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."
HABITAT AND DISTRIBUTION
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).
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.
LIFE HISTORY AND POPULATION BIOLOGY
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.
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).
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).
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.
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).
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 compliment 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.
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).
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).
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
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.
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.
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,
Florida. 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 twenty 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.
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.
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 ten years in the northeast United 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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Berkson J, Shuster Jr CN. 1999. The horseshoe crab: the battle for a true multiple-use resource. Fisheries 24: 6-10.
Botton ML, Loveland RE, Jacobsen TR. 1988. Beach erosion and geochemical factors: influence on spawning success of horseshoe crabs (Limulus polyphemus) in Delaware Bay. Mar Biol 99: 325-332.
Botton ML, Loveland RE, Jacobsen TR. 1992. Overwintering by trilobite larvae of the horseshoe crab Limulus polyphemus on a sandy beach of Delaware Bay (New Jersey, USA). Mar Ecol Prog Ser 88: 289-292.
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Brockmann HJ. 1990. Mating behavior of horseshoe crabs, Limulus polyphemus. Behaviour 114: 206-220.
Cohen JA, Brockmann JH. 1983. Breeding activity and mate selection in the horseshoe crab, Limulus polyphemus. Bull Mar Sci 33: 274-281.
Jegla TC, Costlow JD. 1981. Temperature and salinity effects on developmental and early posthatch stages of Limulus. Prog Clinic Biol Res 81: 103-113.
Penn D, Brockmann HJ. 1994. Nest-site selection in the horseshoe crab, Limulus polyphemus. Biol Bull 187: 373-384.
Provancha J. 1997. Annual report for sea turtle netting in Mosquito Lagoon. NMFS Permit #942, FL Permit #114.
Rudloe A. 1979. Locomotor and light responses of larvae of the horseshoe crab, Limulus polyphemus (L.). Biol Bull 157: 494-505.
Rudloe A. 1980. The breeding behavior and patterns of movement of horseshoe crabs, Limulus polyphemus, in the vicinity of breeding beaches in Apalachee Bay, Florida. Estuaries 3: 177-183.
Rudloe A. 1981. Aspects of the biology of juvenile horseshoe crabs, Limulus polyphemus. Bull Mar Sci 31: 125-133.
Shuster Jr CN. 1982. A pictorial review of the natural history and ecology of the horseshoe crab Limulus polyphemus, with reference to other Limulidae. Prog Clinic Biol Res 81: 1-52.
Shuster CN, Botton ML. 1985. A contribution to the population biology of horseshoe crabs, Limulus polyphemus (L.), in Delaware Bay. Estuaries 8: 363-372.
Widener JW, Barlow RB. 1999. Decline of a horseshoe crab population on Cape Cod. Biol Bull 197: 300-302.
Report by: G. Ehlinger,
Florida Institute of Technology
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Page last updated: July 25, 2001