Ancestors of horseshoe crabs date back over 450 million years--long
before the age of the dinosaurs.
Four species of horseshoe
crabs exist today. Only one species, Limulus polyphemus,
is found in North America along the Atlantic and Gulf coasts from
Maine to Mexico. The other three species are found in Southeast
Asia. Horseshoe crabs are not true crabs at
all. Horseshoe crabs are more closely related to
arachnids (a group that includes spiders and scorpions) than to
crustaceans (a group that includes true crabs, lobsters, and
shrimp). Horseshoe crabs are often called "living fossils"
because fossils of their ancestors date back almost 450 million
years--that's 200 million years before dinosaurs existed.
Despite inhabiting the planet for so long, horseshoe crab body
forms have changed very little over all of those years.
The strange anatomy of the horseshoe crab is one of this
animal's most notable aspects. Unfortunately, the long, thin,
spike-like tail of horseshoe crabs has given this species an
unfavorable reputation. Many people view horseshoe crabs as
dangerous animals because they have sharp tails. In reality,
horseshoe crabs are harmless. Their tails are used
primarily to flip themselves upright if they are accidentally
Horseshoe crabs nest on
beaches in Florida and mid-Atlantic states.
Horseshoe crabs are known for their large nesting
aggregations, or groups, on beaches particularly in
mid-Atlantic states such as Delaware, New Jersey, and Maryland.
These nesting aggregations are commonly observed in
Florida as well. During the nesting season, principally in spring
and summer, male horseshoe crabs move parallel to the shoreline on
sandy flats and intercept females as they pass by. A successful
male attaches himself to a female by using his specialized front
claws, and together they crawl to the beach. Some males do not
attach to females, but still have success in fertilizing the
female's eggs. The male fertilizes the eggs as the female lays them
in a nest in the sand. Males that do not find mates will often
swarm mating couples and try to fertilize some of the females'
eggs. Most of this nesting activity takes place during high tides
in the three days before and after a new or full moon.
Horseshoe crab larvae emerge from their
nests several weeks after the eggs are laid. Juvenile horseshoe crabs
resemble adults except that their tails are proportionally smaller.
The young and adult horseshoe crabs spend most of their time on the
sandy bottoms of intertidal flats or zones above the low tide
mark and feed on various invertebrates.
Why are horseshoe crabs important?
Horseshoe crabs are an important part of the ecology of coastal
communities. During the nesting season, especially in the
mid-Atlantic States, horseshoe crab eggs become the major food
source for migrating birds. Over 50 percent of the diet of
many shorebird species consists of horseshoe crab eggs. Many bird
species in Florida have been observed feeding on horseshoe crab
eggs. In addition, many fish species rely on horseshoe crab eggs
Horseshoe crabs are extremely important to the biomedical
industry because their unique, copper-based blue blood contains a
substance called Limulus amebocyte lysate. The substance, which coagulates in the
presence of small amounts of bacterial toxins, is used to test
for sterility of medical equipment and virtually all intravenous
drugs. Research on the compound eyes of
horseshoe crabs has led to a better understanding of human vision.
The marine life fishery collects live
horseshoe crabs for resale as aquarium, research, or educational
specimens, and the American eel and whelk fisheries use horseshoe
crabs extensively as bait along many parts of the Atlantic
Threats to horseshoe crabs and research
Horseshoe crab numbers are declining throughout much of the
species range. Although scientists are unsure of the exact causes
of this decline, it is probably due to a variety of factors,
including the degradation or decrease of habitat.
In 1998, The Atlantic States Marine Fisheries Commission
developed a Horseshoe Crab Fishery Management Plan that
requires all Atlantic coastal states to identify horseshoe crab
nesting beaches. Currently, with the help of the public, biologists
at the Fish and Wildlife Research Institute are trying to document
nesting sites of horseshoe crabs throughout the state. If you are
interested in becoming more involved with the horseshoe crab
survey, please visit the
Survey for Horseshoe Crab Nesting Beaches in Florida for more