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Spawning densities, spawning indices, egg densities, size distributions, and movement patterns of horseshoe crabs (Limulus polyphemus) were quantified for four coastal embayments (Monomoy National Wildlife Refuge, Pleasant Bay, Nauset Estuary, and Cape Cod Bay) on Cape Cod, Massachusetts from 2000 to 2002. Spawning activity was highest from mid May through mid June, but densities varied throughout the Cape Cod region. Average spawning densities (male and female crabs combined), measured using 25-m2 quadrats, were lower than 1 crab 25 m−2, although certain locations had consistently higher densities averaging 2 to 3 crabs 25 m−2 with individual survey densities recorded as high as 17 crabs 25 m−2. Spawning densities during night surveys were either similar or slightly higher than day surveys, except at a few sites within Pleasant Bay. Spawning indices were considerably lower ranging from 0 to 1.3 females 25 m−2 throughout the Cape Cod region. Spawning sex ratios varied from 1∶1.6 to 1∶3.1 (females:males) throughout the region, except within Pleasant Bay where highly male skewed ratios were observed (e.g., 1∶5.8, 3-yr average). Egg densities were low overall (

Adult horseshoe crabs, Limulus polyphemus, were tagged in the Middle Atlantic Bight area, from New York to Virginia on the continental shelf and within bays, to determine their migratory patterns and longevity. Of 30,432 horseshoe crabs that were tagged during the years 1986-2002, 1,122 were recovered alive, and 1,027 were dead. Many of the live recoveries were observed within 30 d (54.4%) and after years (37.53%) with one tagged animal surviving up to 10 yr. In 9 locations from Great Kills Harbor, New York, to Chesapeake Bay, Maryland, the horseshoe crabs return to their release beach within days during the spawning season. Of the 762 (100%) recoveries from crabs released along the Delaware Bay shoreline, 75.07% traveled 0-20 km, 21.0% traveled 20-50 km, 2.36% traveled 50-100 km, and 1.57% traveled over 100 km. Within Delaware Bay, 327 tagged animals (43.6%) had moved away from the release points to other locations, and 59 of these had moved out of the bay onto the continental shelf along the Mid-Atlantic Bight coastline. Horseshoe crabs migrate into Delaware Bay from waters off Ocean City, Maryland, and adjacent coastal bays. In addition to defining the range of the Delaware Bay spawning populations, 2 neighboring populations were identified by the tagging program. In one, animals tagged in southern New York mingled with those in the Sandy Hook, New Jersey area, comprising a population that ranged from Raritan Bay across New York Harbor to Jamaica Bay. The second confirmed that a discrete population existed in northern Chesapeake Bay in the general vicinity of the Annapolis Bay Bridge.

Atlantic horseshoe crabs,Limulus polyphemus, are currently harvested for biomedical, scientific, and bait purposes. In recent years, changes in population abundance and magnitude of harvesting have raised concerns about the status of this resource. We found horseshoe crab harvest in Pleasant Bay, Massachusetts, was selective by sex and size. Biomedical harvest preferred larger individuals and females, the scientific harvest preferred smaller individuals and males, and the bait harvest preferred females. Total 2001 harvest for all purposes accounted for the mortality of ∼1–2% the adult population. Biomedical harvest accounted for the greatest loss of horseshoe crabs from Pleasant Bay, ∼1–1.6% of the total population. Although biomedical harvest had the lowest associated mortality rate (∼10–15%), many more crabs were harvested from Pleasant Bay for biomedical purposes than for other uses. The scientific harvest accounted for the mortality of ∼0.4% of the population, and bait harvest accounted for the smallest mortality at ∼0.06% of the population. Harvest mortality rate was estimated to be lower in Pleasant Bay than in other Cape Cod areas and may be lower than natural mortality in the population. This study is the first qualitative investigation of commercial harvest on horseshoe crab populations and emphasizes that harvest pressures on different populations need to be individually evaluated.

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.

Estuarine species with wide geographic distributions often experience tidal regimes that vary significantly throughout their range. Plasticity in behaviors associated with the tide is expected to enable synchronization with local tides. The American horseshoe crabLimulus polyphemus typically inhabits estuaries and coastal areas with pronounced semi-diurnal tides that play a role in synchronizing the timing of spawning and larval hatching, but also lives in areas that lack significant tides and associated synchronization cues. We investigated the spatial and temporal pattern of adult spawning and larval hatching ofL. polyphemus in a microtidal coastal lagoon (Indian River Lagoon, Florida, USA). Spawning activity and larval abundance were monitored weekly February 1998–August 2000 at sites spanning 100 km of the lagoon. To identify possible synchronization cues for spawning and hatching success, the presence of adult and larvalL. polyphemus were related to environmental and hydrologic variables using logistic regression. The presence of spawning adults varied significantly among the sub-basins of the lagoon, with the highest densities occurring in the Banana River. Large spawning aggregations were not observed and densities never exceeded 6 m−2. Spawning occurred year-round but varied seasonally with episodes of increased mating activity in the early spring. The occurrence of mating pairs was episodic and was not synchronized among sites. Larval densities were low (4 m−3) and larvae were present at only 12 of the 21 sites. Hatching success was decoupled temporally from spawning activity, with peaks in larval abundance occurring approximately 8 wk after peaks in spawning. Larval abundance was associated with periods of high water. Reproductive activity of horseshoe crabs in the lagoon differs significantly from populations inhabiting areas with semi-diurnal and diurnal tides. These differences are likely due to the lack of periodic tidally-related synchronization cues and regular beach inundation.

Concern for the status of horseshoe crab (Limulus polyphemus) has increased as harvest for conch and eel bait has increased and spawning habitat has decreased. In early 1999 a workshop was held at the behest of the Atlantic States Marine Fisheries Commission to design a statistically valid survey of horseshoe crab spawning in Delaware Bay. The survey that resulted was a redesign of a volunteer-based spawning survey that began in 1990, and its network of volunteers was relied on to implement the three-stage sampling design in 1999. During May and June of 1999, 163 participants surveyed during the highest of the daily high tides on 16 beaches (8 on each site of Delaware Bay). During the first half of the spawning season, spawning was associated with lunar phases, but moderated by wave height. Disproportionately more spawning occurred within 3 d of the first new and full moons, and spawning activity (measured by an index of female density) was correlated inversely to the percent of beaches with waves ≥0.3 m. Spawning was heaviest on the Delaware shore around the full moon in May in spite of low waves in New Jersey during the new and full moons in May. Number of beaches sampled was the most important factor in determining the precision of the spawning index and power to detect a decline. Explicit consideration of statistical power has been absent from the current debate on horseshoe crab status and harvest. Those who argue against harvest restrictions because of a lack of statistically significant declines take on a burden to show that the surveys they cite have high statistical power. We show the Delaware Bay spawning survey will achieve high statistical power with sufficient sampling intensity and duration. We recommend that future Delaware Bay spawning surveys sample on 3 d around each new and full moon in May and June and increase the number of beaches to ensure high statistical power to detect trends in baywide spawning activity.

The Delaware Bay contains the world’s largest population of horseshoe crabs, which constitute an ecologically significant component of this estuarine ecosystem. The North Atlantic speciesLimulus polyphemus has an extensive geographical distribution, ranging from New England to the Gulf of Mexico. Recent assessments of the Delaware Bay population based on beach spawning and trawling data have suggested a considerable decrease in the number of adult animals since 1990. Considerable debate has centered on the accuracy of these estimates and their impact on marine fisheries management planning. Compounding this problem is the lack of information concerning the genetic structure of Atlantic horseshoe crab populations. This study assessed patterns of genetic variation within and between the horseshoe crab populations of Delaware Bay and Chesapeake Bay, using both Random Amplification of Polymorphic DNA (RAPD) and DNA sequence analysis of the mitochondrial cytochrome oxidase I gene (COI). We examined 41 animals from Delaware Bay and 14 animals from the eastern shore of Chesapeake Bay. To provide high quality, uncontaminated genomic DNA for RAPD analysis, DNA was isolated from hemocytes by direct cardiac puncture, purified by spin column chromatography, and quantified by agarose gel electrophoresis. RAPD fingerprints revealed a relative paucity of polymorphic fragments, with generally homogeneous banding patterns both within and between populations. DNA sequence analysis of 515 bases of the 5′ portion of the mitochondrial COI gene showed haplotype diversity in the Chesapeake Bay sample to be significantly higher than in the Delaware Bay sample, despite the larger size of the latter. Haplotype analysis indicates minimal contemporary gene flow between Delaware Bay and Chesapeake Bay crab populations, and further suggests that the Delaware Bay population is recovering from a recent population decline.

Cumulatively scored distributions of the slipper shell gastropodCrepidula sp., the sand-building polychaete wormSabellaria vulgaris, the calcareous meandering tubed polychaeteFilograna implexa, the barnacleSemibalanaus balanoides, and the lacy bryozoanSchizoporella sp. were mapped on a cm-square gridded silhouette of the prosoma, opisthosoma, and movable spines, and telson of the horseshoe crabLimulus polyphemus stranded on Reeds Beach, Cape May County, New Jersey in the Delaware Bay. Statistically tested and contoured epizoan frequencies for 81 males and 59 females revealed that slipper shells and agglutinated worm tubes are concentrated near the movable spines on the opisthosoma, but rare on recessed chilarial or opercular pleurites, the prosoma, and telson. Barnacles are disproportionately concentrated in entapophyseal pits on the opisthosoma and in the longitudinal furrows flanking the cardiac lobe of the prosoma. Bryozoans are concentrated near movable marginal spines of the opisthosoma, but also found on all areas of the prosoma, opisthosoma, and hinge area in between, as well as the telson. Different morphologic sites for their respective modal concentrations may result from competitive exclusion of bryozoans by barnacles. Genal angles have lower statistically predicted frequencies of bryozoans, barnacles, and sand worm tubes. Apertural openings of calcareous tubes are predominantly posteriorly oriented whereas the anterior magin of slipper shells are randomly oriented on the opisthosoma. Eschewing differential mortality, nonrandom distributions indicate possible: rugophilic (groove-seeking) larval settlement behavior for barnacles that preferred the recessed hinge area, longitudinal grooves, and pits; rheophilic (current-seeking) larval settlement behavior for the gastropods and tube-building polychaetes that filter feed in the eddies generated near the movable spines of the downcurrent-sloping opisthosoma; and geotactic (gravity-influenced) larval settlement or post-settlement migration to the vaulted crest (cardiac lobe). Deficiencies of protruding epizoic skeletons on the genal angles and anterior margin of the host may be due to removal by abrasion as the horseshoe crab plowed through sediment or shearing of the epizoans from the carapace substratum in strong currents or waves.

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.

Biological and physical reworking of sediments can profoundly affect the structure and functioning of benthic communities. The depth of the disturbance is an important factor that controls the types of organisms that can exist within the sediments. Large numbers of horseshoe crabs,Limulus polyphemus, spawn each spring on the sandy shores of Delaware Bay beaches. We have used this abundance peak to provide an estimate of the depth of sediment disruption caused by this species on an intertidal flat adjoining a major spawning beach. Vertical columns of marked sediment were placed in three locations of an intertidal flat. Some columns were protected with cages while others remained unprotected. Analysis of variance of the depth of disruption of the marked sediment indicated that different areas of the intertidal zone were disrupted to different depths. Caged marked sediment was disturbed to an average depth of 3.2 cm while unprotected sediment was disturbed to a mean depth of 11.1 cm. Deepest mixing occurred in a trough between sandwave crests and averaged 17.7 cm deep. These mixing depths are greater than the 1.2 cm deep disturbance produced by nonstorm wave action in the study area.

Breeding activity of the horseshoe crab,Limulus polyphemus, was quantitatively monitored in Apalachee Bay, Florida, throughout one breeding season. Breeding peaked at times of full and new moon at the hour of high tide. Breeding activity was heavier on night tides than on corresponding day tides of the same date. Males routinely outnumbered females and indications of sperm competition were present. Many horseshoe crabs buried in the intertidal zone throughout the subsequent low tide and returned to the beach to breed again on the following high tide. A tagging study of the horseshoe crab indicated that male animals return to breeding beaches more frequently than females. Most animals tagged at breeding beaches did not move away from the tagging site during a breeding season and were recovered at the point of release. No long-range movements were noted. The sex ratio of animals tagged near breeding beaches was predominately male while it was predominately female for animals collected and tagged 3–6 miles offshore. A nine percent recovery rate was achieved. Existing localized populations are potentially subject to depletion due to heavy collecting pressure on breeding beaches.

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.

Newly hatched horsehoe crabs (Limulus polyphemus) were exposed to autoclaved Bunker C oil for 8 weeks. Autoclaving served to prevent bacterial degradation of stored oil and to drive off the volatile fraction of the oil. The oil was introduced in the form of a whole oil suspension. Minimum lethal dose was shown to be 2.25 mg per 1. Concentrations greater than 0.25 mg per 1 caused a delay in molting.