Horseshoe Crab Populations Research Articles

Wisdom of Crowds reveals decline of Asian horseshoe crabs in Beibu Gulf, China

AbstractPopulation decline among Asian horseshoe crabs in Asia is increasingly reported, but knowledge of their population and ecological status in China is limited. We conducted community interviews in 30 fishing villages around Beibu Gulf in Guangxi, China, to collect distribution information about the potential spawning/nursery grounds of Tachypleus tridentatus and Carcinoscorpius rotundicauda, and any imminent threats to their populations. Based on the results from 400 respondents we identified 45 potential spawning/nursery grounds distributed widely along the shores of Beibu Gulf. We visited 10 of these sites and verified the presence of juvenile horseshoe crabs by field surveys. Nearly all respondents reported an overall depletion in horseshoe crab populations from these 45 sites, which they attributed mainly to unsustainable fishing practices. Respondents who reported having seen horseshoe crab mating pairs on shores were mostly older people, which may suggest a considerable reduction in horseshoe crabs coming to the shores to spawn in recent years. The mean daily harvest of adult T. tridentatus offshore, as indicated by fishers, has declined from c. 50–1,000 in the 1990s to 0–30 individuals during 2011–2016. Our Wisdom of Crowds approach, supported by confirmatory field surveys, is a cost-effective method for assessing the population status of horseshoe crabs, and the level of threat they face. Similar approaches with other species are likely to be particularly valuable in the Asia–Pacific region, where well-structured population monitoring is largely unaffordable.

Phylogeny and Genetic Variation within Population of <i>Tachypleus gigas</i> (Muller, 1785)

Isolated population of Indian horseshoe crabs, Tachypleus gigas plays an important role in the ecology of several marine organisms, and is being scrutinized for its abundance and morphology by many researchers. However, limited information is available about its genetic variability and evolution. Samples of horseshoe crab were collected from the east coasts of India and analysed for their phylogenetic relationship, genetic variability and structure within population based on the cytochrome oxidase I (COI) gene sequence. Analysis of molecular variance revealed two groups with significant genetic differentiation indices (F ST = 0.544, p < 0.001), and the number of migrants (Nm) was estimated as 0.11 individuals per generation. Maximum likelihood results revealed two distinct clusters, showing that the evolution of the Indian population was genetically diverse forming a separate clade from other Southeast Asian populations, moderately with a low gene flow. Considering the ecological, economic and evolutionary significance of T. gigas and its declining population, there is a pressing need for conservation measures.

Present population and habitat status of potentially threatened Asian horseshoe crabs Tachypleus tridentatus and Carcinoscorpius rotundicauda in Hong Kong: a proposal for marine protected areas

While there are persistent calls for developing more marine protected areas (MPAs) for Asian horseshoe crab conservation in response to population declines in Asia–Pacific region, most existing horseshoe crab MPAs were designated without prior comprehensive population assessment and habitat characterization. This study collected biological and habitat information in Hong Kong, in order to identify priority sites for conservation and management of Asian horseshoe crab populations. The territory-wide surveys at 18 spawning/nursery beaches displayed a persistently low mean juvenile density from 0.16 to 2.19 ind/100 m2 in 2012 and 0.08 to 1.41 ind/100 m2 in 2014. These density data were within the same range of that in 2002 and 2007 (0.10–1.97 ind/100 m2), apart from a low survey return of 0.08–0.31 ind/100 m2 in 2004. The current population of juvenile T. tridentatus and C. rotundicauda in Hong Kong is estimated about 2100–4300 and 2400–3000 individuals, respectively. From the 2012–2014 data, no new recruitments of 1st–3rd instar juveniles were found, as the shores were occupied mostly by 6th–9th instar juveniles of prosomal width between 23 and 45 mm, in which considerably high mortality rates were noted. The present findings of existing small and discrete juvenile populations, coupled with relatively few recruits, suggest that the status of juvenile horseshoe crabs in Hong Kong is fragile and vulnerable to local extirpation, especially if no urgent protection measures are implemented. Based on available population data and habitat characteristics, three MPAs, ranging from 5 to 7 km2, are proposed, so as to conserve over 60 and 70 % of the existing juvenile populations of T. tridentatus and C. rotundicauda, respectively. The approach adopted in this study may serve as a case study for proposing other horseshoe crab MPAs in Asia, where increasing human disturbances and over-harvest are imminent.

Influence of Environmental Factors on Spawning of the American Horseshoe Crab (Limulus polyphemus) in the Great Bay Estuary, New Hampshire, USA

The Great Bay Estuary, New Hampshire, USA is near the northern distribution limit of the American horseshoe crab (Limulus polyphemus). This estuary has few ideal beaches for spawning, yet it supports a modest population of horseshoe crabs. There is no organized monitoring program in the Great Bay Estuary, so it is unclear when and where spawning occurs. In this 2-year study (May through June, 2012 and 2013), >5,000 adult horseshoe crabs were counted at four sites in the estuary. The greatest densities of horseshoe crabs were observed at Great Bay sites in the upper, warmer reaches of the estuary. Peaks of spawning activity were not strongly correlated with the times of the new or full moons, and similar numbers of horseshoe crabs were observed mating during daytime and nighttime high tides. While many environmental factors are likely to influence the temporal and spatial patterns of spawning in this estuary, temperature appears to have the most profound impact.

Population Health of Horseshoe Crabs (Limulus polyphemus) at Harvested and Unharvested Beaches in South Carolina

The horseshoe crab (Limulus polyphemus) is a valuable species environmentally, commercially, and biomedically. The population genetic health of horseshoe crab populations in South Carolina has been relatively unstudied. This project is a part of a greater ongoing study examining the genetic health of horseshoe crab populations along the southern coast of South Carolina. A suite of 11 microsatellites was used to analyze the Coffin Point and Harbor Island horseshoe crab spawning sites. Genetic structure was analyzed along with various genetic health metric tests. Results indicated the two sites represent a single population (Jost's D=0.0019, p=0.2679; FST=0.0058, p=0.2385). The population analyses showed moderately high diversity (HE= 0.762), no inbreeding (FIS= ‐0.0057), and a high Ne value based on the lower 95% confidence interval obtained from linkage disequilibrium‐based estimates of Ne (Ne &gt; 2240.7). These are all positive indicators for the adaptive potential of the population. The results of this study serve as an important baseline for future conservation and management of horseshoe crab populations.

Metals in horseshoe crab eggs from Delaware Bay, USA: temporal patterns from 1993 to 2012.

The health of horseshoe crab (Limulus polyphemus) eggs is important not only to maintain horseshoe crab populations, but because they are a resource for higher trophic levels, such as fish and shorebirds. We examined the concentrations of arsenic, cadmium, chromium, lead, manganese, mercury, and selenium in the eggs of horseshoe crabs from Delaware Bay (between New Jersey and Delaware, USA) in 1993, 1994, 1995, 1999, 2000, and 2012 to determine if there were significant temporal changes and if levels appear to pose a health risk to the crabs themselves, or to predators that consume them. All metal levels declined in horseshoe crab eggs between 1994 and 2012, although the declines were much less consistent for lead and chromium than that for mercury and cadmium. Levels of contaminants found in these eggs are well below those known to cause adverse effects in the crabs themselves or to organisms that consume them, such as migrating shorebirds.

Predicting use of habitat patches by spawning horseshoe crabs (Limulus polyphemus) along a complex coastline with field surveys and geospatial analyses

Abstract This research described coastal habitat characteristics at a landscape scale and followed a geospatial modelling approach to predict the probability of habitat use by the population of Atlantic horseshoe crabs (Limulus polyphemus) in Long Island Sound (LIS, USA). This approach was developed to aid in decision‐making regarding management of horseshoe crab spawning locations. Geospatial data layers were created within which beach slope, wave exposure, surface substrate type, and distance from offshore aggregations of crabs (i.e. hotspots) were summarized for beaches in the western, central, and eastern regions of the Connecticut coast of LIS. Spawning abundances derived from field surveys conducted in May and June of 2009 and 2010 were used with remotely‐sensed habitat characteristics to develop a resource selection function from a candidate model set based on polytomous logistic regression. A single best model (Akaike weight = 0.97) for predicting the probability of habitat use by spawning horseshoe crabs was selected using an information‐theoretic approach. The parameter estimates predicted a greater probability of habitat use with increasing slope, decreasing wave exposure, and decreasing distance from offshore hotspots. Small ‘pocket’ beaches surrounded by rocky headlands, marshes, and developed areas are the typical habitat available to support horseshoe crab spawning in LIS. As spawning densities in LIS are relatively small and suitable spawning habitat is fragmented, geospatial methods are especially useful for identifying high‐use sites and maximizing the conservation value of management actions. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Genetic Diversity of Horseshoe Crab (Tachypleus gigas) in Malaysia Revealed using Microsatellite Markers

The genetic structure of horseshoe crab (Tachypleus gigas) populations were examined using 18 microsatellite markers. Samples were collected from 2 stations of east coast of Peninsular Malaysia and one station from East Malaysia. All the observed allelic frequency showed no significant variation between sampling stations (p>0.05). Mean allelic richness (Ar) was greater in Sarawak population (4.83) followed by Pahang (4.24) and Terengganu (3.57) samples with the overall mean allelic richness of 4.21. Mean inbreeding coefficient (FIS) value was higher in Sarawak population (0.37) followed by Terengganu (0.31) and Pahang population (0.17). The genetic differentiation (FST value) and genetic distance between geographically closer populations was smaller compared to geographically isolated populations. Due to the dwindling population size of horseshoe crabs in Malaysian coast line, present data provides new information in assisting proper management and conservation of this living fossil.

The consequences of climate change at an avian influenza 'hotspot'.

Avian influenza viruses (AIVs) pose significant danger to human health. A key step in managing this threat is understanding the maintenance of AIVs in wild birds, their natural reservoir. Ruddy turnstones (Arenaria interpres) are an atypical bird species in this regard, annually experiencing high AIV prevalence in only one location-Delaware Bay, USA, during their spring migration. While there, they congregate on beaches, attracted by the super-abundance of horseshoe crab eggs. A relationship between ruddy turnstone and horseshoe crab (Limulus polyphemus) population sizes has been established, with a declining horseshoe crab population linked to a corresponding drop in ruddy turnstone population sizes. The effect of this interaction on AIV prevalence in ruddy turnstones has also been addressed. Here, we employ a transmission model to investigate how the interaction between these two species is likely to be altered by climate change. We explore the consequences of this modified interaction on both ruddy turnstone population size and AIV prevalence and show that, if climate change leads to a large enough mismatch in species phenology, AIV prevalence in ruddy turnstones will increase even as their population size decreases.

Three decades of horseshoe crab rearing: a review of conditions for captive growth and survival

AbstractThreats to wild horseshoe crab populations and growing interest in their use for research, education and biomedical applications have prompted demand for improved techniques to rear and maintain crabs in captivity. Although numerous laboratory studies have been conducted to determine growth and survival of horseshoe crabs under various conditions, these data have not been compiled and summarized to inform culture practices. We surveyed the literature and analysed the range of available techniques to identify and define a consistent set of conditions for maximum growth and survival of horseshoe crabs in culture. We considered three age classes; embryo, juvenile and adult, and included all extant species (Limulus polyphemus, Carcinoscorpius rotundicauda, Tachypleus gigas and Tachypleus tridentatus). We discovered relatively few published studies that clearly related husbandry conditions to growth and survival. Comparison among studies was complicated by inconsistent data collection and reporting techniques. Most published sources reported data for younger age classes, and more studies considered L. polyphemus than Asian species. The most commonly reported variables (temperature, salinity, enclosure maintenance and diet composition) showed size dependent and in some cases species‐specific effects on growth and survival that will be important in guiding culture efforts. We suggest that future studies give additional consideration to substrate type, water flow, dissolved oxygen concentrations, diet quality and the quantity and frequency of feeding. If laboratory‐reared stocks are to be used for propagation and restoration activities, future studies will benefit from closing these data gaps and promoting international data sharing.

Isolation by the 5′ anchored PCR technique and characterization of eighteen microsatellite loci in horseshoe crab (Tachypleus gigas)

The horseshoe crab, a well-known ‘living fossil’, is represented by two species (Tachypleus gigas and Carcinoscorpius rotundicauda) in peninsular Malaysia. One of these, T. gigas inhabits shallow marine waters and migrates to intertidal sandy beaches for breeding during high tides at every full and new moon throughout the year (Hajeb et al. 2009). This species is being used as bait by local fishermen to catch crabs or shrimps by netting or cage fishing methods. Due to its low commercial value in comparison to other marine economic species such as fishes, crabs and shrimps, most of the animals caught in fishing nets or cages are released immediately back to sea by fishermen or left to die in the sun. In certain states of Malaysia, such as Kelantan, Penang and Johore, adult female horseshoe crabs are sold to local seafood restaurants and in fish markets due to consumption of their eggs by the local people. Horseshoe crab populations are rapidly declining around the world due to various reasons such as water pollution, loss of their living and spawning habitats and human exploitation of adult horseshoe crabs (Li et al. 2009; Shin et al. 2009). Assessing the genetic variability of horseshoe crab is important as it has direct benefit for the conservation and management of wild populations of the species. However, there is no information available on the population genetic structure and diversity of T. gigas in Malaysia. To address this research need, we have isolated and developed the first set of polymorphic microsatellite markers for T. gigas by using the 5′ anchored PCR technique. A total of 130 individuals of horseshoe crabs were collected from five locations along the coastal areas of peninsular Malaysia, namely Port Dickson (Negeri Sembilan), Pantai Bersih (Penang), Sungai Muar (Johor), Pantai Balok

A Long-Term Study of Spawning Activity in a Florida Gulf Coast Population of Horseshoe Crabs (Limulus polyphemus)

Populations of the American horseshoe crab (Limulus polyphemus) differ in broad areas of their biology. We observed a non-harvested, marked Florida Gulf coast population during their spring spawning (March–May) in 11 years across a 17-year period (1992–2009). Long-term changes occurred in the number of spawning pairs: the population was stable from 1992 to 2000 but increased markedly after 2000. Short-term variation in numbers of spawning pairs, unpaired females, unpaired males, and operational sex ratios was explained by changes over the season and during each week of spring tides and by differences in actual (not predicted) maximum high tide height. Wind direction strongly affected tidal inundation and the number of spawning horseshoe crabs. Tagging individuals revealed that females returned to the nesting beach less often than males and most females were re-sighted only within 1 week of spring tides. No animals were seen across more than 6 years. Implications for management are discussed.

AFLP analysis of genetic variation among three natural populations of horseshoe crab Tachypleus tridentatus along Chinese coast

The AFLP (amplified fragment length polymorphism) technique was used to analyze and compare the genetic diversity of Tachypleus tridentatus from three south-eastern coastal sites of China (Pingtan, Hong Kong and Beihai). Eight pairs of primers generated 361 loci, including 285 polymorphic loci. The ratio of polymorphic loci was 96.97%. Nei’s genetic diversity index was 0.420 8 and the Shannon information index was 0.607 5, both of which were higher than that reported for many other arthropods. These results show that the genetic diversity detected was mainly caused by individual differences within a population. Genetic distance showed that the rational division of the three geographic populations of T. tridentatus along the south-eastern coast of China was not significant, in which the genetic distance was not proportional to the geographic distance. All three horseshoe crab populations may belong to a large group, and had a high degree of genetic similarity. The high level of genetic diversity obtained from the present AFLP analysis may be due to the large effective population size of the species in Chinese waters.

Allozyme polymorphisms in horseshoe crabs, Carcinoscorpius rotundicauda, collected from polluted and unpolluted intertidal areas in Peninsular Malaysia

It has been widely reported that allozyme frequency variation is a potential indicator of heavy metal-induced impacts in aquatic populations. In the present study, wild populations of horseshoe crab (Carcinoscorpius rotundicauda) were collected from contaminated and uncontaminated sites of Peninsular Malaysia. By adopting horizontal starch gel electrophoresis, seven enzyme systems were used to study allozyme polymorphisms. Nine polymorphic loci were observed in C. rotundicauda. The relationships of allozyme variations with the concentrations of Cd, Cu, Ni, and Zn in sediments and in muscle tissues of horseshoe crabs were determined. Based on genetic distance, the lower mean value of Nei's D (0.017) indicated that both of the contaminated populations of Kg. Pasir Puteh and Kuala Juru were very closely related when compared to the relatively uncontaminated Pantai Lido population. Higher heterozygosities were shown by the contaminated populations when compared to the uncontaminated population. Different allelic frequencies could be observed for the aldolase (ALD; E.C. 2.7.5.1) locus between the contaminated and uncontaminated populations of C. rotundicauda. The dendrogram of genetic relationships of the three populations of C. rotundicauda showed the same clustering pattern as the dendrograms are based on heavy metals in the sediments and in the horseshoe crabs' abdominal muscles. From the F statistics, the present study showed that the three populations of horseshoe crabs were considered to have undergone moderate genetic differentiation with a mean F (ST) value of 0.092 .The current results suggest that allozyme polymorphism in horseshoe crabs is a potential biomonitoring tool for metal contamination, although further validation is required.

Age and sex specific timing, frequency, and spatial distribution of horseshoe crab spawning in Delaware Bay: Insights from a large-scale radio telemetry array

Abstract To study horseshoe crab Limulus polyphemus spawning behavior and migration over a large-spatial extent (&gt;100 km), we arrayed fixed station radio receivers throughout Delaware Bay and deployed radio transmitters and archival tags on adult horseshoe crabs prior to their spawning season. We tagged and released 160 females and 60 males in 2004 and 217 females in 2005. The array covered approximately 140 km of shoreline. Recapture rates were &gt;70% with multi-year recaptures. We categorized adult age by carapace wear. Older females tended to spawn earlier in the season and more frequently than young females, but those tendencies were more apparent in 2004 when spawning overall occurred earlier than in 2005 when spawning was delayed possibly due to decreased water temperatures. Timing of initial spawning within a year was correlated with water temperature. After adjusting for day of first spring tide, the day of first spawning was 4 days earlier for every 1 degree (̊C) rise in mean daily water temperature in May. Seventy nine % of spawning occurred during nighttime high tides. Fifty five % of spawning occurred within 3 d of a spring tide, which was slightly higher than the 47% expected if spawning was uniformly distributed regardless of tidal cycle. Within the same spawning season, males and females were observed spawning or intertidally resting at more than one beach separated by &gt;5 km. Between years, most (77%) did not return to spawn at the same beach. Probability of stranding was strongly age dependent for males and females with older adults experiencing higher stranding rates. Horseshoe crabs staging in the shallow waters east of the channel spawned exclusively along the eastern (NJ) shoreline, but those staging west of the channel spawned throughout the bay. Overall, several insights emerged from the use of radio telemetry, which advances our understanding of horseshoe crab ecology and will be useful in conserving the Delaware Bay horseshoe crab population and habitats.

Effects of Horseshoe Crab Harvest in Delaware Bay on Red Knots: Are Harvest Restrictions Working?

Each May, red knots (Calidris canutus rufa) congregate in Delaware Bay during their northward migration to feed on horseshoe crab eggs (Limulus polyphemus) and refuel for breeding in the Arctic. During the 1990s, the Delaware Bay harvest of horseshoe crabs for bait increased 10-fold, leading to a more than 90% decline in the availability of their eggs for knots. The proportion of knots achieving weights of more than 180 grams by 26–28 May, their main departure period, dropped from 0.6–0.8 to 0.14–0.4 over 1997–2007. During the same period, the red knot population stopping in Delaware Bay declined by more than 75%, in part because the annual survival rate of adult knots wintering in Tierra del Fuego declined. Despite restrictions, the 2007 horseshoe crab harvest was still greater than the 1990 harvest, and no recovery of knots was detectable. We propose an adaptive management strategy with recovery goals and annual monitoring that, if adopted, will both allow red knot and horseshoe crab populations to reco...

Seasonal movement and resource-use patterns of resident horseshoe crab (Limulus polyphemus) populations in a Maine, USA Estuary

Knowledge of resource-use and movement patterns is a missing component in the development of horseshoe crab (Limulus polyphemus) management strategies. Available evidence indicates the potential for a variety of possible migratory behaviors, but the lack of high-resolution, spatial-temporal data has hindered development of a year-round profile of ranging behavior. This need was addressed in the present study by using acoustic telemetry to track the movements of adult horseshoe crabs in two subembayments (Egypt and Hog Bays) of the Taunton Bay Estuary, Maine, from June 2003 to June 2005. Estimated mean total home range sizes were 64.1 and 61.4 ha for breeding crabs tagged in Egypt and Hog Bays, respectively. We observed no horseshoe crab dispersal to areas outside of the subembayments where they were tagged, so no mixing was observed between Egypt and Hog Bay individuals despite a < 4-km separation. Observed shifts in movement patterns, resource use (subtidal versus intertidal), and vagility facilitated a profile of seasonally partitioned horseshoe crab activity, which included late April to early May post-wintering, June–July breeding, August–September pre-wintering, and October–April wintering, where space usage represented about 10% of the mean total home range size. The apparent isolation of these resident populations implies a heightened vulnerability to overexploitation and large-scale habitat alteration that might be more easily sustained by larger, more vagile populations. This work underscores the need to apply horseshoe crab conservation, research, and management efforts at scales that are appropriate to the ranging patterns of crabs, which first requires application of high-resolution methods to identify those patterns.

Historical Diet Analysis of Loggerhead Sea Turtles (Caretta Caretta) in Virginia

Digestive tract contents were examined from 297 Loggerhead Sea Turtles (Caretta caretta) in Virginia during 1983–2002. Analyses revealed a shift from predominantly Atlantic Horseshoe Crab (Limulus polyphemus) during the early to mid-1980s to predominantly Common Blue Crab (Callinectes sapidus) during the late 1980s and early 1990s. Diet in the mid-1990s and in 2000–2002 was dominated by finfish, particularly Atlantic Menhaden (Brevoortia tyrannus) and Atlantic Croaker (Micropogonias undulatus). These diet shifts suggest that horseshoe crab and blue crab population declines have caused loggerheads to forage in nets or on discarded fishery bycatch.

Effect of horseshoe crab spawning density on nest disturbance and exhumation of eggs: A simulation study

Because the Delaware Bay horseshoe crab (Limulus polyphemus) population is managed to provide for dependent species, such as migratory shorebirds, there is a need to understand the process of egg exhumation and to predict eggs available to foraging shorebirds. A simple spatial model was used to simulate horseshoe crab spawning that would occur on a typical Delaware Bay beach during spring tide cycles to quantify density-dependent nest disturbance. At least 20% of nests and eggs were disturbed for levels of spawning greater than one third of the average density in Delaware Bay during 2004. Nest disturbance increased approximately linearly as spawning density increased from one half to twice the 2004 level. As spawning density increased further, the percentage of eggs that were disturbed reached an asymptote of 70% for densities up to 10 times the density in 2004. Nest disturbance was heaviest in the mid beach zone. Nest disturbance precedes entrainment and begins the process of exhumation of eggs to surface sediments. Model predictions were combined with observations from egg surveys to estimate a snap-shot exhumation rate of 5–9% of disturbed eggs. Because an unknown quantity of eggs were exhumed and removed from the beach prior to the survey, cumulative exhumation rate was likely to have been higher than the snap-shot estimate. Because egg exhumation is density-dependent, in addition to managing for a high population size, identification and conservation of beaches where spawning horseshoe crabs concentrate in high densities (i.e., hot spots) are important steps toward providing a reliable food supply for migratory shorebirds.

Fecundity and spawning of the Atlantic horseshoe crab,Limulus polyphemus, in Pleasant Bay, Cape Cod, Massachusetts, USA

AbstractThis study provided the first comprehensive analysis of Atlantic horseshoe crab (Limulus polyphemus) fecundity.Limulusappear to be a determinate spawner, maturing all eggs for the breeding season before spawning begins. On average, larger females held a larger number of eggs (63,500) than smaller females (14,500). By the end of the breeding season there was an average of 11,600 mature eggs per female left undeposited, regardless of female size. Larger females laid a higher percentage of the eggs they contained. Thus they not only contain more eggs, but are more effective at laying them as well. Size of spawning females ranged from about 185–300 mm prosomal width, with by far the highest concentration in the mid‐size ranges. Although on an individual basis large females carry and lay the greatest number of eggs, mid‐size crabs as a group contributed more to the horseshoe crab population in Pleasant Bay because they were more plentiful (net fecundity was highest for mid‐size crabs). These results have implications for the management of this important species, which is harvested for bait, scientific, and biomedical uses. Incorporation of these results into models and other management tools can help predict growth rates, effects of size‐selective harvest, reproductive value, and stable stage distribution of populations.