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Sea Level Rise in Delaware Bay, U.S.A.: Adaptations of Spawning Horseshoe Crabs (Limulus polyphemus) to the Glacial Past, and the Rapidly Changing Shoreline of the Bay
Abstract
Horseshoe crabs have proven adept at locating suitable areas of sandy beach spawning habitat throughout their long geological history. Paleogeographic studies have shown that the most recent period of sea level rise (SLR) has been occurring in Delaware Bay for at least 6,000 years. Comparison of aerial photographs from the 1930s with contemporary satellite imagery clearly indicates a landward movement of the shoreline along the New Jersey coastline of Delaware Bay. Habitat for horseshoe crab spawning has been adversely impacted over this period of time by the loss or degradation of spawning beaches, which to some extent has been offset by the deposition of this sand in “marginal habitats” such as tidal creeks and sandy deltas. The well-documented natural landward movement of a beach-marsh system in a time of SLR has been compromised in some locations by the hardening of the coastline through construction of bulkheads, groins and jetties. This directly reduces the productivity of these beaches for horseshoe crabs, and, consequently, their use by shorebirds. The response to SLR and storms in the recent past has emphasized the protection of coastal property; however, there has been some effort to restore beach ecosystems through nourishment. Given that SLR is an ongoing process, beach nourishment projects to protect a developed shoreline will require a long-term commitment at considerable cost. From the perspective of horseshoe crab conservation and habitat preservation, we suggest that consideration be given to the strategy of property buy-outs and abandonment, thus enabling a more natural beach response to SLR.
... shift to low quality spawning and nursery habitats would be expected to reduce carrying capacity and limit recovery potential (Loveland & Botton, 2015). ...
... As expected during the next 100 years, an increase in average water temperature might negatively affect reproductive activity in the Méxican spatial units, shifting or jeopardizing the viability of populations in the southernmost extent of the range. Range-wide, there is potential for SLR to cause widespread alteration of spawning habitat as beaches erode or are converted into developed or hardened shorelines (Loveland & Botton, 2015). However, the net result upon population status is uncertain owing to a lack of reliable projections, the inherent adaptability of horseshoe crabs to varied habitats at a local level, and the potential for phenological shifts to affect communities in 3.2 | Threats to horseshoe crab populations and habitats ...
... However, the impact severity will depend upon whether habitat of sufficient quality can be maintained despite climate impacts. In many locations, the expectation is severe habitat loss because of hardened shorelines and flooding-induced erosion of sediments (Loveland & Botton, 2015;Smith et al., 2020a;Botton et al., 2022). ...
According to an International Union for Conservation of Nature (IUCN) Red List assessment (RLA), the American horseshoe crab ( Limulus polyphemus ), an iconic coastal species, is at risk of extirpation in some regions within its range where small and vulnerable populations occur. However, the RLA does not consider future status beyond viability and does not attempt to identify the conservation necessary to effectively mitigate threats and recover the species to full ecological functionality. To aid in conservation planning for vulnerable species, the IUCN developed the Green Status of Species assessment (GSA) process to complement the RLA.
This paper describes the application of the GSA process to assess the recovery potential of the American horseshoe crab. First, specific Limulus populations within spatial units for conservation were delineated, and their statuses were defined based on viability and ecological functionality. Then conservation actions were identified that would promote recovery and affect their near‐ and long‐term population status under different conservation scenarios.
Horseshoe crab conservation has relied on, and will continue to depend on, effective harvest regulation. However, as currently conceived, conservation is not expected to mitigate habitat loss at the scale required to restore range‐wide ecological functionality, primarily because habitat loss is widespread and affected by climate change. Thus, the GSA results, while indicating that there is potential for near‐term recovery gains, reveal that long‐term recovery is in doubt owing to expected loss of habitat.
To conserve critical habitats for spawning and early life stages and achieve ecological functionality, it is imperative to identify and develop conservation plans at appropriate spatial scales. Unfortunately, such plans do not currently exist and need to be established. The GSA Green Score can then serve as a metric for monitoring recovery and gauging the effectiveness of conservation implementation.
... The avoidance of peat is most likely a consequence of the reduced developmental success of horseshoe crab eggs in low O 2 /high H 2 S environments (Vasquez et al. 2015;Funch et al. 2016). The encroaching bay has also compelled the use of various shoreline armoring practices along Delaware Bay, including the use of revetments and bulkheads, that have diminished the suitability of the habitat for horseshoe crabs (Loveland and Botton 2015). Shoreline armoring is also a major factor in the declining populations of horseshoe crabs in Asia (Akbar John et al. 2018). ...
... As we have recently discussed the importance of sea level rise and beach erosion to horseshoe crabs in Delaware Bay (Loveland and Botton 2015), we will concentrate our discussion here on the central portion of the Cape May Peninsula (New Jersey), in the vicinity of the Rutgers Cape Shore Laboratory (Fig. 2). There are several reasons why we focus on this area. ...
... Although the recession of the shoreline near the Cape Shore Laboratory may have been accelerated by the entrainment of sediments at the mouths of the tidal creeks (Weggel 2011), erosion is not at all atypical. To the contrary, many other Delaware Bay beaches have experienced similar alterations (Loveland and Botton 2015). Some bay shore communities (e.g., Fortescue, East Point, and Pierce's Point) have installed stone revetments, or wooden and sheet steel bulkheads to protect property, which have diminished the suitability of these beaches for horseshoe crabs (Botton et al. 1988;Jackson and Nordstrom 2009). ...
American horseshoe crab populations are imperiled in many locations, with overfishing, loss, and degradation of essential spawning/nursery habitats generally acknowledged to be the most significant factors. Against this backdrop, some have implicated additional local factors as further stressors. Specifically, rack-and-bag oyster aquaculture and biomedical bleeding for the production of Limulus amoebocyte lysate (LAL) have been targeted as factors preventing the full recovery of the Delaware Bay horseshoe crab population, which has special importance because of the significance of horseshoe crab eggs in the diets of migratory shorebirds that use Delaware Bay as a staging area during their spring migration. Rack-and-bag oyster culture along the Cape May Peninsula in lower Delaware Bay, New Jersey, is presently centered in an area that has undergone considerable erosional change in the past four decades and is no longer the prime horseshoe crab habitat that it formerly was. Studies on the abundance and behavior of horseshoe crabs in the vicinity of oyster racks show no difference in the numbers of crabs reaching inshore spawning habitat due to farm gear, suggesting that reproductive behavior and capacity is likely unimpacted by oyster farms. With respect to the potential impacts of LAL production, mortality caused by bleeding crabs is minor compared to the mortality associated with the bait fishery. The potential adoption of a synthetic endotoxin test (e.g., recombinant factor C) would have only a small impact on the Delaware Bay and mid-Atlantic horseshoe crab population. We conclude that the loss of high-quality spawning habitat ultimately poses the greatest threat to horseshoe crabs in Delaware Bay and elsewhere; in comparison, oyster culture and biomedical mortality are lesser concerns. For the future, the focus should be on the preservation and, if feasible, replenishment of the remaining optimal habitats.
... Although harvest records extend into the nineteenth century (Shuster 2003), management plans that regulate harvest are recent and may not adequately address the exploitation for the bait fishery, biomedical harvest, or locally intense marine-life harvest throughout the species range (ASMFC 1998;Millard et al. 2015). Coastal change, particularly from hardened shorelines and erosion, has resulted in a loss of suitable habitat Hapke et al. 2013;Jackson et al. 2015), which will be affected by sea level rise (Loveland and Botton 2015). The geographic variation in the conservation status of the American horseshoe crab and the potential impact of emerging threats have not been comprehensively reviewed and assessed. ...
... Adult horseshoe crabs, as well as embryos and larvae, are eurythermal (Botton and Itow 2009), so direct mortality from rising water temperatures is probably less of a threat to the species than sea level rise. The apparent threat of climate change to coastal habitat is the loss of spawning habitat due to sea level rise and storms (Arkema et al. 2013;Loveland and Botton 2015). Sea level rise will increase the rate at which these habitats disappear, and it will increase the likelihood that horseshoe crab spawning habitat becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). ...
... The apparent threat of climate change to coastal habitat is the loss of spawning habitat due to sea level rise and storms (Arkema et al. 2013;Loveland and Botton 2015). Sea level rise will increase the rate at which these habitats disappear, and it will increase the likelihood that horseshoe crab spawning habitat becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). Over the last century, sea level has risen by 20-40 cm depending on coastal location, due to sea level rise and local sinking of land. ...
Horseshoe crabs have persisted for more than 200 million years, and fossil forms date to 450 million years ago. The American horseshoe crab (Limulus polyphemus), one of four extant horseshoe crab species, is found along the Atlantic coastline of North America ranging from Alabama to Maine, USA with another distinct population on the coasts of Campeche, Yucatán and Quintana Roo in the Yucatán Peninsula, México. Although the American horseshoe crab tolerates broad environmental conditions, exploitation and habitat loss threaten the species. We assessed the conservation status of the American horseshoe crab by comprehensively reviewing available scientific information on its range, life history, genetic structure, population trends and analyses, major threats, and conservation. We structured the status assessment by six genetically-informed regions and accounted for sub-regional differences in environmental conditions, threats, and management. The transnational regions are Gulf of Maine (USA), Mid-Atlantic (USA), Southeast (USA), Florida Atlantic (USA), Northeast Gulf of México (USA), and Yucatán Peninsula (México). Our conclusion is that the American horseshoe crab species is vulnerable to local extirpation and that the degree and extent of risk vary among and within the regions. The risk is elevated in the Gulf of Maine region due to limited and fragmented habitat. The populations of horseshoe crabs in the Mid-Atlantic region are stable in the Delaware Bay area, and regulatory controls are in place, but the risk is elevated in the New England area as evidenced by continuing declines understood to be caused by over-harvest. The populations of horseshoe crabs in the Southeast region are stable or increasing. The populations of horseshoe crabs in the Florida Atlantic region show mixed trends among areas, and continuing population reductions at the embayment level have poorly understood causes. Within the Northeast Gulf of Mexico, causes of population trends are poorly understood and currently there is no active management of horseshoe crabs. Horseshoe crabs within México have conservation protection based on limited and fragmented habitat and geographic isolation from other regions, but elevated risk applies to the horseshoe crabs in the Yucatán Peninsula region until sufficient data can confirm population stability. Future species status throughout its range will depend on the effectiveness of conservation to mitigate habitat loss and manage for sustainable harvest among and within regions.
... Although harvest records extend into the nineteenth century (Shuster 2003), management plans that regulate harvest are recent and may not adequately address the exploitation for the bait fishery, biomedical harvest, or locally intense marine-life harvest throughout the species range (ASMFC 1998;Millard et al. 2015). Coastal change, particularly from hardened shorelines and erosion, has resulted in a loss of suitable habitat Hapke et al. 2013;Jackson et al. 2015), which will be affected by sea level rise (Loveland and Botton 2015). The geographic variation in the conservation status of the American horseshoe crab and the potential impact of emerging threats have not been comprehensively reviewed and assessed. ...
... Adult horseshoe crabs, as well as embryos and larvae, are eurythermal (Botton and Itow 2009), so direct mortality from rising water temperatures is probably less of a threat to the species than sea level rise. The apparent threat of climate change to coastal habitat is the loss of spawning habitat due to sea level rise and storms (Arkema et al. 2013;Loveland and Botton 2015). Sea level rise will increase the rate at which these habitats disappear, and it will increase the likelihood that horseshoe crab spawning habitat becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). ...
... The apparent threat of climate change to coastal habitat is the loss of spawning habitat due to sea level rise and storms (Arkema et al. 2013;Loveland and Botton 2015). Sea level rise will increase the rate at which these habitats disappear, and it will increase the likelihood that horseshoe crab spawning habitat becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). Over the last century, sea level has risen by 20-40 cm depending on coastal location, due to sea level rise and local sinking of land. ...
... We also include habitat quality (beach sand depth) as a random effect to account for changes to habitat quality from early to recent periods. We assumed uniform high habitat quality for early periods (Loveland & Botton, 2015) and for the 2015-21 phase used site-level seasonal averages of proportion of sampling locations with sand depths >40 cm. ...
The largest aggregation of spawning American horseshoe crabs in the world occurs in Delaware Bay and supports one of the largest concentrations of shorebirds in the western hemisphere where the birds feed on horseshoe crab eggs during migration. Unregulated harvest in the 1990s is associated with the decline of shorebird populations using the bay, but corresponding baseline information on the horseshoe crab egg food supply that supported peak shorebird populations has been lacking.
Past and current measurements of horseshoe crab eggs in the bay indicate that abundance in the 1980s was an order of magnitude greater (x̄ = 156,600/m ² ) than present‐day estimates (2015–2021 x̄ = 10,243/m ² ). An additional egg prevalence index, which characterizes the timing and magnitude of horseshoe crab egg output, revealed a similar pattern of higher prevalence in the 1980s (0.89, 95% confidence interval 0.81–0.94) compared with the recent 2015–2021 interval (0.52, 95% confidence interval 0.43–0.60).
Declines of egg and shorebird abundance occurred shortly after horseshoe crab harvest reached its peak. Red knot aerial survey counts have stabilized at 30% of the baseline population while ruddy turnstone counts are 40% of baseline estimates. Initially these species were evenly distributed between the New Jersey and Delaware coasts but both species have trended toward predominately occupying New Jersey beaches.
A lack of recovery of horseshoe crab egg and shorebird abundance suggests that horseshoe crab harvest management has stabilized populations but progress toward population recovery has been limited. Impacts from bycatch, the pharmaceutical industry and other potential population impacts must be better quantified and mitigated, if necessary, to promote the recovery of horseshoe crab populations. Measurements of horseshoe egg prevalence and abundance are essential metrics for tracking the progress toward management goals for improving shorebird habitat quality.
... The American horseshoe crab is classified as a vulnerable species on the IUCN Red List (IUCN, 2016) because of overharvesting for bait and the loss or degradation of essential spawning and nursery habitats resulting from shoreline hardening and sea level rise (Loveland and Botton, 2015). This vulnerable status is consequential because Limulus eggs provide essential food for multiple migratory shorebird species (Botton, 2009). ...
Microplastic transfer between horseshoe crabs (Limulus polyphemus) and migratory shorebirds through consumption of crab eggs was examined in Jamaica Bay, New York. Fertilized and unfertilized crab eggs, shorebird fecal pellets, beach sand, and bay water were processed with a hydrogen peroxide solution to remove organic material, then stained with a Nile Red to identify microplastics using fluorescence microscopy. Microplastics were present in all samples and ranged from approximately26–1300 μm. Unfertilized and fertilized eggs contained significantly higher numbers of microplastic particles per gram than shorebird fecal pellets, beach sand, and bay water. The presence of microplastics in unfertilized egg samples indicates that microplastics undergo maternal transfer during oogenesis. We estimated that 1 g of horseshoe crab eggs could contain approximately 426 to 840 microplastic particles, suggesting that shorebirds feeding on this resource could be ingesting a substantial burden of microplastics during their migratory stopover, much of which appears to be retained by shorebirds, rather than being eliminated in their fecal pellets.
... American horseshoe crab populations have been declining throughout much of their range, which spans the North American coast from Maine to Alabama and parts of Mexico's Yucatan Peninsula (ASMFC 2020). Declines in horseshoe crab populations are due to bait harvest, shoreline hardening, sea-level rise, and biomedical harvest (Botton, Loveland, and Jacobsen 1988;Widener and Barlow 1999;Loveland and Botton 2015). Some eastern states have restricted harvests to reverse population decline. ...
We used citizen science to improve understanding of population trends and behavior in the American horseshoe crab ('Limulus polyphemus') along the entire coast of Florida. First, we used 18 years of public sightings data, beginning in 2002, on horseshoe crab mating to determine which spawning locations were used most. Then, a subset of those locations was more rigorously surveyed through the Florida Horseshoe Crab Watch program. Florida Horseshoe Crab Watch, implemented in 2015 by a collaboration between the Florida Fish and Wildlife Conservation Commission (FWC), University of Florida Institute of Food and Agricultural Science (UF/IFAS), University of Florida Department of Biology (UF), and Florida Sea Grant (FSG), uses trained citizen volunteers to survey beaches with high spawning activity. Volunteers count, weigh, measure, and tag horseshoe crabs that are nesting on the shoreline. These data contribute to a nationwide mark–recapture program managed by the United States Fish and Wildlife Service (USFWS) and are used in Florida to inform species management decisions and to increase general understanding of the species. We developed and adapted both phases of the research by implementing modern technology, improving survey design, and expanding geographic coverage. The quality and accuracy of public reports have improved with technological advancements. The quality of the morphological data collected by Florida Horseshoe Crab Watch volunteers was comparable to that of data collected by professional scientists. Our design can serve as a model for programs regardless of region or taxon of interest and when funding is limited but public interest is high.
... Mishra, 2009;Shin, Li & Cheung, 2009;Behera et al., 2015;Cartwright-Taylor, 2015;Faridah et al., 2015;Mishra, Mishra & Yasmin, 2015;Nelson et al., 2015;Manca et al., 2017). Global climate change is an overarching threat to horseshoe crabs in both North America and Asia, because of the loss of intertidal spawning and juvenile nursery habitats associated with sea-level rise, and the detrimental effects of shoreline armouring (Botton, 2001;Jackson & Nordstrom, 2015;Loveland & Botton, 2015). ...
• Horseshoe crab population sizes and trends have been previously studied using surveys of spawning adults and counts of eggs from surface (top 5 cm) and deep (20 cm) sediment samples. The correlations between surface and deep eggs were studied at two locations, Delaware Bay and Jamaica Bay, USA, and the correlations between egg densities and spawning counts were examined in Jamaica Bay.
• There were significantly higher densities of eggs in deep sediments than in surface sediments. Only about 10% of the variability in surface egg density was explained by deep egg density. The numerical patterns between surface and deep eggs were similar between Delaware Bay and Jamaica Bay and across sampling dates.
• Nearly 20% of the deep samples in the combined data from Delaware Bay and Jamaica Bay with an egg density of ≥100,000 m⁻² had zero surface eggs. Therefore, the use of surface eggs as an indicator of habitat suitability and spawning intensity may seriously underestimate the importance of a beach for spawning horseshoe crabs.
• When paired with nearest survey date, Jamaica Bay spawning indices did not predict deep or surface egg densities. This may be related to a temporal mismatch between survey methods, the extreme overdispersion (patchiness) of the eggs, and/or the dynamics of egg distribution after exhumation.
• Both egg density and spawning surveys can provide useful data on habitat suitability for horseshoe crabs and can offer excellent opportunities for student and citizen scientist engagement. More labour is required for egg surveys than spawning surveys because of the time required to sample, sort, and enumerate the eggs.
... They are also useful as a flagship species to spread public awareness regarding biodiversity conservation [6]. However, their habitats are continually diminishing worldwide due to anthropogenic activities, and potentially due to the sea level rise induced by climate change [7]. Thus, the conservation of horseshoe crabs is ecologically and commercially important, and it is now receiving increased global attention [6,[8][9][10]. ...
The tri-spine horseshoe crab, Tachypleus tridentatus, is a threatened species that inhabits coastal areas from South to East Asia. A Conservation management system is urgently required for managing its nursery habitats, i.e., intertidal flats, especially in Japan. Habitat suitability maps are useful in drafting conservation plans; however, they have rarely been prepared for juvenile T. tridentatus. In this study, we examined the possibility of constructing robust habitat suitability models (HSMs) for juveniles based on topographical data acquired using unmanned aerial vehicles and the Structure from Motion (UAV-SfM) technique. The distribution data of the juveniles in the Tsuyazaki and Imazu intertidal flats from 2017 to 2019 were determined. The data were divided into a training dataset for HSM construction and three test datasets for model evaluation. High accuracy digital surface models were built for each region using the UAV-SfM technique. Normalized elevation was assessed by converting the topographical models that consider the tidal range in each region, and the slope was calculated based on these models. Using the training data, HSMs of the juveniles were constructed with normalized elevation and slope as the predictor variables. The HSMs were evaluated using the test data. The results showed that HSMs exhibited acceptable discrimination performance for each region. Habitat suitability maps were built for the juveniles in each region, and the suitable areas were estimated to be approximately 6.1 ha of the total 19.5 ha in Tuyazaki, and 3.7 ha of the total 7.9 ha area in Imazu. In conclusion, our findings support the usefulness of the UAV-SfM technique in constructing HSMs for juvenile T. tridentatus. The monitoring of suitable habitat areas for the juveniles using the UAV-SfM technique is expected to reduce survey costs, as it can be conducted with fewer investigators over vast intertidal zones within a short period of time.
... This same pattern occurs for fishes in other small estuaries Fahay 2010, Cardoso et al. 2011). In addition, the natural shorelines in the study area, with little or no human alteration such as shoreline armoring, suggests that these nearly natural habitats may provide spawning and nursery habitats for Horseshoe Crabs into the future even as sea levels continue to rise (Loveland and Botton 2015). ...
... Sea-level rise will increase the risk of flooding of spawning habitats and, in turn, the rate at which these habitats disappear, including increasing the likelihood that spawning habit becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). This is a recognized risk in Asia, because many T. tridentatus spawning sites and nursery beaches are shared with resident human populations (Weng et al. 2012b, Schoppe 2002. ...
Taxon Name: Tachypleus tridentatus (Leach, 1819)
Common Name(s): Tri-spine Horseshoe Crab, Chinese Horseshoe Crab, Horseshoe Crab, Japanese Horseshoe Crab, King Crab
Assessment Information:
Red List Category & Criteria: Endangered A4bcd
Year Published: 2019
Date Assessed: July 22, 2018
Justification:
This assessment integrates information on major threats, habitat and population responses, population genetics, past and predicted future levels of exploitation and management and conservation actions. Population responses, in terms of abundance, geographic range and viability inform risk at the country level, which in turn, informs the assessment of the species' risk of extinction.
... Xiphosurid horseshoe crabs are rare in the fossil record (Rudkin and Young, 2009;Sekiguchi and Shuster Jr., 2009;Loveland and Botton, 2015;Lamsdell, 2016). Their unmineralized exoskeleton and predilection for dynamic marginal environments may be the factors responsible for their scarcity in the stratigraphic record (Babcock et al., 2000); however, their preservation potential seems to be relatively better than some other marine arthropod clades from among crustacean and chelicerate species (Klompmaker et al., 2017). ...
Approximately 10% of Late Jurassic (Tithonian) horseshoe crabs collected from the Kcynia Formation in the Owadów-Brzezinki Quarry, Central Poland, display microborings covering the entire carapace of specimens. Detailed examination of the surrounding sediment indicates the occurrence of short-term fluctuations in the oxygenation level of bottom waters and their salinities. The identified palynomorph assemblage from the sediment suggests pathogenic algal blooms as the probable cause for mass mortality. Anoxic conditions resulting from algal blooms in restricted environments facilitated fungal infection evidenced by the damage (microborings) caused by the hyphae on the surface of the exoskeletons. Associated biochemical factors contributed to the excellent preservation of diversified fauna with soft tissues.
... Rising sea levels could reduce suitable horseshoe crab breeding habitats, since the retreating shorelines would bring the horseshoe crabs closer to the human infrastructure near the coasts. This is the case for Delaware Bay, New Jersey, USA, where sea levels have risen at a high rate during the twentieth century and hence decreased horseshoe crab breeding habitat (Loveland and Botton, 2015). Climate change would also affect ecological factors that influence the developmental success of horseshoe crab eggs and larvae, such as salinity and temperature (Jegla and Costlow, 1982;Laughlin, 1983;Ehlinger and Tankersley, 2004;Zaleha et al., 2011), oxygen levels (Palumbi and Johnson, 1982;Funch et al., 2016), beach geochemistry, wave energy, and erosion (Botton et al., 1988;Penn and Brockmann, 1994;Jackson et al., 2008). ...
Conservation of horseshoe crabs has recently received increasing attention as several populations are in decline. However, scarce information on their distributions in Southeast Asia is impairing conservation efforts. In this study, we sought to improve our understanding of the geographical range and distinct populations of the three Asian horseshoe crabs species in order to identify optimal conservation areas. We mapped the geographic range of Carcinoscorpius rotundicauda, Tachypleus gigas, and T. tridentatus using recent data from field work, literature, Global Biodiversity Information Facility (GBIF), and unpublished data from our scientific network. The data were correlated with 23 different environmental variables of potential ecological importance for horseshoe crabs using the openModeller webservices, including new tidal variables. Ecological niche models were generated using two algorithms, Maximum Entropy and support vector machine, for the three species under present conditions, and projected into a climate change scenario of 2050. The niches of the Asian horseshoe crabs were mostly determined by tidal regime, chlorophyll A concentrations, depth, distance to land, and sea surface temperature. According to our predictions, horseshoe crabs in Southeast Asia are not expected to experience any severe change in extent and distribution of suitable habitat in the future. In order to conserve Asian horseshoe crabs, we suggest establishing Marine Protected Areas at locations where distinct populations and several species occur, such as northern Vietnam, China, Borneo, and southern Japan.
... The chelicerate order Xiphosurida, generally known by the colloquial misnomer 'horseshoe crabs,' is among one of the rarest of invertebrate taxa, mostly owing to their unmineralized exoskeleton and predilection for marginal environments that are so rare in the stratigraphic record (Babcock et al., 2000;Loveland and Botton, 2015;Lamsdell, 2016). Thus, the discovery of Middle Triassic horseshoe crab material adds significantly to our understanding of the group. ...
Numerous well-preserved remains of a new limulid species from the Anisian-lower Ladinian (Middle Triassic) of the Tejra section of southern Tunisia are described. Comparisons are made with limulids from the Triassic deposits of Europe and Australia. The new specimens are congeneric with the type species of Limulitella , but show some morphological differences. Here we describe Limulitella tejraensis new species, a small limulid with semicircular prosoma, small and triangular opisthosoma, well-defined axial ridge, and pleurae along both ridges of the opisthosoma. The Tunisian Limulitella fossils are associated with conchostracans, bivalves, gastropods, and microconchids. Sedimentological and paleontological data from the Tejra section suggest freshwater to brackish-water conditions during the formation of the fossil-bearing interval and the influence of marine transgression into a playa-like environment. Supposed adaptation to the stressful environment sheds new light on the origin and survival of the extant limulines. This is the first report of limulid body fossils from the Triassic of North Africa and the first documentation of Limulitella in the Middle Triassic of northern Gondwanaland.
... Mapping and classification of suitable spawning habitat in Delaware Bay revealed a total of 91 km of habitat of which 42 km (46%) were Boptimal^or Bsuitable^beaches (Lathrop et al. 2006). Due to sea level rise and storms (such as hurricane Sandy in 2010), there has been significant erosion of the Delaware Bay shoreline and some locations have had replacement of a once suitable habitat with soupy clay-silt and finely ground detritus mixed with silty clay (Loveland and Botton 2015;Shuster 2015) which are not suitable for L. polyphemus nesting. ...
The American horseshoe crab, Limulus polyphemus, is found along the Atlantic and Gulf of Mexico coasts in genetically-isolated populations. Eggs are laid in shoreline beaches that expose developing embryos to combinations of environmental stressors. Whether populations of L. polyphemus differ in multi-stressor tolerance had never been tested. We assessed the multi-stressor tolerance of L. polyphemus embryos from a population in Delaware Bay (DE) and determined whether these differed from the multi-stressor tolerance of embryos from a more southerly Florida Gulf Coast (FGC) population. We monitored the field sediment temperatures and determined multi-stressor tolerance of DE embryos, then compared these to published data for FGC embryos. For multi-stressor tolerance, we assessed development success of embryos in two-week exposures to 36 fully-factorial combinations of temperature (20, 25, 30, 35 °C), salinity (5, 15 and 34 ppt) and ambient O2 (5, 13 and 21% O2), followed by two-weeks in recovery conditions. Sediment temperatures in the DE site ranged from 9.5-46 °C, with extended periods exceeding 35 °C. Development success was similar between the DE and FGC populations in 14 of 26 multi-stressor combinations. The DE embryos were generally more successful in conditions that included high temperature or moderate hyposalinity, whereas the FGC embryos were generally more successful in conditions that included extreme hyposalinity. This suggests that although multi-stressor tolerances are generally similar between the two populations, specific differences exist that correlate more with differences in nest microenvironment than latitude.
This chapter summarizes and expands on a research roundtable co-sponsored by the IUCN Horseshoe Crab Specialist Group, Nature Society (Singapore), and the Mangrove Action Project during the 2016 IUCN World Congress in Honolulu, Hawaii. Our central premise is that horseshoe crabs can serve as a flagship species for the conservation and restoration of several ecologically significant nearshore habitats, including estuarine beaches, mangroves, salt marshes, and tidal flats, all of which are being lost or degraded from coastal development and climate change. Horseshoe crabs are suitable as a flagship species because of their importance as a “living fossil,” their role in the nearshore food web, and their significant economic importance. Workshop participants from North America and Asia highlighted case histories and discussed the ways that horseshoe crab science and public education can join together to promote coastal marine conservation.KeywordsBeachMangroveSaltmarshMudflatsSeagrassHorseshoe crabsFlagship species conservation
Horseshoe crabs are archetypal marine chelicer-ates with an exceptionally long fossil record. Due to the historical nature of the genus Limulus, which extends back to Linnaeus' descriptions, many horseshoe crab fossils were traditionally placed in Limulus and the family Limulidae. Despite continued research into the accurate placement of species within Limulidae, a systematic outline of characteristics that define limulid genera, specifically using exclusively dorsal characteristics, does not yet exist. However, such an approach is essential as appendage data is rarely preserved in fossil horseshoe crabs. Here we present a systematic review of Limulidae with a focus on dorsal features , and illustrate all accepted limulid species across the 12 genera. Through this descriptive lens, we consider the validity of supposed Limulus species outlined in a recent xiphosurid review. We find evidence for only one fossil Limulus species: Limulus coffini. This revision therefore excludes Limulus from Jurassic-aged deposits. We refer 'Limulus' darwini from the Upper Jurassic (Upper Titho-nian) of Poland to Crenatolimulus darwini comb. nov. and 'Limulus' woodwardi from the Middle Jurassic (Aalenian) of England to Mesolimulus woodwardi comb. nov. This highlights that the Limulus evolutionary record is highly constrained and started as recently as the Late Cretaceous. The rare Limulus fossil record emphasizes the current need for conservation of extant species and the importance of thoroughly scrutinizing the morphology of fossil specimens to uncover all facets of the limited limulid evolutionary record.
This study provides regional-scale data on drivers of horseshoe crab (Limulus polyphemus) presence along the northcentral Gulf of Mexico coast and has implications for understanding habitat suitability for sparse horseshoe crab populations of conservation concern worldwide. To collect baseline data on the relationship between environmental factors and presence of horseshoe crabs, we surveyed four sites from the Fort Morgan peninsula of Mobile Bay, Alabama (AL) to Horn Island, Mississippi (MS). We documented number, size and sex of live animals, molts, and carcasses as metrics of horseshoe crab presence and demographics for two years. Data were compared to in situ and remotely sensed environmental attributes to assess environmental drivers of occurrence during the time of study. Overall, greater evidence of horseshoe crab presence was found at western sites (Petit Bois and Horn Islands) compared to eastern sites (Dauphin Island, Fort Morgan peninsula), mediated by a combination of distance from areas of high freshwater discharge and interannual variation in weather. Higher sex ratios also were found associated with higher occurrence, west of Mobile Bay. Land cover, particularly Bare Land and Estuarine Emergent Wetland classes that are common to western sites, was most predictive of live animal and to some extent carcass occurrence. Our findings suggest that small-scale variation in habitat quality can affect occurrence of horseshoe crabs in sparse populations where density is not a limiting factor. Data from molts and carcasses were informative to supplement live animal data and may be useful to enhance ecological assessment and support conservation and management in regions with sparse populations.
Beach nourishment is commonly conducted to protect human infrastructure but rarely for the primary purpose of improving wildlife habitat. To improve horseshoe crab spawning and shorebird feeding habitat in the Delaware Bay, New Jersey (USA), we removed 2000 t of shoreline rubble and placed sand on 16 ha of degraded shoreline spanning 8 beaches. Horseshoe crab egg cluster abundance varied annually, seasonally and spatially. Beaches restored using sand with grain size fractions similar to native sand had horseshoe crab egg cluster abundances matching or exceeding those of high-quality reference beaches. Deeper sand with a higher coarse-grain fraction resulted in the highest egg cluster abundance across all sites and beaches, while finer-grained sand used on a subset of restored beaches was associated with lower egg cluster abundances. These patterns were also reflected in shallow egg availability for shorebirds, with egg cluster abundance correlating positively with shallow egg abundance. Over time, sand placed on beaches moved cross-shore and longshore, and overwashed into marshes. Longshore sand movement nourished adjacent beaches and ebb shoals at creek mouths. Such shoals attract spawning horseshoe crabs and have high densities of surface eggs available for shorebird feeding, but experience high attrition of egg clusters. This study demonstrates that investments in beach restoration provide important benefits for horseshoe crabs and shorebirds. Outcomes can be further improved by expanding project scope and integrating other coastal restoration strategies. Restoration will be critical for the conservation of coastal species as sea levels rise and current and past coastal management practices continue to degrade habitats.
Conservation is increasingly in conflict with human activities due to global human population growth, particularly in areas that support threatened species. Conflicts often impede effective implementation of needed conservation measures and also have implications for social inequality, resource use and economic development. Bivalve molluscan shellfish aquaculture is commonly considered one of the least impactful forms of protein production worldwide but, in some locations, may interfere with essential activities of threatened species such as the stopover ecology of migrating shorebirds. Here we assess the impact of oyster aquaculture as practiced in Delaware Bay (New Jersey, USA) on the presence and foraging behavior of migratory shorebird species of conservation concern. We conducted counts and behavioral observations of shorebirds across a 4.8‐km stretch of the Delaware Bay and tested the effect of regulated aquaculture structures and activities on shorebird presence relative to various environmental factors. We also evaluated differences in mean peck rates for each species within and away from aquaculture areas, and we examined multiple factors influencing foraging rates for each species. For all species, we found that oyster tending reduced the probability of shorebird presence by 1–7%, whereas the untended aquaculture structures had no detectable impact. Foraging rates were mostly influenced by environmental conditions, particularly the presence of competitors (gulls or other shorebirds), and the foraging substrate. None of the focal species substantially altered their time budget or foraging rates in the presence of tended or untended oyster aquaculture. This evidence suggests that intertidal oyster aquaculture and migrating shorebirds can co‐utilize the resource rich intertidal areas on which they occur.
Table of Content: Geographic Range, Population, Habitat, Biology & Ecology
Tables and Figures
Concern has been raised about the ability of horseshoe crabs (Limulus polyphemus) to traverse intertidal rack-and-bag oyster farms, and how farms may change shorebird foraging activity. During the 2016 horseshoe crab spawning season, experiments conducted in Delaware Bay (New Jersey, USA) assessed crab ability to move among oyster farms and access landward nesting grounds, and surveyed distribution of dislodged eggs upon which many shorebirds feed. Experiments included testing (1) for impairment of crab passage by racks, (2) differences in crab abundance among paired farm/control transects, (3) whether farms affect crab stranding rate on nesting beaches, and (4) the spatial distribution of dislodged eggs along the wrack zone among farm and non-farm areas. All crabs, regardless of size, passed beneath racks 10 cm (4 inches) tall or more, indicating that the regulated rack height of 30.5 cm (12”) is abundantly precautious to allow crab movement beneath racks. Farm/control census observed 853 crabs in total, with no evidence of differing crab numbers among farmed and control transects. Only 2 of 853 (<0.5%) crabs were obstructed by farm gear, and more crabs were present on nesting beaches inshore of farms compared to adjacent farm-free areas. The proportion of crabs flipped (stranded) at low tide within nesting habitats was constant regardless of farm presence. Dislodged eggs in the wrack zone were observed most frequently in the center of the survey area, and were not concentrated near farms suggesting that in 2016, shorebird foraging opportunities were not coincident with farm locations.
This paper describes the discovery of new, exceptionally well preserved Late Jurassic (Late Tithonian) horseshoe crabs belonging to the genus Limulus, which are considered to be the oldest known representatives of the genus. The three-dimensionally preserved fossil horseshoe crabs, belonging to two species: Limulus darwini and Crenatolimulus sp. nov., recently were discovered in the Owadów-Brzezinki Quarry in Central Poland. The state of preservation of the fossil horseshoe crabs is described and the evolutionary lineage of both extinct and extant limulids is discussed. The palaeoenvironment inhabited by the new species and the trophic relationships of living horseshoe crabs are investigated in an attempt to determine the potential relationships between Mesozoic and recent xiphosurans.
The 1977 peak population of spawning horsehoe crabs, Limulus polyphemus, in Delaware Bay, was comprised of about 222,000 males and 51,000 females. This estimate, based upon a shoreline survey of spawning intensity along Delaware and New Jersey beaches at the time of full moon tides in June, was corroborated by a quantification of egg clusters in a beach. Fecundity of gravid females was used, in conjunction with the egg cluster estimate, to approximate the number of females responsible for the observed quantity of eggs. The present spawning population of Delaware Bay is several fold larger than that which existed during the 1960's. From a longer historical perspective, however, the population is far from approaching the numbers and spawning intensity reported a century ago.
A new horseshoe crab species, Limulus darwini, is described from the uppermost Jurassic (ca. 148 Ma) near-shore sediments of the Kcynia Formation, central Poland. The only extant species Limulus polyphemus (Linnaeus) inhabits brackish-marine, shallow water environments of the east coast of the United States. Here it is shown that there are no important morphological differences between the Kcynia Formation specimens and extant juvenile representatives of the genus Limulus. The palaeoecological setting inhabited by the new species and the trophic relationships of extant horseshoe crabs are discussed in an attempt to determine the potential range of food items ingested by these Mesozoic xiphosurans. In this paper we propose the adoption of a new term stabilomorphism, this being: an effect of a specific formula of adaptative strategy among organisms whose taxonomic status does not exceed genus-level. A high effectiveness of adaptation significantly reduces the need for differentiated phenotypic variants in response to environmental changes and provides for long-term evolutionary success.
A rich assemblage of exceptionally preserved marine and terrestrial fossils occurs in fine‐grained limestones in the upper part of the Late Tithonian (Middle Volgian) shallowing upward carbonate sequence in Central Poland. The richest horizon, a deposit known locally as the Corbulomima horizon, is named after the shallow burrowing suspension feeding bivalve Corbulomima obscura, moulds of which occur in densities of up to 500 per square metre on some bedding planes. The fauna in this bed also includes organic and phosphatic remains of a wide range of other creatures including the exuviae of limulids and decapods, disarticulated fish skeletons and rare isolated pterosaur bones and teeth. There are also perfectly preserved dragonfly wings and beetle exoskeletons. The average stable carbon and oxygen isotope values for ostracod shells and fine‐grained sediment from this horizon suggest precipitation of the calcium carbonate from warm seawater of normal marine salinity. The carbonate sediments overlying the fossiliferous horizon have been interpreted as nearshore to shoreface facies. These pass abruptly into coarse reworked intraclastic sediments interpreted as possible tsunami or storm surge over‐wash deposits. The clasts in this deposit have more positive oxygen isotope values than those in the underlying limestone, which may indicate that they were lithified in a slightly more evaporative, perhaps intertidal, setting. The succession terminates with silicified fine‐grained limestones likely to have formed in extremely shallow lagoonal environments. In contrast with the Solnhofen limestones of Lower Tithonian age in south‐central Germany the Corbulomima horizon is interpreted as a transitional deposit formed in a shallow marine setting by rapid burial with elements of both Konservat‐ and Konzentrat‐Lagerstätte preservation. □Konzentrat and Konservat‐Lagerstätte, Taphonomy, Palaeoenvironment, Paleogeography, Late Jurassic, Poland.
The purpose of this article is to highlight the impact of sea level rise on coastal landscape protections. To begin, a summary is provided of how coastal land is both utilized and protected. The utilization of coastal land includes a discussion of the values associated with coastal zones, including the development value of coastal land and the intrinsic ecological values that exist within coastal landscapes. From this summary, the issue of coastal landscape protection is discussed in relationship to sea level rise. This discussion focuses on two main choices relative to sea level rise response: staying at the coastline or, alternatively, retreating from the coastline. The factors impacting a stay or retreat deci- sion are identified and explored. Next, the legal considerations associated with the two choices—staying or retreating—are summarized. The relative benefits and costs of each choice are reviewed in relation to maintaining coastal landscape fea- tures, as are the potential legal impacts of government policies that affect private property rights in coastal land. Finally this article concludes with some recommendations on planning for the impacts of sea level rise from a programmatic stand- point, including the need to prioritize a response to sea level rise, understand the implications of choosing between staying at the shore and retreating from the shore, and understanding how these choices help to define and impact both the physical makeup and use of the coastal landscape in the future.
A systematically excavated track site in a 243.5 Myr old Middle Triassic (Karlstadt Formation, Pelsonian, middle Anisian) intertidal carbonate mud-flat palaeoenvironment at Bernburg (Saxony-Anhalt, central Germany) has revealed extensive horseshoe crab trackways attributable to the KouphichniumNopsca, 1923 ichnogenus. The exposed track bed of a Germanic Basin-wide spanned intertidal megatrack site is a mud-cracked biolaminate surface on which detailed tracks have been preserved because of rapid drying and cementation as a result of high temperatures, followed by rapid covering with a protective layer of arenitic storm or tsunami sediments. The different trackway types and their orientations have allowed a tidal sequence to be reconstructed, with the initial appearance of swimming horseshoe crabs followed by half-swimming/half-hopping limulids under the shallowest water conditions. The Bernburg trackways, which have mapped lengths of up to 40 m, were all produced by adult animals and exhibit a variety of shapes and patterns that reflect a range of subaquatic locomotion behaviour more typical of mating than of feeding activities. The closest match to the proportions and dimensions of the horseshoe crab tracks at Bernburg is provided by the largest known Middle Triassic limulid Tachypleus gadeai, which is known from the north-western Tethys in Spain. The horseshoe crab body fossils recognized in the German Mesozoic intertidal zones, instead, are from juveniles. The uniformly adult size indicated by the trackways therefore suggests that they may record the oldest intertidal reproductive zones of horseshoe crabs known from anywhere in the world, with the track-makers having possibly migrated thousands of kilometres from shallow marine areas of the north-western Tethys to reproduce in the intertidal palaeoenvironments of the Germanic Basin. Chirotherium trackways of large thecodont archosaurs also appeared on these flats where they appear to have fed on the limulids. With the tidal ebb, smaller reptiles such as Macrocnemus (Rhynchosauroides trackways) appeared on the dry intertidal flats, probably feeding on marine organisms and possibly also on horseshoe crab eggs. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 76–105.
We used results from a survey of horseshoe crab reproductive activity that was conducted in 1999 throughout Delaware Bay to examine the relationship between estimates of spawning females and egg deposition and analyze how that relationship varies with geography, time within a spawning season, beach morphology, and wave energy. We found that beach morphology and wave energy interacted with density of spawning females to explain variation in the density and distribution of eggs and larvae. For example, the quantity of eggs in surface sediment (i.e., eggs that are potentially available to foraging shorebirds) was associated with the density of spawning females, beach morphology, and wave energy. The association between beach morphology and live eggs in surface sediment was strong especially in late May (Percent Reduction in Error = 86% from regression tree model) where egg density was an order of magnitude higher on beaches <15 m wide (3.38*105 m-2; 90% CI: 2.29*105, 4.47*105) compared to wider beaches (1.49*104 m-2; 90% CI: 4.47*103, 2.53*104). Results also indicate that, among bay-front beaches, horseshoe crabs prefer to spawn on narrow beaches, possibly because of reduced wave energy. At peak periods of spawning activity, density of spawning females was inversely related to foreshore width on mid-latitude beaches within Delaware Bay (t = -2.68, 7 df, p = 0.03). Because the distribution of eggs across the foreshore varied with beach morphology and widened as the spawning season progressed, methods used to sample eggs need to be robust to variation in beach morphology and applicable regardless of when the samples are taken. Because beach morphology and wave energy were associated with the quantity of eggs in surface sediment, certain beach types may be critical to the conservation of shorebird foraging habitat.
This field study evaluates the effect of nourishing an estuarine beach with gravel to enhance spawning rates by horseshoe crabs. A total of 564 m3 of coarse sand and gravel were emplaced in two 90 m-long treatment segments at Bowers Beach, Delaware, USA from 9 to 11 April 2002. Field data were gathered between 6 April and 24 May 2002 to characterize the two fill segments and the un-nourished segments between them as well as two control segments at the adjacent Ted Harvey Beach.Sediment samples were taken from the foreshore surface and at depth before and after the nourishment. Bay water levels, wave heights, and beach ground water characteristics were monitored over a 12-hour tidal cycle at one of the nourished (15 May 2002) and the unnourished segment (16 May 2002) at Bowers Beach and at one of the control segments at Ted Harvey Beach (21 May 2002) using piezometers and pressure transducers inserted in wells. The beaches were cored to estimate the density of horseshoe crab eggs deposited during the spawning season. Horseshoe crab eggs were buried in pouches at 0.15 to 0.20 m depth for 30 to 40 days to evaluate their survival in developing into embryo or larval stage. Bulk sediment samples were taken to evaluate moisture characteristics near locations where egg pouches were buried.Density of spawning females at Bowers Beach was 1.04 m− 2 in 2001 and 1.20 m− 2 in 2002. These rates are lower than at Ted Harvey Beach but reveal an increase in spawning while Ted Harvey Beach underwent a considerable decrease (2.63 m− 2 to 1.35 m− 2). Sediments low on the foreshore remained nearly saturated throughout the tidal cycle at both beaches. The average hydraulic conductivity on the upper foreshore at the non-treatment section at Bowers Beach (0.19 cm s− 1) was less than at Ted Harvey Beach (0.27 cm s− 1), and the finer, better sorted sediments at depth at Bowers Beach resulted in a higher porosity, creating greater moisture retention potential. Egg development was greatest at mid foreshore at all sites. Eggs at the lower foreshore elevation remained viable, but did not develop to the embryo stage. Between-beach differences were limited to high elevations where higher mortality occurred at Ted Harvey Beach due to desiccation. Adding small amounts of gravel to a sand beach may change the appearance of the surface but may not appreciably increase mean grain size and sorting at depth or the hydraulic conductivity over the spawning season. The pebble fraction may be important for site selection, but finer sizes may be more important for egg survival because of moisture retention.
One of the most certain consequences of global warming is an increase of global (eustatic) sea level. The resulting inundation
from rising seas will heavily impact low-lying areas; at least 100 million persons live within one meter of mean sea level
and are at increased risk in the coming decades. The very existence of some island states and deltaic coasts is threatened
by sea level rise. An additional threat affecting some of the most heavily developed and economically valuable real estate
will come from an exacerbation of sandy beach erosion. As the beach is lost, fixed structures nearby are increasingly exposed
to the direct impact of storm waves, and will ultimately be damaged or destroyed unless expensive protective measures are
taken. It has long been speculated that the underlying rate of long-term sandy beach erosion is two orders of magnitude greater than the rate of rise of sea level, so that any significant increase of sea level has dire consequences for coastal inhabitants.
We present in this paper an analytical treatment that indicates there is a highly multiplicative association between long-term
sandy beach erosion and sea level rise, and use a large and consistent data base of shoreline position field data to show
that there is reasonable quantitative agreement with observations of 19th and 20th century sea levels and coastal erosion.
This result means that the already-severe coastal erosion problems witnessed in the 20th century will be exacerbated in the
21st century under plausible global warming scenarios.
We assessed the suitability of intertidal habitats for spawning by horseshoe crabs (Limulus polyphemus) at 12 proposed restoration sites identified by the United States Army Corps of Engineers along the shore of Jamaica Bay,
a highly developed estuary in New York City. Based on beach geomorphology, we chose to quantify horseshoe crab activity at
five of the sites during the May–July 2000 breeding season. Horseshoe crabs spawned intensively on small patches of suitable
sand within larger areas of eroding shoreline with bulkheads and rubble fill. Small areas of sand behind grounded barges at
Brant Point and Dubos Point had densities of over 100,000 eggs m−2, which was equal to or greater than the egg densities on longer, more natural appearing beaches at Spring Creek and Dead
Horse Bay, or at a sand spit at Bayswater State Park. There were no significant differences in the percentage of Jamaica Bay
horseshoe crab eggs that completed development when cultured using water from Jamaica Bay or lower Delaware Bay, a less polluted
location. Only 1% of the embryos from Jamaica Bay exhibited developmental anomalies, a frequency comparable to a previously
studied population from Delaware Bay. We suggest that the distribution and abundance of horseshoe crabs at our study areas
in Jamaica Bay is presently limited by the availability of suitable shoreline for breeding, rather than by water quality.
Restoration efforts that increase the amount of sandy beach in this urban estuary have a good likelihood of benefiting horseshoe
crabs and providing additional value to migrating shorebirds that use horseshoe crab eggs as food.
A panel of five international experts was convened during the International Symposium on the Science and Conservation of Horseshoe
Crabs to compare and contrast horseshoe crab management in their countries. The panel members each responded to a series of
questions prepared by a facilitator. All five speakers stated that habitat degradation and destruction were a major threat
to the horseshoe crab population(s) in their country. Pressure for economic development often hindered efforts to preserve
and protect coastal habitats. Public education was viewed as an important step toward the implementation of effective management
actions. The urgent need for conservation was viewed to be a strong, motivating factor to strengthen international management
efforts.
Horseshoe crab spawning activity is spatially patchy within the Delaware Estuary. This study investigated the importance of geochemical and erosional factors to the selection of breeding beaches. Two sandy beaches in Cape May county, New Jersey, USA, were studied; one beach had been subjected to considerable erosion, exposing underlying peat; the second beach, less than 1 km away, had only traces of peat. Reduced sediments with high levels of hydrogen sulfide were correlated with the presence of peat, and significantly fewer crabs utilized sediments in the proximity of peat beds for reproduction. The lower spawning activity on the beach in the vicinity of exposed peat, suggests that crabs may detect, at a distance, the nature of sediments and the quality of beach for spawning activity. Active salt marsh and peat-bank sediments dominate the upper bay shore; these sediments are unsuitable, or at best marginal, for horseshoe crab reproduction. Extensive bulkheading of eroding sandy beach along several New Jersey shore communities has further restricted the availability of suitable spawning habitat, making the remaining stretches of optimal sandy beach critical to the reproductive success of this species.
The 1977 peak population of spawning horsehoe crabs,Limulus polyphemus, in Delaware Bay, was comprised of about 222,000 males and 51,000 females. This estimate, based upon a shoreline survey of
spawning intensity along Delaware and New Jersey beaches at the time of full moon tides in June, was corroborated by a quantification
of egg clusters in a beach. Fecundity of gravid females was used, in conjunction with the egg cluster estimate, to approximate
the number of females responsible for the observed quantity of eggs. The present spawning population of Delaware Bay is several
fold larger than that which existed during the 1960’s. From a longer historical perspective, however, the population is far
from approaching the numbers and spawning intensity reported a century ago.
Recent and dramatic declines have been documented in several shorebird populations that stage in Delaware Bay during spring
migration. As a result, considerable attention has been given to issues such as the reliance of sandpipers on Delaware Bay
American horseshoe crab (Limulus polyphemus) eggs for refueling and how to best manage the horseshoe crab fishery to insure adequate resources for migratory shorebirds.
In this chapter, we synthesize over 25 years of shorebird research and monitoring data in Delaware Bay to support the premise
that horseshoe crab eggs are an essential element for migrating sandpipers during northbound passage through the bay. We then
discuss long- and short-term trends in American horseshoe crab populations resulting from changes in demand and harvest regulations,
and how this has affected shorebird population viability. Regulatory actions have led to recent increases in some demographic
elements of the Delaware Bay crab population, but such changes have not yet translated into increased crab egg availability
or population recovery indices in shorebirds. Because reduced availability of horseshoe crab eggs has severe consequences
for migratory sandpipers at the individual and population levels, current conservation strategies that include harvest reductions
on American horseshoe crabs in the Delaware Bay region must persist into the foreseeable future to insure the recovery of
horseshoe crab populations and the long-term health of migratory sandpipers in Delaware Bay.
As sea level rises, coastal communities will face increased risks of flooding, storm surge, and inundation. In some areas, structural protective measures will be built, and for some properties, accommodation to sea level rise may be possible. For other areas, however, some form of retreat will be either preferred on economic or sociopolitical grounds or required given fiscal constraints. This paper considers how society can proactively manage shoreline retreat in those locations where it is deemed the preferable policy. A three-part strategy is proposed: (1) reduce new development in the highest-risk areas; (2) adopt policies that allow for expected and orderly removal or modification of development as inundation occurs; and (3) take advantage of disasters to implement managed retreat approaches. Specific policies are recommended and the challenges of institutional change discussed.
Rapidly growing populations and expanding development are intensifying pressures on coastal ecosystems. Sea-level rise and other predicted effects of climate change are expected to exert even greater pressures on coastal ecosystems, exacerbating erosion, degrading habitat, and accelerating shoreline retreat. Historically, society’s responses to threats from erosion and shoreline retreat have relied on armoring and other engineered coastal defenses. Despite widespread use on all types of shorelines, information about the ecological impacts of shoreline armoring is quite limited. Here we summarize existing knowledge on the effects of armoring structures on the biodiversity, productivity, structure, and function of coastal ecosystems.
It is established fact that sea level is rising slowly and irregularly; also, it seems to be true that erosion on most seashores built up of alluvial materials greatly exceeds accretion; relationship between rise of sea level and erosion.
Beach replenishment has been proposed to increase nesting habitat for horseshoe crabs, but its environmental consequences may compromise the egg development and viability of this declining species. Horseshoe crab habitat requirements were used to build a habitat suitability model in STELLA to predict the potential impacts of beach replenishment on horseshoe crab eggs. A habitat suitability index (H.S.I.) comprised of six variables (dissolved oxygen, sediment grain size, sand temperature, sand moisture, wave energy, and salinity) was developed and compared between replenished and natural beaches. Sediment grain size and dissolved oxygen were higher in the natural beach, whereas sand temperature and moisture were higher in the replenished beach, resulting in significantly higher suitability of the natural beach ( p = 5.39 × 10 −15 , df = 30). The model was most sensitive to air temperature, rainfall, tide, and sediment grain size. This model is useful for understanding the processes affecting horseshoe crabs and predicting impacts of coastal management activities on habitat suitability. Based on the results of this model, beach replenishment is not recommended for increasing or improving horseshoe crab habitat, unless care is taken to match fill sediment to natural grain size and color.
Analyses of an extensive grid of seismic reflection profiles along with previously published core data and modern sedimentary environment information from surrounding coastal areas permit an outline of the paleogeography of the large Delaware Bay estuary during the last transgression of sea level. During late Wisconsinan times, the Delaware River system eroded a dendritic drainage pattern into the gravelly and muddy sands of Tertiary and younger age beneath the southern half of the lower bay area. This system included the trunk valley of the ancestral river and a large tributary valley formed by the convergence of secondary streams along the Delaware coast. The evolution of the estuary from this drainage system proceeded as follows: (1) When local relative sea level was at −50 m, the head of the tide reached the present bay-mouth area. (2) At −40 m (possibly 15,000–12,000 yrs ago), the trunk valley of the drainage system was a tidal river that extended more than 30 km up the bay, and a small contiguous inlet existed at the bay mouth. (3) At −30 m (approximately 11,000−10,000 yrs ago), the estuary comprised two narrow passages formed by the drowning of the main and tributary river valleys, and the bay-mouth inlet was 5–6 km wide. (4) At −20 m (between 8000 and 7000 yrs ago), the two passages of the estuary were joined, except for a series of small islands on top of a low intervening ridge, and the inlet channel was 11 km wide. (5) At −10 m (between 6000 and 5000 yrs ago), the estuary was nearly continuous and encompassed about 60% of the present lower bay area. Thin, coarse-grained fluvial deposits accumulated initially within the main channels of the former drainage system as base level was elevated by rising sea level. During the subsequent development of the estuary, clayey silts were deposited rapidly beneath the nontidal estuarine depocenter (turbidity maximum) as it migrated through the bay area, and organic muds accumulated in tidal wetlands that occupied the mouths of tributaries and small marginal embayments. As the fetch and tidal prism of the estuary increased, narrow barrier and headland beaches, composed of fine to coarse sands, were formed locally along the bay shorelines. In the later stages of development, sediment scour, reworking and transport became the dominant processes within the open estuary.Data from this study demonstrate the great temporal and spatial variability of sedimentary deposits within large drowned river-valley estuaries and outline a model that can be used to interpret ancient estuarine strata.
Horseshoe crab (Limulus polyphemus) harvest on Delaware Bay is not a new event. Early settlers to the New World reported use of horseshoe crabs by Native Americans
for food, tools, and to enrich soils for growing crops. Literature from the mid- to late 1800s documents the use of horseshoe
crabs for fertilizer and to supplement livestock feed. By the 1870s and for almost a century thereafter, well over a million
crabs were harvested annually from Delaware Bay, in support of a regionally significant “cancerine” (fertilizer) industry.
Subsequent to the cessation of the cancerine industry in the mid-twentieth century, relatively low-scale use of horseshoe
crabs as bait for American eel and other fisheries existed. This use exploded in the 1990s, as eel markets expanded and use
of horseshoe crabs for bait in a rapidly emerging whelk (Busycon spp.) pot fishery intensified along the East Coast of the United States. With horseshoe crabs spawning in mass along the
shores of Delaware Bay, and little or no regulations in place, harvest pressure once again approached levels of the fertilizer
use days. Simultaneously, an ongoing need for bleeding of horseshoe crabs to provide Limulus amebocyte lysate (LAL) for biomedical use, and growing recognition of the importance of horseshoe crab eggs on Delaware Bay
as a key stopover food source for migratory shorebirds prompted concerns about observed declines in the population, resulting
in implementation of significant management measures to ensure the sustainability of the species.
The fossil record of the basic xiphosurid horseshoe crab body plan has been extended back to the Late Ordovician Period, about
445 million years ago, demonstrating an origin that lies outside of the paraphyletic ‘synziphosurines.’ Horseshoe crab body
fossils are exceptionally rare and are found mostly in shallow coastal and marginal marine Konservat-Lagerstätten deposits.
Their sporadic occurrences document a post-Cambrian history of low overall diversity with a modest morphological and taxonomic
peak in the Late Paleozoic Era. Survival of a single xiphosurid lineage through the end-Permian mass extinction events was
followed by a minor secondary radiation during the Triassic Period. The Jurassic to Recent fossil record of horseshoe crabs
is relatively impoverished in both taxa and known occurrences. Overall, the rarity of fossil xiphosurids reflects both taphonomic
biases inherent in the unusual conditions required for preservation of their non-biomineralized exoskeletons and complex ecological
factors related to a long-term association with shallow marginal aquatic habitats. Focused paleontological investigations
should yield additional fossil horseshoe crab discoveries that will in turn inform research on their phylogeny, morphological
stasis, and ecological persistence.
This study presents an assessment of the potential impacts of sea level rise on the New Jersey, USA coastal region. We produce
two projections of sea level rise for the New Jersey coast over the next century and apply them to a digital elevation model
to illustrate the extent to which coastal areas are susceptible to permanent inundation and episodic flooding due to storm
events. We estimate future coastline displacement and its consequences based on direct inundation only, which provides a lower
bound on total coastline displacement. The objective of this study is to illustrate methodologies that may prove useful to
policy makers despite the large uncertainties inherent in analysis of local impacts of climate and sea level change. Our findings
suggest that approximately 1% to 3% of the land area of New Jersey would be permanently inundated over the next century and
coastal storms would temporarily flood low-lying areas up to 20 times more frequently. Thus, absent human adaptation, by 2100
New Jersey would experience substantial land loss and alteration of the coastal zone, causing widespread impacts on coastal
development and ecosystems. Given the results, we identify future research needs and suggest that an important next step would
be for policy makers to explore potential adaptation strategies.
The ability to successfully manage estuarine shorelines requires balancing ecological function with societal demands. The
sandy barriers of Delaware Bay provide important spawning habitat for horseshoe crabs but they are modified for shore protection.
This chapter provides a review of the sandy shoreline resources within Delaware Bay; describes the spatial and temporal scales
of processes that govern their dimensions, location, morphology, and sedimentary characteristics; compares management programs
in the state of Delaware and New Jersey for managing sandy shorelines in the estuary; and provides examples of the status
of developed sandy barriers where high levels of horseshoe crab spawning occur.
A study of the physical properties of horseshoe crab (Tachypleus tridentatus) nursery grounds indicated that juveniles preferred sediments consisting of fine sand with median grain size of 0.14–0.27mm
in diameter, 16.9–23.2% water content when tides receded, 0.23–0.41% TOC content, 0.04–0.07% TN content, 2.3–2.8 µg/cm2 chlorophyll a content, and poorly sorted substrates. Juvenile horseshoe crab density increased based on the amount of chlorophyll a content in the sediment and infaunal polychaete density, suggesting that the juveniles prefer nursery grounds containing
abundant prey and its supporting food web. An effort to restore horseshoe crab spawning grounds was conducted by covering
20cm deep mud substrate (0.23mm in diameter) with coarse sand (1.10mm in diameter). Although adults transferred to this
site succeeded in laying eggs, the hatching rate was only 33.9%. This low rate may be attributed to small tidal amplitude
at this restoration site.
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