The Incredible Horseshoe Crab

The Incredible Horseshoe Crab They tell us if the surgeon's equipment is clean because it is very sensitive to bacteria. It costs 60000 a gallon Inside the horseshoe crab's blood cell (called the amebocyte) are the proteins of its blood clotting system. These proteins are released in response to the presence of unwanted organisms like Gram-negative bacteria and cause its blood to clot around the injury and bacteria, protecting the animal from further harm. Horseshoe crab blood is unusual for two reasons.

Firstly, it gets its incredible blue color from the copper used to carry oxygen around the crab’s body, in the same way, the iron in hemoglobin makes our blood red. Secondly, horseshoe crab blood reacts to the presence of bacterial endotoxins, coagulating around the contamination and trapping it in a gel-like substance. The blood is so sensitive that it will react to a contaminant concentration of one part per trillion. Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata, and are closely related to arachnids. Prosoma (cephalothorax) - The largest section of the horseshoe crab. From a top view, it is shaped like a horse's shoe. Several eyes are found on the exterior of the prosoma. Opisthosoma (abdomen) - The abdomen is the center section of the shell and attaches to the cephalothorax using a hinge. From a top view, moveable spines are visible along the edge of the abdomen. Telson (tail) - The tail is attached to the abdomen at the terminal base. The horseshoe crab uses its telson to steer and right itself if it becomes inverted in the tidal zone.

Contrary to popular belief, the tail is not a poisonous stinger. Occasionally, horseshoe crabs are found with a misshapened telson. This is usually due to a physical injury of the telson. Eyes Horseshoe crabs have a total of 10 eyes used for finding mates and sensing light. The most obvious eyes are the 2 lateral compound eyes. These are used for finding mates during the spawning season. Each compound eye has about 1,000 receptors or ommatidia. The cones and rods of the lateral eyes have a similar structure to those found in human eyes but are around 100 times larger in size. The ommatidia are adapted to change the way they function by day or night.

At night, the lateral eyes are chemically stimulated to greatly increase the sensitivity of each receptor to light. This allows the horseshoe crab to identify other horseshoe crabs in the darkness. The horseshoe crab has an additional five eyes on the top side of its prosoma. Directly behind each lateral eye is a rudimentary lateral eye. Towards the front of the prosoma is a small ridge with three dark spots. Two are the median eyes and there is one endoparietal eye. Each of these eyes detects ultraviolet (UV) light from the sun and reflected light from the moon. They help the crab follow the lunar cycle. This is important to their spawning period that peaks on the new and full moon. Two ventral eyes are located near the mouth but their function is unknown. Multiple photoreceptors located on the telson constitute the last eye. These are believed to help the brain synchronize to the cycle of light and darkness. (Check out a diagram of the horseshoe crab's 10 eyes).

Gills A horseshoe crab absorbs oxygen from the water using gills that are divided into 5 distinct pairs located under the abdomen. Each pair of gills has a large flap-like structure covering leaf-like membranes called lamellae. Gaseous exchange occurs on the surface of the lamellae as the gills are in motion. Each gill contains approximately 150 lamellae that appear as pages in a book. They are commonly called book gills. The gills also function as paddles to propel juvenile horseshoe crabs through the water. Mouth & Legs The horseshoe crab has 6 pairs of appendages on the posterior side of the prosoma. Five pairs of walking legs or pedipalps enable the horseshoe crab to easily move along benthic sediments. Each has a small claw at the tip except the last pair. The last pair of legs has a leaflike structure at the terminal end that is used for pushing and clearing away sediments as the crab burrows into the marine bottom.

The base of each leg is covered with inward pointing spines called gnathobases that move food towards the mouth located between the legs. As the legs are moving, food is crushed and macerated. There are also 2 small chelicera appendages that help guide food into the mouth. Circulatory System The horseshoe crab has a developed circulatory system. A long tubular heart runs down the middle of the prosoma and abdomen. The rough outline of the heart is visible on the exoskeleton and at the hinge. Blood flows into the book gills where it is oxygenated in the lamellae of each gill. The flapping movement of the gills circulates blood in and out of the lamellae. Oxygenated blood is returned to the heart for distribution throughout the horseshoe crab. the tidal mouth of a large river, where the tide meets the stream. The horseshoe crab has been described as an armored box that moves. Their appearance is similar to the prehistoric and extinct trilobite. Looking at the exterior of the crab, the body is divided into three sections. These three sections comprise the horseshoe crab's hardened exoskeleton.

The exoskeleton is shed periodically as the crab grows. The tail or telson cannot be used to endanger another animal, the horseshoe crab uses it to flip itself over if it has been flipped onto its back. Adult horseshoe crabs gather on beaches in large numbers to dig nests and lay and fertilize eggs. This process is known as spawning. The start of their inshore movement from deep bay and coastal waters appears to be triggered by lengthening daylight hours. Spawning in the Chesapeake and Delaware Bays usually begins during the latter part of May. The peak in spawning activity usually coincides with the high tide during the full moon and new moon in May and June in Delaware and New Jersey. However, in Florida, breeding activity continues between March and November with peak spawning occurring as early as April (Brockmann, 1990) and continuing through August (Rudloe, 1980). In Massachusetts, spawning occurs between May and July (Barlow et al., 1986).

Breeding activity is consistently higher during the full moon than the new moon and is also greater during the night than through the day (Rudloe, 1980; Thompson, 1998). Thompson (1998) found that spawning horseshoe crabs responded to optimum tidal and solar conditions available during each lunar phase, rather than to the lunar phase itself. Barlow et al. (1986) in Massachusetts and Penn and Brockmann (1994) in Delaware found spawning activity greatest during the highest tides regardless of whether it was day or night. Brockmann and Penn (1992) found a significant tendency for horseshoe crabs tagged during the day to return to spawn during the day, while horseshoe crabs tagged during the night tend to return to spawn at night. Lunar cycle, day of the year, and wave height are significantly correlated with horseshoe crab spawning activity (Rudloe, 1980). Shuster and Botton (1985) observed that horseshoe crabs avoid spawning during rough weather, no matter what the phase of the moon, possibly because fighting the surf would only serve to exhaust the animals during an already energy-draining activity (spawning). 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 The average number of eggs per cluster is 3,650 to 4,000 (Shuster 1982; Shuster and Botton, 1985). Several egg clusters are deposited during each trip, and females will return on successive tides to lay more eggs. A female will lay about 20 egg clusters each season in the Delaware Bay (Botton, 1995). However, Brockmann (1990) only identified up to 15 egg clusters each season in Florida. Approximately 88,000 eggs are produced per female per year (Shuster, 1982). The density of egg clusters has been reported to be as high as 50 egg clusters per linear meter (Shuster and Botton, 1985) and up to 500,000 eggs per square meter. (Botton et al., 1994). Egg development is dependent on temperature, moisture and oxygen and usually takes a month or more. 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: - The egg clusters are moistened by water with salinity of at least 8 parts per thousand. - The substrate around the egg clusters is well oxygenated. - Tides are sufficient to keep incubating eggs moist. - The beach surface is exposed to direct sunlight to provide sufficient incubation. - The slope of the beach is adequate for larvae to orient and travel downslope to the water upon hatching. 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 Horseshoe crab eggs typically hatch 14 to 30 days after fertilization (Sekiguchi, et al., 1982; Jegla and Costlow, 1982; Botton, 1995), but factors such as overcrowding or high-density egg clusters can prolong the incubation period (Barber and Itzkowitz, 1982). The optimum temperature for egg development has been estimated to be between 30C and 35C (Jegla and Costlow, 1982). 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 year’s 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. Adults The horseshoe crab must molt (shed its chitinous exoskeleton) to grow. Molting occurs several times during the first two to three years. As the horseshoe crab grows larger, the time between molts increases. Horseshoe crabs will molt 16 to 17 times over a period of nine to 11 years before they are fully-grown and sexually mature (Shuster, 1950). It should be noted that the often-cited age of sexual maturity is based on a series of molted shells from a single captive specimen. Females reach maturity one year later than males and, consequently, experience one additional molt (Shuster, 1955). 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. The horseshoe crab is endangered meaning its almost extinct

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