relative to qPCR, suggesting the former may offer more power for detecting spatial variation in eDNA concentration. When applying this assay in management programs, we suggest including a panel of eDNA samples from sites with trapping data as standards to estimate relative abundance at sites with no a priori information. Results presented here indicate the tNGS approach has great promise for surveillance of green crab and could easily be adopted for surveillance of any species of high interest to management, including endangered species, new incursions of invasive species, and species with low eDNA shedding rates. Pros and cons of this approach compared to qPCR are discussed.

Engaging the importance of community scientists in the management of an invasive marine pest

The introduction of nonnative invasive pests is among the many threats facing coastal ecosystems worldwide. Managing these pests often requires considerable effort and resources, and community scientists can be essential for providing the capacity needed for management and monitoring activities. In response to the invasion of a Northern California estuary by the predatory European green crab, a collaborative team of academic researchers and community scientists initiated a local eradication program. The green crab is listed among the world's 100 worst invaders, and threatened both native species and commercial shellfisheries. The program dramatically reduced the green crab population over a 5-year period, but it rebounded, which necessitated a switch in project goals from eradication to population suppression. Community scientists were essential for facilitating this switch by providing the necessary capacity to quantify population characteristics and maintain reduced crab populations. The result was a sustainable program that successfully maintained low green crab densities, which will likely improve habitat for native species.