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Is Limulus an Arachnid?
A. S. Packard, Jr. The American Naturalist, Vol.
16, No. 4. (Apr., 1882), pp. 287-292.
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Photo University of Delawarewww.ocean.udel.edu
HORSESHOE CRABSSome Facts

• Horseshoe crabs, common along the Delaware
  coast, are estimated to be at least 300 million
  years old. They have survived because of their
  hard, curved shells, which have made it difficult
  for predators to overturn them and expose their
  soft, vulnerable underbellies.
• The horseshoe crab has also survived because it
  can go a year without eating and endure extreme
  temperatures and salinity.

• Called "Horsefoot Crabs" because of the
  resemblance of its shell to a horse hoof.
• The Horseshoe Crab isn't really a crab. It is
  related to scorpions, ticks and land spiders,
  horseshoe crabs have their own classification
  (Class Merostomata).

without eating and endure extreme temperatures
and salinity.
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Life Stages
From Egg to Trilobite Larva
After fertilization, the eggs begin to develop
into trilobite larvae. By day five, miniature
legs are visible inside the translucent egg.
Upon hatching, the trilobite larvae dig their way
out of the sand.They are approximately 3 mm (1/8
inch) across and look just like miniature adults,
but lack a movable tail and functional compound
eyes. Their digestive system is also not yet
functional, and the baby crabs swim around for
about a week absorbing the yolk sac as their
digestive systems mature.
It Takes Eight to Ten Years for Horseshoe Crabs
to Reach Adulthood!
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Living Fossils
Most scientists believe that horseshoe crabs are
the closest living relative of the trilobite, a
marine animal that has long been extinct.
Scientists believe that horseshoe crabs were
among the dominant creatures some 300 million
years ago.
                                 Name Paleomerus hamiltoni   (cast)   Locality Kinnekulle, Sweden   Age Early Cambrian   Showcase Chelicerata showcase   The oldest known Horseshoe Crab.   Other chelicerata in the exhibition      
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Human Use
Farmers in the years 1800 - 1920 used horseshoe
crabs for fertilizer
Some farmers also used horseshoe crabs as a cheap
source of food for chickens and hogs. However,
the crabs gave the meat a "fishy" taste that
required weeks of purging on grain to remove.
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Dr. Bang and Dr. Levin had created Limulus
amoebocyte lysate, or LAL, and a new method to
test for gram-negative bacteria. It was so
effective that the U.S. Food and Drug
Administration (FDA) accepted it as a standard
test for endotoxins in 1983. Since then, LAL has
gained widespread use, replacing rabbit tests for
clinical and biomedical applications
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The Critique Of these days
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HORSESHOE CRABSToday
Limulus polyphemus is not presently endangered,
but harvesting and habitat destruction have
reduced its numbers at some locations and caused
some concern for these animals' future. Since the
1970s, the horseshoe crab population has been
decreasing in some areas, owing to several
factors, including the use of the crab as bait in
conch trapping.
In 1995, the nonprofit Ecological Research and
Development Group (ERDG) was founded with the aim
of preserving the four remaining species of
horseshoe crab.
Every year, around 10 of the horseshoe crab's
breeding population dies when rough surf flips
the creatures onto their backs, a position from
which they often cannot right themselves. In
response, the ERDG (Ecological Resource and
Development Group) launched a "Just Flip 'Em"
campaign, in the hopes that beachgoers will
simply turn the crabs back over.
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Originally thought to be crustaceans (hence the
common name), xiphosurans were recognized as
aquatic chelicerates late in the 19th
century (Lankester 1881). The fossil record of
horseshoe crabs goes back to the Devonian, and
modern-looking horseshoe crabs first appear in
the mid-Mesozoic (Størmer 1952). Their apparently
slow rate of morphological change since has led
to their being dubbed living fossils (Fisher
1984) and regarded as a keystone group for
studies of evolution and of arthropod
phylogeny. (The Complete Mitochondrial DNA
Sequence of the Horseshoe Crab Limulus polyphemus)