There are four species of horseshoe crabs, and
all are members of the class Merostomata,
aquatic animals with two body segments and a
spikelike telson at the tail end. Perhaps the bestknown
representative is Limulus polyphemus, the
common horseshoe crab native to the northwest
Atlantic coast and the Gulf of Mexico. These animals
live in shallow water to depths of one hundred
feet and prefer soft sand or mud bottoms,
through which they slowly plow as they scavenge
Horseshoe crabs, unlike true crabs, do not have
antennae. However, like crabs, they do have
jointed appendages and a hard shell, or exoskeleton,
made of chitin, which must be periodically
shed to accommodate the growing body of the individual.
Horseshoe Crab Anatomy
The body of horseshoe crabs is divided into two segments: a large, helmet-shaped, forward section called the cephalothorax or prosoma, and a rear abdomen or opisthosoma, to which is attached the lancelike telson. Despite its threatening appearance, the telson is not used for defense, but rather for pushing and righting the body if the animal is overturned. There are two lateral and two median eyes on the upper surface of the prosoma. Although horseshoe crabs may be able to detect movement, there is little evidence that they can form images. The unique and relatively simple anatomy of horseshoe crab eyes make them favorite subjects for nervous system research. Under the cephalothorax there is a pair of small, pincerlike chelicerae, followed by five pairs of walking legs. The first four pairs are chelate and the fifth pair is for pushing away mud and silt during burrowing. The first four pairs also have spines along the joints closest to the body. These gnathobases are used to shred and macerate food and move it toward the mouth. The abdomen has six pairs of appendages, five of which are modified as thin, flaplike gills. In addition to providing oxygen to the animals, the gills function as paddles during upside-down swimming in small individuals.
Reproduction in Horseshoe Crabs
Horseshoe crabs are dioecious, meaning there are separate sexes. During warm months, females migrate into the intertidal zone to rendezvous with the smaller males. The males crawl onto the shell of the females and cling to them while the females scoop out depressions in the sand and deposit two hundred to three hundred small green eggs, which the male then fertilizes. The location of this egg burying is of critical importance: too high up the beach and the eggs will dry out; too low and they will die in the oxygen-poor sand. After a lunar month a small (about one centimeter), swimming larva hatches which little resembles the adult. After successive molts the adult body form is eventually achieved, with sexual maturity being reached after three years.
Economic and Scientific Importance of Horseshoe Crabs
The economic value of horseshoe crabs has been recognized since at least the nineteenth century, when millions were harvested annually from Delaware Bay to be ground up as fertilizer. By the 1950’s, the population of horseshoe crabs had decreased to the tens of thousands. Since then, controls have been put in place to protect this animal. In Japan, it has been declared a national monument to shield it from extinction. Today the horseshoe crab is used for bait in the fishing industry. It is also valuable in biomedical research because of its blue, copper-based blood. Anextract of this blood, called limulus amebocyte lysate (LAL), is used in detecting bacterial contamination of drugs and medical devices. There are many other chemicals derived fromhorseshoe crabs that may prove useful against human diseases.
Genus and species: Limulus polyphemus
Geographical location: L. polyphemus, the most common species, is distributed along the northwestern Atlantic coast and the Gulf of Mexico; other species are native to Asian coasts, from Japan and Korea south through the East Indies and the Philippines Habitat: Shallow water with soft bottoms Gestational period: One lunar month for the eggs to hatch
Life span: Approximately thirty-five years Special anatomy: Two distinct body segments, a rigid telson at the tail end, four eyes, a median frontal organ
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