This horseshoe crab life history was developed by the Atlantic States Marine Fisheries Commission (ASMFC) Horseshoe Crab Plan Development Team from available scientific literature and state natural resource agency documents. It has been edited to provide the reader with the basic information to understand the life cycle and habitat requirements of the Limulus polyphemus species of horseshoe crab.
Spawning activity is significantly greater at water temperatures of 20C or greater in South Carolina (Thompson, 1998). At temperatures below 20C, a state of dormancy is initiated and production of ecdysone (a substance that stimulates molting and development) is curtailed (Jegla, 1982).
In Massachusetts, New Jersey, and Delaware, horseshoe crabs often spawn during neap tides (Penn and Brockmann, 1994; Cavanaugh, 1975; Barlow et al., 1986). However, in Florida, horseshoe crabs almost never spawn during neap tides (Rudloe, 1980). Penn and Brockmann (1994) conclude that the dissimilarity is due to differences in grain size (aerobic sediments occur at higher elevations in Florida than in Massachusetts, New Jersey, and Delaware). Additionally, neap tides are lower in Florida, and flood tides rarely reach the aerobic zone of the beach, which further explains why horseshoe crabs in Florida do not nest during neap tides (Penn and Brockmann, 1994).
While current tagging studies in New Jersey and South Carolina have not discounted the possibility of spawning site fidelity, horseshoe crabs are probably not loyal to one spawning site over successive years and generations (Thompson, 1998). However, spawning animals do display short-term fidelity to a spawning site, and they return to the same site on numerous high tides until spawning is complete (Thompson, 1998; Brockmann, 1990). Shuster (1994) reports that while horseshoe crabs probably do not return to their natal beaches, the majority does return to the same estuary to spawn.
Adults prefer sandy beach areas within bays and coves that are protected from the rough action of the surf. However, spawning has been observed on mud, sod, and peat banks. In addition, horseshoe crabs may be capable of spawning in subtidal areas (Rudloe, pers. comm., 1998).
Low-energy embayments preferred by horseshoe crabs include Tom's Cove (Chincoteague Bay, Virginia), Sandy Hook Bay (New Jersey), and Great Bay (New Hampshire) (Botton and Loveland, 1989). Optimum spawning areas are limited by the availability of sandy beach habitat. Eggs are laid in clusters or nest sites along the beach, usually between the tide marks.
Eggs and Nests
Egg nests are located in a broad area from the spring high tide line down to three meters above the low-water line (Shuster, 1982). However, the geochemical characteristics of the beach are more relevant to egg nest placement than the distance from the tide marks (Penn and Brockmann, 1994). Differences in the distribution of egg nests within a beach may be dependent (in part) upon the amplitude of the tides and beach morphology (Shuster, 1982; Penn and Brockmann, 1994). Specifically, beach morphology (sediment type and grain size) affects oxygen, temperature, and moisture gradients on the beach. Delaware Bay beaches are characterized as coarse-grained and well-drained, whereas Florida beaches are fine-grained and poorly drained (Penn and Brockmann, 1994).
Horseshoe crabs select locations for their nests that will maximize egg development; Penn and Brockmann (1994) found the mean nesting location for horseshoe crabs on Delaware Bay beaches to be about equal to the mean high tide line. However, horseshoe crabs in Florida nest much higher up on the beach (than in the Delaware Bay) to avoid the anaerobic conditions at the mean high tide line (Penn and Brockmann, 1994). Ultimately, eggs buried too high on the beach are subject to desiccation and those buried too low are subject to anoxic conditions (i.e., insufficient interstitial oxygen concentrations). Eggs are deposited in clusters in the upper portion of the intertidal zone. Depth of eggs in the sediment range from five to 20 centimeters below the surface (mean 11.5 ± 2.8 centimeters) (Rudloe, 1979; Brockmann, 1990). The mean nest depth in Delaware was found to be 9.3 ± 3.9 centimeters (Penn and Brockmann, 1994).
Shuster (1994) found that horseshoe crab reproductive success is greatest under the following conditions:
In examining the moisture requirement, Penn and Brockmann (1994) found that in Delaware, horseshoe crabs tended to place their nests in sand that was about three percent saturated. Eggs that were buried above this zone were more likely to desiccate, and the saturated sediments of the lower beaches contained insufficient interstitial oxygen concentrations for egg development to occur.
The moisture content of the sediment is determined largely by the size of the grains in the sediment. The grain size of the beaches in Delaware that had the greatest horseshoe crab spawning concentrations, as reported by Shuster and Botton (1985), had grain sizes ranging from 0.5 to 2.0 mm in diameter (Botton et al. 1994), with a median grain size of 0.7 mm. Beaches used by spawning horseshoe crabs in South Carolina and Florida have much smaller grain sizes. In South Carolina, grain sizes on study beaches used by horseshoe crabs are between 0.2 and 0.4 mm in diameter (Thompson, 1998).
The mechanism by which horseshoe crabs locate preferred spawning habitat is not completely understood. While horseshoe crabs spawn in greater numbers and with greater fecundity along sandy beaches, horseshoe crabs can tolerate a wide range of physical and chemical environmental conditions, and they will spawn in less suitable habitats if ideal conditions are not encountered. Therefore, the presence of large numbers of horseshoe crabs on a beach is not necessarily an indicator of habitat suitability (Shuster, 1994).
Interestingly, it appears that horseshoe crabs can detect hydrogen sulfide, which is produced in the anaerobic conditions of peat substrates. These anaerobic conditions reduce egg survivability, and horseshoe crabs avoid peat substrates (Botton et al., 1988; Thompson, 1998). Jacobsen (pers. comm., 1996) believes that horseshoe crabs need at least eight inches of sand over peat to avoid anaerobic conditions that could prevent egg development, with 16 inches or more being optimal.
Beach slope is also thought to play an important role in determining the suitability of beaches for horseshoe crab spawning (Shuster, pers. comm., 1995). Horseshoe crabs generally travel downslope after spawning and appear to become disoriented on flat areas (Jacobsen, pers. comm., 1995). Field experiments by Botton and Loveland (1987) determined that beach slope is more significant than vision in orientation behavior and identified poor orientation performance on flat beaches. Horseshoe crabs show rapid seaward orientation on beaches with slopes of approximately six degrees (Botton and Loveland, 1987).
Although the optimal beach slope is unknown, beaches commonly used by horseshoe crabs in New Jersey have slopes of three to seven degrees seaward (U.S. Fish and Wildlife Service, 1995). Jacobsen (pers. comm., 1996) estimates the optimal slope to be about seven percent. However, Thompson (1998) concluded that while parameters controlling site selection for spawning would normally favor beaches with an optimal slope (i.e., gentle seaward slope), beach slope itself is not likely to be the determining criteria for selection.
Erosion is also an important component in spawning success. Erosion of the substrate in which eggs are deposited would increase egg and larval mortality. Thompson (1998) suggested that short-term, seasonal erosion characteristics may be more important than long-term conditions.
In addition to the intertidal zone used for spawning, horseshoe crabs also use shallow water areas (less than 12 feet deep) such as intertidal flats and shoal water as nursery habitat in their juvenile life stages. The presence of offshore intertidal flats may also influence the use of certain beaches by spawning horseshoe crabs. Horseshoe crabs may congregate on intertidal flats to wait for full moon high tides because these flats provide protection from wave energy. Thompson (1998) identified that preferentially selected spawning sites were located adjacent to large intertidal sand flat areas. In addition to providing protection from wave energy, sand flats typically provide an abundance of available food for juvenile horseshoe crabs. Since several tidal cycles may be required to complete spawning, offshore intertidal flats may provide safe areas to rest between tide cycles.
Growth and Diet
The larval stage begins when the eggs hatch and the larvae emerge. The larvae swim and feed for a period of approximately six days. Although this free-swimming period provides the possibility of wide dispersion, when it is over, most larvae settle in shallow, intertidal areas near the beaches where they were spawned to complete their first molt (Shuster, 1982). This molt into the first juvenile instar occurs approximately 20 days after emergence (Jegla and Costlow, 1982).
Some larvae, while still in the egg capsule, delay emergence, overwinter within beach sediments, and hatch the following spring (Botton et al., 1992). This phenomenon was observed during the winters of 1989 to 1992, and densities of 1,000 to10,000 live trilobites per square meter were observed in sediment depths greater than 15 centimeters (Botton et al., 1992). While overwintering in beach sediment carries a risk of mortality associated with erosion from coastal storms, this strategy does minimize avian predation and provides insurance in the event that the previous years hatchlings had poor survivorship (Botton et al., 1992). Upon hatching, these larvae follow the same cycle described above.
Juvenile horseshoe crabs generally spend their first and second summer on the intertidal flats, usually near breeding beaches (Rudloe, 1981; Shuster, 1982). Thompson (1998) found significant use of sand flats by juvenile horseshoe crabs in South Carolina. Older crabs move out of intertidal areas and are found a few miles offshore except during breeding migrations (Botton and Ropes, 1987). After the larvae and young juveniles leave the beach environment, they do not return to the beach until they are sexually active adults (Rudloe, 1979).
Horseshoe crabs swim or crawl as their primary means of locomotion. Both larvae and juveniles are more active at night than during the day (Rudloe, 1979; Shuster, 1982: Thompson, 1998). Juveniles typically feed prior to the daytime low tide, then burrow into the sand, remaining inactive for the remainder of the day (Rudloe, 1981; Thompson, 1998). Because horseshoe crabs lack jaws, they crush and pulverize their food with the spiny bases of their legs and then place the food in the mouth.
Larvae feed on a variety of small polychaetes, nematodes, and nereis (Shuster, 1982). Juvenile and adult horseshoe crabs feed mainly on molluscs, including razor clam (Ensis spp.), macoma clam (Macoma spp.), surf clam (Spisula solidissima), blue mussel (Mytilus edulis), wedge clam (Tellina spp.), and fragile razor clam (Siliqua costata). However, horseshoe crabs also prey on a wide variety of benthic organisms including arthropods, annelids, nemertean, and polychaete worms (Botton, 1984; Botton and Haskin, 1984). In the Delaware Bay, horseshoe crabs prefer soft-shell clam (Mya arenaria) and small surf clam (Mulinia lateralis) over gem clam (Gemma gemma) despite the numerical dominance of the gem clam in the Delaware Bay (Botton, 1984). The horseshoe crab is also an important predator of soft-shell clams in Massachusetts.
Shuster (1950) reported the consumption of sand worm (Nereis spp.), sand ribbon worm (Cerebratulus spp.), gem clam, macoma clam, razor clam, and soft-shell clam by horseshoe crabs in Cape Cod Bay, Massachusetts. Botton (1984) found 56.4 percent of prey was infaunal burrowers, which included bivalves and polychaetes. Botton (1984) also found vascular plant material in nearly 90 percent of all individuals. Botton and Ropes (1989) hypothesized that horseshoe crabs may control species diversity, richness, and abundance in areas where they prey upon small molluscs and polychaetes. No differences between diet and food preference are apparent between male and female horseshoe crabs. Not surprisingly, Shuster (1996) identified that food for the horseshoe crab is abundant on the continental shelf in areas where horseshoe crabs abound.
Once sexual maturity is reached, horseshoe crabs no longer molt (or molt rarely). It is estimated that their lifespan beyond this point can be up to eight years. Once they stop molting, the horseshoe crabs provide an ideal surface to which epifaunal slipper shells (Crepidula fornicata) can attach themselves. By determining the age of these univalves, the age of the horseshoe crab can also be established. Therefore, the lifespan of horseshoe crabs may be 17 to 19 years in the northern part of their range, accepting the estimate of 9 to 11 years to reach sexual maturity (Shuster, 1950).
Like many animals, horseshoe crabs exhibit sexual dimorphism. Males are generally smaller than females at maturity, which is most likely a result of the females undergoing one more molt than males. The mean prosomal widths of the adult males is only 75 to 79 percent of that of the adult females (Shuster, 1982). In addition, males have specialized clasper claws to aid them in attaching to females during egg fertilization.
Males versus Females
Shuster (1996) suggested that a shift in the normal 1:1 sex ratio toward less than one female per male becomes an important indicator, pointing specifically to overharvesting of females. In South Carolina, the 1997 male-to-female ratio in each estuary sampled was higher than in the preceding years (i.e., 1996 and 1995) (Thompson, 1998), indicating a population changing due to environmental conditions or overharvesting. Trawling in the Delaware Bay by the Delaware Division of Fish and Wildlife (1997) identified annual sex ratios of approximately 1:1 for 1990 through 1996, except in 1993 and 1994 when 1.6:1 was noted (significant at p<0.05 from 1:1).
Sources Of Natural Mortality
Horseshoe crab eggs are also eaten by shorebirds, including semipalmated plover (Charadrius semipalmatus), black-bellied plover (Pluvialis squatarola), red knot (Calidris canutus), pectoral sandpiper (Calidris melanotos), least sandpiper (Calidris minutilla), semipalmated sandpiper (Calidris pusilla), dowitcher (Limnodromus spp.), sanderling (Calidris alba), ruddy turnstone (Arenaria interpres), and laughing gull (Larus atricilla). The willet (Catoptrophorus semipalmatus) is a predator of both horseshoe crab eggs and larvae (Rudloe, 1979). For more information about shorebirds, click here.
Adult horseshoe crabs provide food for sharks (Squaliformes), gulls (Larus spp.), and boat-tailed grackles (Quiscalus major) (Shuster, 1982). In addition, adult and juvenile horseshoe crabs make up a portion of the loggerhead sea turtle's (Caretta caretta) diet in the Chesapeake Bay (Musick, et al. 1983). Shuster (1996) also identifies red fox (Vulpes vulpes) and raccoon (Procyon lotor) as potential predators of adult and juvenile horseshoe crabs. Despite potential predation, Loveland et al. (1996) identify that natural mortality among subtidal adults is probably low. However, the exact percentage of horseshoe crab mortality from sea turtles and other marine animals remains unknown.
Human activity probably accounts for the greatest proportion of adult horseshoe crab mortality. Between the 1850s and the 1920s, over one million horseshoe crabs were harvested annually for fertilizer and livestock feed (Shuster, 1982; Shuster and Botton, 1985). Reported harvests in the 1870s were four million horseshoe crabs annually. Between 1880 and 1930, the harvest was 1.5 to 1.8 million animals (Finn et al., 1991). Shuster (1960) reports that in the late 1920s and early 1930s 4 to 5 million crabs were harvested annually. Shuster (1960) reports over one million crabs were harvested annually during the 1940s and 500,000 to 250,000 horseshoe crabs were harvested each year in the 1950s. By the 1960s, only 42,000 horseshoe crabs were reported to be harvested annually (Finn et al., 1991). For more information on the harvest of horseshoe crabs, click here.
More recently, horseshoe crabs have been taken in substantial numbers to provide bait for other fisheries, including (primarily) the American eel and conch (Busycon carica and B. canaliculatum) fisheries. Horseshoe crabs, particularly females, are sectioned and placed in American eel pots as bait. The conch fishery uses horseshoe crabs of either sex. Horseshoe crabs are also collected by the biomedical industry to support production of Limulus Amebocyte Lysate (LAL). For more information on Medical Uses, click here. Although this industry bleeds individuals and then releases the animals, two studies estimate 10 to 15 percent of animals do not survive the bleeding procedure (Rudloe, 1983; Thompson, 1998).
Diseases and Parasites
External parasites and ectocommensals do not usually attach to sub-adult horseshoe crabs due to the frequency of molting (Shuster, 1982). Thompson (1998) and Rudloe (pers. comm., 1998) identify the Bdelloura candida flatworm as common passenger on horseshoe crab gills and appendages, but the flatworm does not appear to be parasitic. A variety of other marine organisms including mussels, gnathobases, barnacles, and other sessile organisms may attach to horseshoe crabs. These species may be harmful if they attach to the ventral surface, which can interfere with feeding or locomotion (Shuster, 1982). Internal parasites such as metacercariae may cause intense and massive internal infections (Shuster, 1982).
Horseshoe crabs may share a commensal relationship with pinfish (Lagodon rhomboides) and juvenile blue crabs (Callinectes sapidus). The pinfish and blue crab stay in close proximity to horseshoe crabs and feed on particles of detritus and small organisms stirred into the water column from the "ploughing" action of the horseshoe crabs (Rudloe, 1985).
Botton and Loveland (1989) report that at least 190,000 horseshoe crabs died from beach stranding along the New Jersey shore of the Delaware Bay during the 1986 spawning season (May to June). This represents nearly 10 percent of the adult horseshoe crab population and is considered a substantial source of natural mortality. Rudloe (pers. comm., 1998) identifies that stranding mortality in Florida may be much lower based on personal observations. Natural mortality was estimated by Swan (pers. comm., 1998) to be up to 8 percent based on a mark and recapture study where 860 individuals were tagged. Telson abnormalities may contribute to stranding deaths because crabs with broken or shortened telsons cannot right themselves (Botton and Loveland, 1989). Stranding is also related to mating tactics and the ability of male horseshoe crabs to right themselves. Males not attached to a female are more likely to become stranded than attached males because they do not have the larger female as an "anchor." Additionally, on average, older males are more likely to become stranded than younger males, probably due to senescence and parasitism (Penn and Brockmann, 1995).