Limulus Polyphemus Research Articles

The Long-Term Effect of Bleeding for Limulus Amebocyte Lysate on Annual Survival and Recapture of Tagged Horseshoe Crabs

In the U.S., 525,000 horseshoe crabs (Limulus polyphemus) per year have been captured during 2013–2017, brought to biomedical facilities, and bled to produce Limulus amebocyte lysate (LAL), then mostly released to the area of capture. The Atlantic States Marine Fisheries Commission estimates short-term bleeding-induced mortality to be 15% (4% to 30%), resulting in mortality of approximately 78,750 horseshoe crabs annually in recent years comprising a minor portion (<13%) of the up to one million annual coastwide landings dominated by harvest for bait. However, the long-term effect of bleeding for LAL on annual survival and spawning behavior is unknown; thus, results from short-term studies alone might underestimate bleeding effects at the population level. To address this knowledge gap, we analyzed data from the U.S. Fish and Wildlife horseshoe crab tagging database to estimate the differences in survival and recapture rates of bled and not bled horseshoe crabs tagged in the same years and geographic area. Contrary to expectation, survival was not lower for bled crabs compared to unbled crabs. Differences varied, but survival estimates tended to be higher for bled crabs than for unbled crabs. However, biomedical culling and selection for younger or healthier animals could have resulted in biomedically tagged individuals representing a healthier subset of the overall population with subsequent higher survival. Furthermore, the tagging analysis revealed a post-bleeding reduction in capture probability, which could indicate decreased spawning activity, evident in males more than females. Continued tagging of bled and unbled crabs in the same geographic area while recording age class and sex will contribute to the further resolution of LAL production’s effect on horseshoe crab populations.

Distribution of juvenile American horseshoe crabs Limulus polyphemus in the Great Bay Estuary, New Hampshire, USA

In order to effectively manage a fishery, it is important to know the essential habitats used by all life history stages of the species being managed. However, this is often a challenge for animals such as the American horseshoe crab Limulus polyphemus, whose juvenile stages are difficult to find and quantify. Visual surveys and suction sampling have been used in previous studies, but in the Great Bay Estuary, New Hampshire, they are less effective, and so little is known about the habitats occupied by juvenile horseshoe crabs in this area. To fill this knowledge gap, we used SCUBA-dive surveys to locate juvenile horseshoe crabs. We found them on subtidal and some intertidal mudflats in the upper regions of the estuary, both adjacent to, and at some distance from, known spawning beaches. However we did not find them in either intertidal or subtidal areas adjacent to spawning beaches just 1-2 km closer to the coast. These data should be useful for future conservation efforts designed to protect essential horseshoe crab habitats in the Great Bay and other similar estuaries.

The American Horseshoe Crab (Limulus polyphemus)

American horseshoe crabs (<em>Limulus polyphemus</em>) look prehistoric and in fact really have not changed very much in the 200 million years they have been around. This 3-page fact sheet written by Savanna Barry, Holly Abeels, and Shelly Krueger and published by the UF/IFAS Program in Fisheries and Aquatic Sciences, School of Forest Resources and Conservation tells the story of these interesting and valuable "living fossils," including their importance both to ecology and human medicine. It provides tips on how to find horseshoe crabs and a few ways you can help them.

Atlantic Horseshoe Crabs and Endotoxin Testing: Perspectives on Alternatives, sustainable Methods, and the 3Rs (Replacement, Reduction, and Refinement).

Endotoxin testing is a vital part of quality and safety control in pharmaceutical production. The primary method for this testing in North America and Europe is the limulus amebocyte lysate (LAL) test, a critical component of which is the blood of Atlantic horseshoe crabs (Limuius poiyphemus). Procuring blood for LAL testing involves capturing and bleeding over 500,000 crabs from wild marine populations each year. Whilst efforts are made by manufacturers to return crabs to the sea following the collection of blood, there is a level of mortality and sub-lethal impact involved, prompting increasing discussions about welfare and ethics. The 3Rs – the ambition to where possible, replace, reduce, and refine the use of animals – are established and accepted worldwide as the best framework for governing animal-dependent science. However, the biomedical utilization of horseshoe crabs to produce the LAL test has rarely been viewed through a 3Rs framework. More recently, there has been a renewed attention on sustainable methods and alternatives to the LAL test. Drawing on in-depth qualitative interviews, this article examines stakeholder perspectives on opportunities for thinking with the 3Rs, considering current appetites to replace, refine, and reduce contemporary biomedical reliance on horseshoe crabs. The shape of conversations about the biomedical utilization of horseshoe crabs has shifted significantly in recent years, and the 3Rs are an important driver of change, offering the potential to advance the use of more sustainable methods, and realize the welfare considerations increasingly expected across science and society.

The relative influence of nature vs. nurture on the expression of circatidal rhythms in the American horseshoe crab Limulus polyphemus

The horseshoe crabLimulus polyphemusexhibits rhythmic locomotor behavior that is controlled by both internal clocks and external tidal cycles. Recent findings demonstrate that adult horseshoe crabs from a 2 tides d-1environment usually express 2 bouts of activity d-1, while adults from areas with 1 dominant tide d-1, or with negligible tides (atidal), generally express 1 bout of activity d-1. Horseshoe crabs from these different environments are genetically distinct, so it is not known if these behavioral differences are driven by genetics or by the tides they experience early in life. To address this question, freshly spawned horseshoe crab eggs from populations that experience these 3 distinct environmental tidal regimes were reared in the laboratory under 1 of the above 3 artificial tidal conditions. Then the activity of individual larval and 2ndinstar juveniles was recorded over 3 wk using video tracking software. Endogenous circatidal rhythms were present in most (88/108) of the freshly molted juveniles from all 3 populations, even when they were raised in atidal conditions, indicating a strong genetic influence. To our knowledge, this is the first demonstration of the presence of genetically driven endogenous circatidal rhythms in an individual juvenile marine organism. Nevertheless, the early environment was also capable of having an impact, because it significantly affected rhythm expression in crabs in the 1 tide d-1population. Thus, overall, the tidal environment to which horseshoe crab eggs are exposed during development appears to have less of an influence than genetics on their expression of tidal rhythms of locomotion.

Effects of the biomedical bleeding process on the behavior and hemocyanin levels of the American horseshoe crab (Limulus polyphemus)

Effects of the biomedical bleeding process on the behavior and hemocyanin levels of the American horseshoe crab (Limulus polyphemus)

Seasonal Variation in Reproduction of Horseshoe Crabs (Limulus polyphemus) from the Gulf Coast of Florida.

AbstractThe timing of reproduction is often governed by environmental variables, such as temperature or rainfall. Understanding how environmental variables affect mating dynamics is necessary to predict how systems and populations may adapt to changing environmental conditions and is crucial for management of threatened species. The American horseshoe crab (Limulus polyphemus) ranges from the Yucatan to Maine in distinct populations that differ in their timing of reproduction; while most populations have only one breeding period during the spring, some southern populations have two breeding periods. Here we discuss seasonal patterns of reproduction in a Florida Gulf coast population where horseshoe crabs have two periods of breeding: one in the spring and another in the fall. We used environmental measurements, spawning surveys, mark-recapture, and measurements of adult traits and spawning behavior to compare reproductive parameters between the two spawning seasons over three years. We then evaluated whether environmental conditions affect fall and spring horseshoe crab nesting patterns similarly and whether fall and spring horseshoe crabs should be considered two separate populations. We found significant differences in environmental conditions across seasons and in a wide variety of horseshoe crab traits and nesting parameters. Furthermore, environmental conditions affected nesting behaviors of fall and spring horseshoe crabs differently. However, some individuals spawn during both seasons, suggesting that trait differences may be attributable to environmental effects during development or seasonal plasticity, rather than genetic differences, although further study is necessary. Finally, our results suggest that management practices should be tailored to each population, because environmental conditions may have different effects even on genetically similar groups.

Microbiome Shifts Associated With the Introduction of Wild Atlantic Horseshoe Crabs (Limulus polyphemus) Into a Touch-Tank Exhibit.

The Atlantic horseshoe crab (Limulus polyphemus) is a common marine aquarium species and model organism for research. There is potential monetary and conservation value in developing a stable captive population of horseshoe crabs, however, one major impediment to achieving captivity is a lack of knowledge regarding captive diseases. We utilized 16S rRNA gene amplicon sequencing to track changes in the microbiomes of four body locations in three wild-caught (tracked over 14 months in captivity) and three tank-acclimated (>2 years in captivity) adult L. polyphemus in a touch tank at Shark Reef Aquarium at Mandalay Bay in Las Vegas, NV. The wild population hosted diverse and distinct microbiomes on the carapace (260 ± 96 amplicon sequence variants or ASVs), cloaca (345 ± 77 ASVs), gills (309 ± 36 ASVs), and oral cavity (359 ± 37 ASVs), which were dominated by classes Gammaproteobacteria, Bacteroidia, and Alphaproteobacteria. A rapid decline in richness across all body locations was observed within 1 month of captivity, with tank-acclimated (>2 years) animals having <5% of the initial microbiome richness and a nearly completely restructured microbial community. Tank-acclimated horseshoe crabs possessed distinct microbiomes that were highly uneven and low in species richness on the carapace (31 ± 7 ASVs), cloaca (53 ± 19 ASVs), gills (17 ± 2 ASVs), and oral cavity (31 ± 13 ASVs). The carapace, oral cavity, and gills of the tank-acclimated animals hosted abundant populations of Aeromonas (>60%) and Pseudomonas (>20%), both of which are known opportunistic pathogens of aquatic animals and can express chitinases, providing a plausible mechanism for the development of the carapace lesion pathology observed in this and other studies. The cloaca of the tank-acclimated animals was slightly more diverse than the other body locations with Aeromonas, Enterococcus, Shewanella, and Vagococcus dominating the community. These results provide an important baseline on the microbiomes of both wild and tank-acclimated horseshoe crabs and underscore the need to continue to investigate how native microbial populations may protect animals from pathogens.

Beach restoration improves habitat quality for American horseshoe crabs and shorebirds in the Delaware Bay, USA

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.

Effects of the octopus fishery on the American horseshoe crab population in the Ría Lagartos Biosphere Reserve, Mexico

The American horseshoe crab, Limulus polyphemus, has been reported to be illegally caught for use as Octopus spp. bait in some localities of the Yucatán Peninsula, Mexico. To learn if the octopus fishery could be negatively influencing the limulus populations in the area, L. polyphemus individuals were sampled twice a year, before and after the annual octopus fishing season, over 4 consecutive years (2015–2018) in Río Lagartos and San Felipe, both in the Ría Lagartos Biosphere Reserve (RLBR), Yucatán. Quadrants were drawn perpendicular to the mangrove edge, physicochemical parameters in the water (depth, temperature, dissolved oxygen, pH, salinity, and electrical conductivity) were determined, and granulometry of marine sediments was measured (Folk technique). The number of live specimens, number of females, and average length were recorded. No significant differences were found for the physicochemical properties of water or the recorded sedimentology (P &gt; 0.05). Samplings revealed a low number of organisms per site: 22 in Río Lagartos and 24 in San Felipe, both from data before the start of the 2017 octopus fishing season. In the samplings after the start of the 2018 octopus fishing season, only 2 organisms were observed in Río Lagartos and none in San Felipe. The results showed a significant decrease in the number of live L. polyphemus specimens after the beginning of the octopus fishing season and a decrease in the average length of organisms, suggesting a negative effect of mollusk fishing activities on the horseshoe crab populations. More studies are required that integrate other physicochemical, biological, and socioeconomic factors to elucidate the potential threat of this fishery on L. polyphemus populations in the RLBR.

Molecular Identification, Characterization, and Expression Analysis of a Gonadotropin-Releasing Hormone Receptor (GnRH-R) in Pacific Abalone, Haliotis discus hannai.

A full-length cDNA sequence encoding a GnRH receptor was cloned from the pleuropedal ganglion of the Pacific abalone, Haliotis discus hannai. The cloned sequence is 1499-bp in length encoding a protein of 460 amino acid residues, with a molecular mass of 52.22 kDa and an isoelectric point (pI) of 9.57. The architecture of HdhGnRH-R gene exhibited key features of G protein-coupled receptors (GPCRs), including seven membrane spanning domains, putative N-linked glycosylation motifs, and phosphorylation sites of serine and threonine residues. It shared 63%, 52%, and 30% sequence identities with Octopus vulgaris, Limulus polyphemus, and Mizuhopecten yessoensis GnRH-R II sequences, respectively. Phylogenetic analysis indicated that HdhGnRH-R gene was clustered with GnRH-R II of O. vulgaris and O. bimaculoides. qPCR assay demonstrated that the mRNA expression level of this receptor was significantly higher in the pleuropedal ganglion than that in any other examined tissue. Transcriptional activities of this gene in gonadal tissues were significantly higher in the ripening stage. The mRNA expression of this gene was significantly higher in pleuropedal ganglion, testis, and ovary at higher effective accumulative temperature (1000 °C). In situ hybridization revealed that HdhGnRH-R mRNA was expressed in neurosecretory cells of pleuropedal ganglion. Our results suggest that HdhGnRH-R gene synthesized in the neural ganglia might be involved in the control of gonadal maturation and gametogenesis of H. discus hannai. This is the first report of GnRH-R in H. discus hannai and the results may contribute to further studies of GPCRs evolution or may useful for the development of aquaculture method of this abalone species.

Sediment microbiomes associated with critical habitat of the juvenile American horseshoe crab; limulus polyphemus.

Plumb Beach, Brooklyn, New York in USA is an important horseshoe crab breeding and nursery ground that has experienced substantial anthropogenic influence, including pollution, erosion and subsequent restoration. Since little is known about the relationship between sediment microbial communities and juvenile horseshoe crab survival, next generation sequencing was used to characterize and compare the sediment microbiome of three distinct areas of Plumb Beach:- a tidal creek with abundant juveniles, East Beach with moderate number of juveniles, and West Beach- a highly disturbed area where juvenile crabs are rarely seen. The microbiome of juvenile crab intestinal content (both dissected gut content and fecal flush content) from the tidal creek site was also examined. The results showed that in our 2017 survey, the overall dominant sediment orders at all beach sites were Vibrionales (30%), Flavobacteriales (22%) and Alteromonadales (21%). Although alpha diversity was similar among the three beach sites, Bray-Curtis distances assessed by Permanova revealed significant differences in Beta diversity, with a unique microbial assemblage found in the tidal creek. Both crab gut and fecal flush samples did not sequence well, showing low species diversity and very high variability. This study is the first to use next generation sequencing to characterize Plumb Beach sediment microbes and the first attempt to examine the gut microbiome of juvenile horseshoe crabs. This information will contribute to understanding the relationships between sediment microbial assemblages and juvenile crab populations within this important urban habitat.

Post-translational protein deimination signatures and extracellular vesicles (EVs) in the Atlantic horseshoe crab (Limulus polyphemus)

The horseshoe crab is a living fossil and a species of marine arthropod with unusual immune system properties which are also exploited commercially. Given its ancient status dating to the Ordovician period (450 million years ago), its standing in phylogeny and unusual immunological characteristics, the horseshoe crab may hold valuable information for comparative immunology studies.Peptidylarginine deiminases (PADs) are calcium dependent enzymes that are phylogenetically conserved and cause protein deimination via conversion of arginine to citrulline. This post-translational modification can lead to structural and functional protein changes contributing to protein moonlighting in health and disease. PAD-mediated regulation of extracellular vesicle (EV) release, a critical component of cellular communication, has furthermore been identified to be a phylogenetically conserved mechanism. PADs, protein deimination and EVs have hitherto not been studied in the horseshoe crab and were assessed in the current study. Horseshoe crab haemolymph serum-EVs were found to be a poly-dispersed population in the 20–400 nm size range, with the majority of EVs falling within 40–123 nm. Key immune proteins were identified to be post-translationally deiminated in horseshoe crab haemolymph serum, providing insights into protein moonlighting function of Limulus and phylogenetically conserved immune proteins. KEGG (Kyoto encyclopaedia of genes and genomes) and GO (gene ontology) enrichment analysis of deiminated proteins identified in Limulus revealed KEGG pathways relating to complement and coagulation pathways, Staphylococcus aureus infection, glycolysis/gluconeogenesis and carbon metabolism, while GO pathways of biological and molecular pathways related to a range of immune and metabolic functions, as well as developmental processes. The characterisation of EVs, and post-translational deimination signatures, revealed here in horseshoe crab, contributes to current understanding of protein moonlighting functions and EV-mediated communication in this ancient arthropod and throughout phylogeny.

Cryptic speciation in the ectocommensal Bdelloura candida (Platyhelminthes, Tricladida, Maricola) follows habitat specialization of the American horseshoe crab, Limulus polyphemus

AbstractIn strict symbiotic associations, the genetic structure of the symbiont often mirrors that of its host, with interesting implications for population dynamics and phylogeography. An unresolved case of symbiotic specificity and phylogeographic consequence is the relationship between the marine triclad Bdelloura candida and its host, the American horseshoe crab, Limulus polyphemus. A recent study by Riesgo et al. (2017, Marine Biology, 164, 111) identified a strong genetic break between populations of B. candida in the Gulf of Mexico and the Atlantic Ocean but had minimal sampling around the Florida peninsula such that the exact location of the boundary zone was not specified. To solve this, a comprehensive analysis of 16S rRNA and ITS2 genetic markers was conducted from new collections around the Florida peninsula. A clear and significant genetic break was identified between populations of supposed B. candida between Cumberland Island, Georgia, and Mosquito Lagoon, Florida. This genetic break establishes two cryptic lineages, an Atlantic population as far south as Georgia and a Floridian population inclusive of the entire peninsula and Gulf of Mexico, potentially due to niche partitioning of the unique intertidal habitats of its horseshoe crab hosts in Florida. This result directly refutes the previous hypothesis that a population break exists between the coasts of the Atlantic Ocean and Gulf of Mexico, and instead matches the genetic break of its host. Furthermore, a third cryptic lineage was identified in Key West. Overall, this work demonstrates the challenges in maintaining genetic connections between populations of both B. candida and L. polyphemus across their distributions, and poses meaningful implications for both species in the larger context of marine conservation and biodiversity.

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.

The draft genome of horseshoe crab Tachypleus tridentatus reveals its evolutionary scenario and well-developed innate immunity

BackgroundHorseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems. However, the genetic mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear.ResultsHere, we describe the 2.06 Gbp genome assembly of Tachypleus tridentatus with 24,222 predicted protein-coding genes. Comparative genomics shows that T. tridentatus and the Atlantic horseshoe crab Limulus polyphemus have the most orthologues shared among two species, including genes involved in the immune-related JAK-STAT signalling pathway. Divergence time dating results show that the last common ancestor of Asian horseshoe crabs (including T. tridentatus and C. rotundicauda) and L. polyphemus appeared approximately 130 Mya (121–141), and the split of the two Asian horseshoe crabs was dated to approximately 63 Mya (57–69). Hox gene analysis suggests two clusters in both horseshoe crab assemblies. Surprisingly, selective analysis of immune-related gene families revealed the high expansion of conserved pattern recognition receptors. Genes involved in the IMD and JAK-STAT signal transduction pathways also exhibited a certain degree of expansion in both genomes. Intact coagulation cascade-related genes were present in the T. tridentatus genome with a higher number of coagulation factor genes. Moreover, most reported antibacterial peptides have been identified in T. tridentatus with their potentially effective antimicrobial sites.ConclusionsThe draft genome of T. tridentatus would provide important evidence for further clarifying the taxonomy and evolutionary relationship of Chelicerata. The expansion of conserved immune signalling pathway genes, coagulation factors and intact antimicrobial peptides in T. tridentatus constitutes its robust and effective innate immunity for self-defence in marine environments with an enormous number of invading pathogens and may affect the quality of the adaptive properties with regard to complicated marine environments.

Smells like home: The use of chemically-mediated rheotaxes by Limulus polyphemus larvae

The distribution of horseshoe crab (Limulus polyphemus) larvae and early benthic juveniles is often non-random, with higher abundances typically found in areas of dense submerged vegetation (e.g., seagrass and algae) in coastal and estuarine nursery habitats. To determine if these distribution patterns are mediated by behavioral responses to chemical odors associated with potential settlement sites, we examined whether habitat chemical cues influence the swimming behavior (chemically-mediated rheotaxis) of horseshoe crab larvae at two different periods in their development: immediately after hatching (early-stage) and just prior to metamorphosis (late-stage). The hypothesis that larvae exhibit a positive rheotaxis (upstream movement) in the presence of chemical cues from submerged vegetation and negative rheotaxis (downstream movement) in response to odors from predators was tested in a laboratory T-maze. Larvae were exposed to seawater containing chemical odors from one of four sources: (1) water collected directly from the estuary, and odor water prepared with (2) aquatic vegetation, (3) potential predators, or (4) conspecifics. Behavioral responses of larvae varied with the source of the odor and the developmental stage of the larva. Both larval stages responded to estuarine water and water containing cues from the seagrass Halodule wrightii, a known nursery habitat, but response rates were greater for early-stage larvae. The response of larvae to H. wrightii odor was dose-dependent; upstream swimming response increased with increasing H. wrightii odor concentration. Though juvenile horseshoe crabs are also found within beds of Syringodium filiforme and Acanthophora spicifera, in our experiments early-stage larvae swam downstream away from both cues. In contrast, late-stage larvae exhibited a positive rheotactic response to the odor of A. spicifera. Conspecific odor had no effect on the rheotaxis of either developmental stage tested. Both larval stages unexpectedly responded to predator odors (blue crabs, Callinectes sapidus; killifish, Fundulus grandis) with a positive rheotaxis. When combined with a known positive cue (H. wrightii), killifish predator odor did not alter the rheotactic response of early-stage larvae; however, the addition of the predator odor reversed the upstream swimming behavior of late-stage larvae, likely indicating an alarm response to the presence of the predatory fish. Collectively, our results indicate that the distribution patterns of early benthic horseshoe crabs are a consequence of developmental shifts in oriented swimming behaviors (rheotaxes) that are mediated by chemical odors associated with settlement habitats. These behaviors should aid in the larva's ability to seek out and settle in advantageous habitat, thereby reducing predation and increasing survivorship.

Egg Exhumation and Transport on a Foreshore Under Wave and Swash Processes

Several aquatic species use beach foreshores to lay their eggs. Understanding the dynamics of egg entrainment versus egg retention in foreshore sediment is important to delivering eggs to foraging species, and to predicting fecundity and population responses of beach nesting species to harvest and habitat management. Exhumation of these eggs is primarily by bioturbation and wave action. This study isolates effects of wave and swash processes on exhumation, entrainment, and transport of horseshoe crab eggs by conducting a 1-day field study designed to avoid effects of bioturbation by spawning and migratory shorebird predation. Dyed sand and horseshoe crab (Limulus polyphemus) eggs were injected into the foreshore of an estuarine beach in Delaware Bay, USA and sampled during 110 swash events using streamer traps. Significant wave heights ranged from 0.30 to 0.47 m. The quantity of eggs exhumed from the mid-foreshore by waves and swash was 3.8 times greater than from the upper foreshore where activation was by swash alone. Swash flows were skewed offshore, but egg transport was predominantly onshore from bore collapse. Egg release was greatest in early rising tide, making this the most productive time for shorebird foraging. Release of eggs from mid-foreshore may be dominated by wave action during high energy days with little spawning or by bioturbation by horseshoe crabs during intensive spawning. Activation depths in the swash were low, indicating that eggs buried on the upper foreshore can reach later stages of development after spawning ceases.

Population Structure and Genetic Diversity of Spawning Atlantic Horseshoe Crabs (Limulus polyphemus) along the South Carolina Coast

The Atlantic horseshoe crab (Limulus polyphemus), a long-lived marine arthropod, is a prime example of a common-pool resource across its native range from Maine to the Yucatan Peninsula, being both a vital ecological food source and a valuable commercial commodity. Genetic differentiation and diversity of horseshoe crabs have been assessed on a regional level, but no fine-scale examinations of these measures had been conducted in South Carolina. A suite of 12 microsatellite loci was used to evaluate the population structure and genetic diversity of spawning adult horseshoe crabs (n = 740) collected from seven sites along the coast of South Carolina from the Georgia/South Carolina border to Bulls Bay. No significant genetic differentiation was detected among the sites, indicating that some degree of gene flow is occurring along the South Carolina coast and that spawning adult horseshoe crabs in South Carolina form a single, unified stock. The horseshoe crab stock in South Carolina contains moderately high levels of genetic diversity (Ho = 0.775,He = 0.778) and fairly low levels of inbreeding (FIS = 0.005). The effective population size is on the order of several thousand individuals and within the values recommended to maintain evolutionary potential. The information generated here will be useful for management purposes to aid in the regulation and preservation of the horseshoe crab stock and fishery in South Carolina.

Insight into intraspecific niche divergence and conservatism in American horseshoe crabs (Limulus polyphemus)

Limulus polyphemus is the sole extant representative of horseshoe crabs distributed along the Atlantic coast, and is threatened by human activities. Conservation priority to safeguard its habitat usually treats L. polyphemus as a whole, without consideration of different intraspecific ecological requirements. We seek to identify intraspecific ecological niche divergence or conservatism in L. polyphemus. In this study, ocean and coastal environmental conditions occupied by the six distinct metapopulations of L. polyphemus were collected, and compared using correlative approaches, niche overlaps among these six metapopulations were compared with geographical and genetic distances, the realized niche divergence/conservatism among these six metapopulations were tested, and ecological niche models were fitted for the six metapopulations separately and for L. polyphemus as a whole using state-of-the-art techniques. We found that mean sea surface temperature, tidal regime, chlorophyll a concentration, distance to shore and depth of seafloor were limiting factors in the geographic distribution of L. polyphemus. In addition to the former hypothesis that tidal amplitude and type account for the absence of L. polyphemus in the western Gulf of Mexico, our niche model predictions suggest that the narrow and high-depth seafloor in the western Gulf of Mexico also explain the absence. We found that realized niche overlaps among six metapopulations were not correlated with their geographic and genetic distances; rather the six metapopulations each occupied a unique combination of habitat conditions, showing different responses to environmental factors, although these divergences might be due a background effect. Results here showed that the distribution of one metapopulation of L. polyphemus cannot be inferred from changes in distribution of another, highlighting the inadequacy of one-size-fits-all conservation actions. We recommend that conservation priority assignment by IUCN and Natureserve for L. polyphemus would better consider the six metapopulations separately rather than L. polyphemus as a whole, which will deliver more useful information on guiding regional conservation efforts.