Mark L. Botton

A contribution to the population biology of horseshoe crabs,Limulus polyphemus (L.), in Delaware Bay

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.

Beach erosion and geochemical factors: Influence on spawning success of horseshoe crabs (Limulus polyphemus) in Delaware Bay

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.

Biology and conservation of horseshoe crabs

Biology.- Limits on the Global Distribution of Horseshoe Crabs (Limulacea): Lessons Learned from Two Lifetimes of Observations: Asia and America.- Horseshoe Crabs - An Ancient Ancestry Revealed.- The Ecological Importance of Horseshoe Crabs in Estuarine and Coastal Communities: A Review and Speculative Summary.- Relationships Between Sandpipers and Horseshoe Crab in Delaware Bay: A Synthesis.- Horseshoe Crabs, Their Eco-biological Status Along the Northeast Coast of India and the Necessity for Ecological Conservation.- American Horseshoe Crabs, Limulus polyphemus, in Mexico: Open Possibilities.- Basic Habitat Requirements of the Extant Species of Horseshoe Crabs (Limulacea).- The Relationship Between Small- and Large-Scale Movements of Horseshoe Crabs in the Great Bay Estuary and Limulus Behavior in the Laboratory.- Ecology of Horseshoe Crabs in Microtidal Lagoons.- Phylogeography, Demographic History, and Reserves Network of Horseshoe Crab, Tachypleus tridentatus, in the South and East China Seaboards.- Genetic Structure of Japanese Populations of Tachypleus tridentatus by mtDNA AT-Rich Region Sequence Analysis.- Reproductive Competition and Sexual Selection in Horseshoe Crabs.- Vision in Horseshoe Crabs.- Sperm Attachment on the Egg of Malaysian King Crab, Carcinoscorpius rotundicauda.- Distribution and Development of Limulus Egg Clusters on Intertidal Beaches in Delaware Bay.- Comparisons in Prosomal Width and Body Weight Among Early Instar Stages of Malaysian Horseshoe Crabs, Carcinoscorpius rotundicauda and Tachypleus gigas in the Laboratory.- Emergence Behavior of Juvenile Tachypleus tridentatus Under Simulated Tidal Conditions in the Laboratory and at Two Different Sediment Temperatures.- Distribution of Juvenile Horseshoe Crabs in Subtidal Habitats of Delaware Bay Using a Suction-Dredge Sampling Device.- Conservation.- History of Horseshoe Crab Harvest on Delaware Bay.- Biomedical Applications of Limulus Amebocyte Lysate.- The Effect of Hemolymph Extraction Volume and Handling Stress on Horseshoe Crab Mortality.- Horseshoe Crabs in Hong Kong: Current Population Status and Human Exploitation.- Comparative Status and Assessment of Limulus polyphemus with Emphasis on the New England and Delaware Bay Populations.- An Integrative Approach to Horseshoe Crab Multiple Use and Sustainability.- Strategies to Conserve and Enhance Sandy Barrier Habitat for Horseshoe Crabs (Limulus polyphemus) on Developed Shorelines in Delaware Bay, United States.- Conservation Program for the Asian Horseshoe Crab Tachypleus tridentatus in Taiwan: Characterizing the Microhabitat of Nursery Grounds and Restoring Spawning Grounds.- The Effects of Water Quality on Horseshoe Crab Embryos and Larvae.- Heavy Metal Concentration in Horseshoe Crab (Carcinoscorpius rotundicauda and Tachypleus gigas) Eggs from Malaysian Coastline.- A Discussion of Horseshoe Crab Management in Five Countries: Taiwan, India, China, United States, and Mexico.- Clinical Evaluation, Common Diseases, and Veterinary Care of the Horseshoe Crab, Limulus polyphemus.- Aquaculture Methods and Early Growth of Juvenile Horseshoe Crabs (Limulus polyphemus).- Larval Culture of Tachypleus gigas and Its Molting Behavior Under Laboratory Conditions.- Diet Composition of Juvenile Horseshoe Crabs: Implications for Growth and Survival of Natural and Cultured Stocks.- Effect of Sediment Type on Growth and Survival of Juvenile Horseshoe Crabs (Tachypleus tridentatus).- The Conservation Network of Horseshoe Crab Tachypleus tridentatus in Taiwan.- The History of Horseshoe Crab Research and Conservation in Japan.- Public Awareness and Community-Based Conservation for the Horseshoe Crab at Saikai National Park in Nagasaki Prefecture, Japan.- Public Participation in Studies on Horseshoe Crab Populations.- Green Eggs and Sand: A Collaborative Effort of Scientists, Teachers, Resource Managers, and Stakeholders in Educating About Limulus polyphemus.- Community Building: An Integrated Approach to Horseshoe Crab Conservation.

Reproductive risk: high mortality associated with spawning by horseshoe crabs (Limulus polyphemus) in Delaware Bay, USA

It has been presumed that intertidal spawning by Limulus polyphemus minimizes the loss of egges to subtidal predators; however, this strategy involves considerable risks. Massive beach strandings of adults accompany seasonal spawning migrations of crabs along Cape May in Delaware Bay, (USA). At least 190000 horseshoe crabs, approximating 10% of the adult population, died from beach stranding along the New Jersey shore of Delaware Bay during the 1986 (May to June) spawning season. Abnormalities of the telson (which is used in righting behavior) were significantly more common among stranded crabs than among individuals actively spawning on the intertidal beach. The number of stranded crabs per day was not correlated with tidal height or environmental variables (wind speed, wave height) which characterized the conditions at spawning. A complex suite of factors, including the size of the available spawning population, tidal and weather conditions, and beach slope, influence the number stranded during the breeding season. Horseshoe crab stranding results in a large loss of gravid females from the population, and may represent a major input of organic matter to intertidal sandy beaches in certain regions of Delaware Bay.

Abundance and dispersal potential of horseshoe crab (Limulus polyphemus) larvae in the Delaware estuary

The distribution, abundance, and dispersal patterns of horseshoe crab (Limulus polyphemus) trilobite larvae were determined from 671 plankton tows taken near a spawning beach in lower Delaware Bay, New Jersey, in 1998 and 1999. In both years, peaks in larval abundance occurred during periods of rough surf (>30 cm wave heights). Planktonic larvae were significantly more abundant nocturnally than during the day, but there was no evidence of a lunar component to larval abundance. Larvae were strongly concentrated inshore; trilobites were 10–100 times more abundant in the immediate vicinity of the shoreline than they were 100–200 m offshore. The strong tendency ofLimulus larvae to remain close to the beach suggests that their capability for long-range dispersal between estuaries is extremely limited. We suggest that limited larval dispersal potential may help explain previously observed patterns of genetic variation among the Mid-Atlantic horseshoe crab populations.

Size dimorphism and the mating system in horseshoe crabs Limulus polyphemis L.

Abstract This study asks whether male-male competition and female choice are important in the mating system of sexually dimorphic, adult horseshoe crabs (Chelicerata: Limulidae, Limulus polyphemus L.). Amplexed pairs approaching a beach for spawning are generally accompanied by a number of unattached, sexually mature suitor males. For the amplexed pair, the average pairwise ratio of male to female carapace width remains constant at 0·78 throughout the spawning season. Morphological characters that relate to mating in amplexed males were compared with unpaired males. Mated and suitor male crabs could not be distinguished on the basis of morphology or body size. Male clasper dimensions, relative to the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the ability of males to amplex with females. Although multiple males participate in the mating process, via the formation of spawning clusters of up to 15 males to one female, there is no evidence of successful male-male displacement. Size-assortative mating does not occur, and large females do not attract either larger males or larger numbers of suitor males in a cluster. A simulation model, using data from 1800 individual horseshoe crabs, generated mated pairs based on the assumption that amplexus was a completely random process with respect to prosoma width. The frequency distribution of actual mated pairs was indistinguishable from the output of the simulation model. The role of size dimorphism and the male-biased sex ratio in the mating system of horseshoe crabs are discussed.

The Ecological Importance of Horseshoe Crabs in Estuarine and Coastal Communities: A Review and Speculative Summary

Beyond their commercial importance for LAL and bait, and their status as a living fossil, it is often asserted that horseshoe crabs play a vital role in the ecology of estuarine and coastal communities. How would the various ecological relationships involving horseshoe crabs be affected if these animals were no longer abundant? Attempts to understand and generalize the ecological importance of horseshoe crabs are hampered by several constraints. We know relatively little about the ecology of juvenile horseshoe crabs. Most ecological studies involving adult Limulus polyphemus have been conducted at only a few locations, while much less is known about the three Indo-Pacific species. Furthermore, we are attempting to infer the ecological importance of a group of animals whose numbers may have already declined significantly (the so-called “shifting baseline syndrome”). Horseshoe crab shells serve as substrate for a large number of epibionts, such as barnacles and slipper limpets, but the relationships between these epibionts and horseshoe crabs appear to be facultative, rather than obligatory. Horseshoe crabs are dietary generalists, and adult crabs are ecologically important bivalve predators in some locations. The most notable predator–prey relationship involving horseshoe crabs is the migratory shorebird–horseshoe crab egg interaction in Delaware Bay. After hatching, the first and second instars are eaten by surf zone fishes, hermit crabs, and other predators. Virtually nothing is known about predator–prey relationships involving older juveniles, but adult L. polyphemus are important as food for the endangered loggerhead turtle, especially in the mid-Atlantic region.

Horseshoe crabs (Limulus polyphemus) in an urban estuary (Jamaica Bay, New York) and the potential for ecological restoration

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.

High tolerance to tributyltin in embryos and larvae of the horseshoe crab,Limulus polyphemus

The effects of acute and chronic exposure to tributyltin (TBT) were examined in bioassays using horseshoe crab (Limulus polyphemus) embryos and “trilobite” larvae. Larvae had>95% survival after 24-h exposure to nominal concentrations of 1–500 μg l−1 TBT. Survival was also high following 48-h and 72-h exposure to ≤100 μg l−1 TBT; >50% mortality was seen only after 48-h and 72-h exposure to 500 μg l−1 TBT. Estimated median lethal concentrations (LC50) were >1000 μg l−1, 742 μg l−1, and 594 μg l−1 for 24-h, 48-h, and 72-h exposure, respectively. Much higher toxicity LC50=42 μg l−1) was seen following chronic exposure of larvae to TBT. Acute exposure to TBT significantly increased the time required by larvae to molt into the first-tailed stage. LC50 for horseshoe crab embryos exposed to TBT were 44 μg l−1, 20 μg l−1, and 14 μg l−1 for 24, 48, and 72 h acute exposure, indicating that this earlier developmental stage was about 30–40 fold more susceptible to TBT than larvae. Horseshoe crabs are highly tolerant of TBT in comparison to early developmental stages of other marine arthropods. The ability of horseshoe crab embryos and larvae to survive in the presence of organotin pollution suggests the possibility of bioaccumulation and movement into the estuarine food chain via shorebirds, gulls, and fish.

Eye and clasper damage influence male mating tactics in the horseshoe crab, Limulus polyphemus

In the horseshoe crab mating system, mated pairs are frequently accompanied by unattached satellite males as they spawn on intertidal beaches. Previous studies have shown that males locate females visually using their lateral (compound) eyes, and that attached (mated) males generally have less heavily worn or damaged carapaces than unattached males. The purpose of this study was to investigate the influences of lateral eye condition and clasper abnormalities on male mating tactics. Sexually mature males had two kinds of eye damage: deterioration caused by disease, and overgrowth by sessile invertebrates, such as bryozoans, mussels, and tube-building polychaetes. The lateral eyes of attached males had significantly less decay than unattached males. On the other hand, coverage of the lateral eyes by encrusting invertebrates was more extensive among attached than unattached males. Although overgrowth did not appear to impair a male’s ability to pair with a female as severely as eye decay, it is conceivable that amplexus may have occurred before epibiont coverage was sufficient to obscure vision. Male crabs that were experimentally “blindfolded” by painting their lateral eyes with black nail polish were less likely to reattach to a female than controls. Appendage injuries were more frequent among unattached males than among attached males; in particular, 6.4% of unattached males but 0.0% of attached males had damaged claspers (the modified first legs required for amplexus). Unattached males in the population were “older,” as judged by the degree of carapace wear, than attached males. Severe visual impairment and/or clasper damage may explain the reduced pairing success of older male horseshoe crabs, and underlie their choice of the alternative satellite male mating tactics.