Species Description: The horseshoe crab, Limulus polyphemus, is more closely related to chelicerates such as spiders, scorpions, ticks and mites than it is to true crabs and other crustaceans. Horseshoe crabs are considered to be "living fossils" that have evolved little in the past 250 million years. Limulus is an ancient genus which has probably existed since the Silurian period (440 to 410 million years ago), and shows little morphological change from the now-extinct genus Paleolimulus that lived about 200 million years ago. Limulus polyphemus is believed to be the closest living relative of trilobites (Shuster 1982).
Like all chelicerates, members of the Order Xiphosurida have a two-part body consisting of a prosoma, or head region; and an opisthosoma, or abdominal region. The prosoma contains 6 pairs of legs, all of which bear claws except the last pair. The prosoma also contains 2 types of eyes: 2 compound eyes, or ommatidia, are located on either side of the head; and 2 simple eyes, or ocelli, are located in the center of the head. The opisthosoma contains an additional 6 pairs of appendages which aid in respiration, reproduction, and locomotion.
The first pair of abdominal appendages form a genital operculum which houses the genital pores. The remaining 5 pairs of appendages are modified into a series of overlapping plates which function as gills. The underside of each plate is highly folded into leaf-like folds, or lamellae, which provide the actual surface for gas exchange. Due to their morphology, the abdominal plates have become known as book gills. In addition to their respiratory function, the opisthosomal appendages also function as paddles in locomotion. A long spine, called a telson, is located behind the opisthosoma and gives this order its name: Xiphos being Greek for "sword", and uros meaning "tail."
Regional Occurrence: Limulus polyphemus is distributed geographically from approximately 19° N to 42° N along the east coast of North America from Maine through south Florida and the Gulf of Mexico to the Yucatan peninsula, with peak abundance in Delaware Bay (Botton and Ropes 1987). Distinct populations occur along this range (Shuster 1982).
IRL Distribution: Limulus polyphemus is found in all three water bodies of the Indian River Lagoon (Indian River, Banana River, Mosquito Lagoon). The greatest abundance of horseshoe crabs in the IRL is found in the northern Indian River, southern Banana River, and southern Mosquito Lagoon.
Age, Size, Lifespan: Horseshoe crabs are long-lived and slow to mature in comparison to most other invertebrate groups. Males reach sexual maturity between 9 - 11 years of age, and females between 10 - 12 years of age (Cohen and Brockmann 1983). The average life span is believed to be approximately 20 - 40 years; however, it is difficult to accurately assess age in horseshoe crabs (Botton and Ropes 1988). The adult size of Limulus polyphemus shows a distinct latitudinal gradient, with larger animals found toward the center of the range, and smaller animals found at the extremes of the range, north of Cape Cod, along the Florida coast, and in the Gulf of Mexico. Limulus polyphemus shows distinct sexual dimorphism, with males approximately 1/3 the size of the females (Shuster 1982). Adult females in the Indian River Lagoon have an average prosomal width of 189 mm, while the average adult male has a prosomal width of 136 mm.
Abundance: Population sizes of Limulus polyphemus show a distinct latitudinal gradient, with the largest population centers found in the central portion of the distributional range along the mid-Atlantic coast of the United States, especially in the Delaware Bay region of New Jersey. Population size decreases north of Cape Cod, along the Florida coast, and in the Gulf of Mexico (Botton and Ropes 1987).
Locomotion: Adults and juvenile L. polyphemus use crawling as their primary means of locomotion. Horseshoe crabs also commonly bury themselves under the surface of the sand (Rudloe 1981). Occasionally a horseshoe crab will turn onto its back and swim upside-down, using its book gills to propel itself through the water (Shuster 1982). Larvae of this species, when first emerging from nests or when first exposed to water, exhibit a "swimming frenzy" similar to that of neonate sea turtles, swimming vigorously and continuously for hours (Rudloe 1980). Despite the possibility for wide dispersion during their free-swimming period, many larvae have been shown to settle in shallow waters near the beaches where they were spawned (Shuster 1982).
Reproduction: Limulus polyphemus is generally dispersed sub-littorally, but spawns on sandy beaches. The movement of mature Limulus to spawning areas is most likely triggered by a sensory system which detects seasonal changes in light patterns (Shuster 1982). Horseshoe crabs spawn during the spring and early summer on beaches along the Atlantic and Gulf coasts of the United States, and in Yucatan, Mexico (Penn and Brockmann 1994). In spring, Limulus males, which often outnumber females many times over, patrol along the foot of the beach awaiting females. Females move from deeper water directly to the beach where they nest. This directional movement of males and females, along with the numbers of males involved, reduces the likelihood of females reaching the beach without becoming paired (Shuster 1982). Horseshoe crabs typically locate mates, achieve amplexus, then migrate to the high tide mark in the intertidal zone to deposit and fertilize eggs before returning to deeper water after the spawning season. When waters are calm, many males may cluster around individual females, and large spawning assemblages can occur. Under rough conditions, only one or two males are able to grasp onto a female while also avoiding being washed away. Rough waters may drive spawning horseshoe crabs off beaches, or may keep them from moving onto the beach entirely (Shuster 1982). While nesting, females bury themselves in the sediment near the water's edge and lay a series of discrete egg clusters, each containing 2,000-20,000 eggs (Brockmann 1990). These eggs are fertilized by sperm released by an attached male and by one or more satellite males that typically congregate around the nesting pair (Rudloe 1980).
The reproductive cycle of horseshoe crabs has been found to be related to lunar activity in some areas. In one Florida study, Rudloe (1980) found that breeding in adults and hatching of larvae in Apalachee Bay, Florida was most prevalent on spring nights at the full moon. However, in St. Joseph Bay, FL spawning peaks occurred at the first and last quarter moons rather than around the new and full moons. This observation lead Rudloe (1980) to suggest that water depth may be a greater influence than lunar phase.
Embryology: The eggs of L. polyphemus develop in sediments 5 to 25 cm below the beach surface. Embryonic development is primarily temperature-related and varies according to the location of nests in the beach (Shuster 1982). The microclimate significant to the development of the eggs is a combination of temperature, moisture, and oxygen (Shuster 1982). Newly laid eggs are sticky and occur as tightly clumped balls, with larvae hatching approximately five weeks later after 4 embryonic molts (Rudloe 1979). Embryos hatch as trilobite larvae and remain in distinct aggregations at depths comparable to those of newly laid eggs (Penn and Brockmann 1994). The larvae may remain in the sand for several weeks, but are capable of feeble swimming, which most often occurs during the night. Buried larvae eventually reach the surface of the sand and emerge into the water column. When larvae first emerge from the nest or when they are first exposed to water, they exhibit a "swimming frenzy" similar to that of neonate sea turtles, swimming vigorously and continuously for hours (Rudloe 1980).
Larvae then swim freely for about six days before settlement and molt into the first juvenile instar, which measures approximately 5 mm in prosomal width. This first instar is morphologically similar to all subsequent instars, and generally resembles the adult except for telson length. The behavioral patterns of the animal change abruptly with the molt to the first juvenile instar. At this point in the lifecycle, L. polyphemus ceases the nocturnal swimming characteristic of trilobite larvae (Rudloe 1979) and becomes a benthic animal that alternatively crawls at the surface of the substratum and buries itself in the sand (Rudloe 1981).
Temperature: Developmental rates of Limulus embryos tend to be somewhat temperature dependent (Jegla and Costlow 1982), but under typical field conditions in Delaware Bay, trilobite larvae commonly appear within beach sediments 3 - 4 weeks after fertilization (Botton et al. 1992). Botton et al. (1992) showed that trilobite larvae of Limulus polyphemus are capable of overwintering, and can emerge after spending up to eight months in beach sediments. Should embryos become exposed to water temperatures below 20° C, the developmental sequence ceases in the first posthatch stage. A larva can survive for at least 6 months on its complement of yolk, and then continue to develop to the second stage when water temperatures increase and the natural environment is again suitable for finding food, growth, and molting to subsequent instars (Jegla and Costlow 1982). Under laboratory conditions, development can be stopped at the beginning of premolt, and the animals can be stored at 13-15° C for 6-8 months (Jegla 1982). Development can then be restarted when the animals are returned to environmental temperatures of at least 20° C.
Salinity: L. polyphemus are most often found in the more saline portions of estuaries, but they demonstrate some euryhaline tendencies (Shuster 1982). Both embryos and the posthatch larval stages develop and molt in the shortest times when exposed to salinities in the range of about 20-30 ppt. Although Limulus survives in salinities as low as 10 ppt and as high as 40 ppt, growth rates are slowed significantly at these salinity extremes (Jegla and Costlow 1982).
Other Physical Tolerances: Horseshoe crabs are capable of surviving physical extremes in temperature, salinity, pH, dissolved oxygen, and anoxic sediments (Shuster 1982).
Trophic Mode: The larvae of horseshoe crabs are non-feeding. Upon the molt to the first juvenile instar, feeding behavior is initiated (Rudloe 1980). The diet of immature and adult L. polyphemus includes bivalve mollusks and Polychaete worms such as Cerebratulus, Nereis, and Cistenides (Shuster 1982). To feed, L. polyphemus typically digs its food from sediments, grasping the prey with its legs. The prey is moved to the gnathobases where it is crushed before being pushed forward toward the mouth (Shuster 1982).
Habitat: L. polyphemus spends most of its life in the subtidal zone, except for annual spawning migrations (Botton and Ropes 1987). Horseshoe crabs require a sloping sandy beach upon which to lay their nests. Horseshoe crabs in Florida (Rudloe 1980) and Massachusetts (Barlow et al. 1986) nest in a narrow band in the upper middle quarter of the beach, whereas crabs in the Delaware Bay nest in a wide band over most of the beach (Botton et al. 1992, Shuster and Botton 1985). Botton et al. (1988) suggests that even subtle alteration of sediment may affect the suitability of the habitat for horseshoe crab reproduction.
Activity Time: Larvae of L. polyphemus remain buried in the sand during daylight hours, and Rudloe (1979) found that larval activity begins suddenly near the same hour each evening and terminates abruptly near the same hour each morning. This finding suggests that larval activity may be triggered very precisely by some environmental factor such as light intensity. After the molt to the juvenile stage, Limulus ceases the nocturnal swimming characteristic of trilobite larvae (Rudloe 1979) and becomes a benthic animal that alternatively crawls at the surface of the substratum and buries itself in the sand (Rudloe 1981). Both adults and juveniles demonstrate a diurnal activity pattern. However, while adults can be active during the evening, juveniles tend to bury themselves at night.
Associated Species: The carapaces of L. polyphemus adults are suitable habitat for a number of species. Most of the sessile organisms that colonize Limulus also attach to other hard surfaces in the environment (Shuster 1982). Several species of algae and protozoa, as well as bryozoans, coelenterates, annelids, barnacles, and tunicates are typical colonizers of Limulus carapaces.
Special Status: A great deal of research has been done on the American horseshoe crab, Limulus polyphemus, in the northeast United States, but little is known about the populations on the east coast of Florida. However, a widespread decline in the abundance of L. polyphemus in the last 20 years may be particularly severe in the Indian River Lagoon (IRL) system. While the horseshoe crab is not currently listed as threatened, there is a rising concern about the fact that it is absent from turtle nets in the northern IRL, particularly around the Mosquito Lagoon area where it has historically been common. Previous qualitative studies noted large numbers of Limulus weighing down turtle nets on a regular basis in Mosquito Lagoon in 1978-79 (Provancha 1997). However, a 1994 study of loggerhead sea turtles in the IRL revealed that while Limulus were common in the northern Indian River, the number of Limulus caught per survey in Mosquito Lagoon ranged from 0 - 4 animals, with 0 being most common (Provancha 1997).
Benefit in the IRL: L. polyphemus and its eggs are an important component of the IRL ecosystem, providing food for threatened loggerhead sea turtles, wading birds, alligators and many species of fish. Its plowing action while feeding supports species diversity, richness and abundance by aerating substrata, thereby affecting infaunal community structure. Because of the horseshoe crab's role in maintaining diversity and productivity in IRL, the alarming decline in numbers over the past 20 years is of serious concern and may serve as an indication of profound environmental disturbance in the lagoon. For example, the noticeable decrease in the number of loggerhead sea turtles being captured during netting surveys is potentially attributable to the decline of L. polyphemus (Provancha 1997), though more research in this area will be needed before definitive causes and effects can be identified.
Broad-scale Cost/Benefit: Loss of the horseshoe crab would negatively impact species which feed on the animal and its eggs and decrease biodiversity of the lagoon. The decrease may also indicate serious ecological disturbance in the lagoon.
Economic Importance: Horseshoe crabs are used extensively in the biomedical and pharmaceutical industries. Horseshoe crabs have blue, copper-based blood that clots when exposed to endotoxins, a dangerous class of chemicals released by certain bacteria. The clotting feature of Limulus blood serves as a commercially important alarm system to pharmaceutical companies which need to test the sterility of fluids intended for use on human patients. The blood enzyme responsible for clotting is called Limulus Amebocyte Lysate (LAL). Pharmaceutical companies that manufacture intravenous solutions, antibiotics, and kidney dialyzers use LAL to test the safety of their products (Widener 1999). Pharmaceutical companies have developed a method whereby they are able to extract up to one-third the total blood volume from an individual horseshoe crab, and then return it unharmed to the water. Limulus bled in this manner have a 90% survival rate. Most animals taken for pharmaceutical use are later returned to the water, but those used for biomedical purposes are not (Berkson 1999).
The use of horseshoe crabs as eel and conch bait over the past 10 years in the northeast United States is in part responsible for a drastic decline in the population (Botton et al. 1994). The extent of human harvest of Limulus polyphemus in Florida and in the IRL has not been stringently documented; however, observations indicate that overharvesting is a potential problem in the IRL.
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