Abundance And Active Patch Selection Modulate Reproductive Connectivity And Fitness of Pea Crabs Living On Sand Dollars

Connectivity is paramount for population stability, but the mechanisms underlying the distribution of populated patches and how they affect reproductive connectivity and individual fitness remain elusive. Here, we mapped the distribution of sand dollars – as habitat patches for obligate-commensal pea crabs – at several sites. At occupied patches, we assessed whole-crab population structure and the fitness of ovigerous females. While sand-dollar supply did not limit the size of crab populations, overall crab abundance limited reproductive connectivity and the potential for offspring production. However, except for sites of extremely low and high connectivity, crab aggregations at sand-dollar clusters countervailed the overall random distribution of sand-dollar populations, greatly enhancing the reproductive potential of whole-crab populations. Crab interactions, likely controlled by larger females, added to reproductive connectivity by increasing the frequency of mating pairs in hosts. Differently from the population-level case, effects of crab abundance on individual fitness were dual and only detectable when abundance was lowest (positive) or highest (negative), so that fitness remained high at intermediate crab abundance, decreasing when it became either too low (e.g. Allee effects) or too high (e.g. energetic costs of intraspecific competition). This study indicates that connectivity may affect different levels of biological organization in specific ways.

Targeted Next Generation Sequencing improves detection and quantification of rare species from eDNA

Targeted species detection from eDNA is central to identifying and quantifying rare (i.e. invasive or endangered) species to inform conservation and resource management. Here we introduce a new targeted Next Generation Sequencing (tNGS) assay that shows improved detection relative to quantitative (q)PCR at low eDNA concentrations and increased precision to detect spatial variation in eDNA concentration related to species abundance. We compare the tNGS and qPCR methods using invasive European green crab (Carcinus maenas) in the northeast Pacific Ocean as a test case, and find that crab abundance measured by traditional trapping is significantly correlated with eDNA concentration across multiple sites for both methods. However, the tNGS assay outperformed qPCR in all tests: (1) increased precision of eDNA concentration estimation; (2) a 7-10% increase in detection probability at low abundance sites; and (3) greater power to detect spatial variation in eDNA concentration. The accuracy of predicting green crab abundance from eDNA concentration increased with the number of field replicates sampled and did not change appreciably over a tidal cycle. Green crab eDNA concentration behaving similarly to abundance measured from trapping demonstrates great promise for this tool to be implemented for early detection and routine monitoring surveys. The tNGS assay is easily accessible for surveying other species with existing qPCR assays and can thus be potentially important for detection and quantification of any species of high interest to management.

Characterizing the impact of recovering sea otters on commercially important crabs in California estuaries

Protective legislation and management have led to an increase in California’s sea otter Enhydra lutris nereis population. While sea otter recovery has been linked to ecosystem benefits, sea otter predation may negatively affect commercially valuable species. Understanding the potential influence of sea otters is of particular importance as their range expands into estuaries that function as nurseries for commercially valuable species like Dungeness crab Metacarcinus magister. We consider how sea otter predation has affected the abundance and size of juvenile Dungeness crab in Elkhorn Slough, California, USA, and analyzed cancrid crab abundance and size across 4 California estuaries with and without sea otters to understand how biotic and abiotic factors contribute to observed variation in crab size and abundance. We compared trends in southern sea otters relative to Dungeness crab landings in California to assess whether increasing sea otter abundance have negatively impacted landings. In Elkhorn Slough, juvenile Dungeness crab abundance and size have declined since 2012, coinciding with sea otter population growth. However, the impact of sea otters on juvenile Dungeness crab size was habitat-specific and only significant in unvegetated habitat. Across estuaries, we found that cancrid crab abundance and size were negatively associated with sea otter presence. While abiotic factors varied among estuaries, these factors explained little of the observed variation in crab abundance or size. Although we found evidence that sea otters can have localized effects on cancrid crab populations within estuaries, we found no evidence that southern sea otters, at recent population sizes, have negatively impacted Dungeness crab landings in California from 2000-2014.

Reduced horseshoe crab abundance and feeding activity beneath intertidal oyster aquaculture structures in the Delaware Bay

Around the world, tidal flats play a unique ecological role in estuaries and are a primary feeding habitat for shorebirds and other benthic feeding organisms. Development and economic use of tidal flats can exclude species that depend on this habitat and disrupt ecological processes. In this study we examine patterns of abundance and feeding activity of American horseshoe crabs among oyster aquaculture structures on tidal flats that are adjacent to one of the most important horseshoe crab spawning sites in the world. We used custom-designed traps to sample horseshoe crab abundance beneath rack and bag aquaculture structures and adjacent areas without structures. In addition, we developed predictive spatial models representing three hypotheses regarding the movement of horseshoe crabs through aquaculture structures when transiting to and from spawning beaches. We tested the predictive power of each model using data from traps and found the strongest support for an avoidance model, where on average, horseshoe crabs are avoiding arrays of aquaculture structures when moving across inundated tidal flats. The best-supported spatial model also indicates that patterns of structure avoidance by horseshoe crabs can potentially affect abundance on spawning beaches, particularly with larger gear arrays that are closer to shore. We found additional support for aquaculture structure avoidance by examining an independent data set of horseshoe crab feeding pits on the tidal flats. Patterns of feeding pit density mirrored our trapping results, with fewer pits beneath and among aquaculture structures when compared to adjacent control areas. Horseshoe crabs are important constituents of the benthic food web and their displacement by aquaculture may translate to significant disruptions to the ecological function of tidal flats. This impact can be limited through deliberative spatial planning that seeks to balance ecological and economic management objectives.

The relationship between blue swimming crab (Portunus pelagicus) abundance and environmental parameters in Spermonde Archipelago

Management and conservation of the blue swimming crabs by the marine protected area method require the abundance and environmental parameter information of the blue swimming crab. The aim of this study is to analyze the changes in the relative abundance of the blue swimming crab and its environmental parameters. The study was conducted from March to July 2015, in the waters of Salemo Island, Spermonde Archipelago. The specimens were collected at the three fishing locations around the mangrove, seagrass, and coral reef ecosystems. The variable of relative abundance of the blue swimming crab is determined catch per effort. Samples of environmental parameters such as temperature and current speed are measured in situ. However, for the salinity, dissolved oxygen, ammonia, nitrate, plankton, and chlorophyll-a were analyzed in the laboratory. A comparison of the blue swimming crab abundance in each ecosystem was conducted by One Way ANOVA. Moreover, the relationship between blue swimming crab abundance with environmental parameters was analyzed by multiple regression. The results show that the blue swimming crab is abundant in the seagrass and coral reefs. The environmental parameter that significantly influences the abundance of the blue swimming crab is salinity. The suitable area for blue swimming carb protection based on abundances are seagrass and coral reef ecosystems.

A computer vision approach for studying fossorial and cryptic crabs

SummaryDespite the increasing need to catalogue and describe biodiversity and the ecosystem processes it underpins, these tasks remain inherently challenging. This is particularly true for species that are difficult to observe in their natural environment, such as fossorial and cryptic crabs that inhabit intertidal sediments. Traditional sampling techniques for intertidal crabs are often invasive, labour intensive and/or inconsistent. These factors can limit the amount and type of data that can be collected which in turn hinders our ability to obtain reliable ecological estimates and compare findings between studies. Computer vision and machine learning algorithms present an opportunity to innovate and improve sampling approaches. Moreover, cheaper and tougher recording devices and the diversity of open source software further boost the possibilities of achieving rigorous image-based sampling, which can broaden the range of questions that can be addressed from the data collected. Despite its significant potential, the software and algorithms associated with image-based sampling may be daunting to researchers without expertise in computer vision. Therefore, there is a need to develop protocols and data processing workflows to showcase the value of embracing new technologies. This paper presents a non-invasive computer vision and learning protocol for sampling fossorial and cryptic crabs in their natural environment. The image-based protocol is underpinned by fit-for-purpose and off-the-shelf software. We demonstrate this approach using fiddler crab and sediment recordings to study and quantify crab abundance, motion patterns, behaviour, intraspecific interactions, and estimate bioturbation rates. We discuss current limitations in this protocol and identify opportunities for improvement and additional data stream options that can be obtained from this approach. We conclude that this protocol can overcome some of the limitations associated with traditional techniques for sampling intertidal crabs, and could be applied to other taxa or ecosystems that present similar challenges. We believe this sampling and analytical framework represents an important step forward in understanding the ecology of species and their functional role within ecosystems.

Oyster aquaculture does not impede spawning beach access for Atlantic horseshoe crabs Limulus polyphemus

Farms for eastern oyster Crassostrea virginica, which are commonly located along shallow estuarine shores of the eastern USA, use a range of farm equipment and require regular access to care for and harvest oyster livestock. In some cases, these farms are located in areas used by Atlantic horseshoe crabs Limulus polyphemus as they come ashore during spring to spawn. The sandy shores of the Delaware Bay host the largest spawning aggregations of this species in the world. Limited studies have examined interactions between horseshoe crabs and intertidal oyster farms, and concern has been raised about the horseshoe crab’s ability to traverse oyster farms to reach spawning habitat. This study examines potential farm interactions with horseshoe crabs in Delaware Bay during the 2018 and 2019 crab spawning season. Our studies included a range of experiments and surveys during high and low tide to observe crab abundance and behavior at rack-and-bag oyster farm and non-farm sites. In all cases, results indicated that crabs can successfully traverse rack-and-bag farms and reach spawning beaches. Crabs do not differentially use farm versus non-farm areas, and crab behavior is relatively unaltered by farm gear. These results provide important context for developing frameworks for managing ecological interactions among farms and wildlife species of concern.

A comparative assessment of salt marsh crabs (Decapoda: Brachyura) across the National Estuarine Research Reserves in New England, USA

Salt marsh degradation and loss is accelerating in many regions of the United States as well as worldwide. Multiple stressors are often responsible, sometimes including crab burrowing and herbivory. A recent national assessment identified stark differences in crab indicators between northern and southern New England, with the latter exhibiting intense signs of impacts by crabs, but more details on crab patterns across the entire region are needed beyond this “broad-brush” assessment. Our study used green crab (Carcinus maenas (Linnaeus, 1758)) traps, intensive marsh platform burrow counts, and a new multi-metric index of relative crab abundance to examine patterns in marsh crabs across four National Estuarine Research Reserves in New England. Crab indicators from the multi-metric index and burrow counts were higher in southern New England marshes; patterns from trapping of green crabs were less clear. At the marshes examined, green crabs were very abundant in Maine, lower in New Hampshire, and intermediate in southern New England. Our study confirms that abundance and impacts by crabs vary dramatically between sites in northern and southern New England, and provides improved context for managers and researchers when considering impacts to marshes from multiple crab species across New England and elsewhere.