Horseshoe crabs

Horseshoe Crab Facts

Classification:

• Kingdom: Animalia
• Subkingdom: Bilateria
• Phylum: Arthropoda
• Subphylum: Chelicerata
• Class: Merostomata
• Subclass: Xiphosura
• Order: Xiphosurida
• Suborder: Limulina
• Family: Limulidae
• Genus and species:Limulus polyphemus, Carcinoscorpius rotundicauda, Tachypleus gigas, and Tachypleus tridentatus
• 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, ten light-sensing organs

There are four species of horseshoe crabs, and all are members of the class Merostomata, aquatic arthropods with two body segments and a spikelike telson at the tail end. Perhaps the best-known 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 for food.

Horseshoe crabs are not in fact crabs and are not closely related to crustaceans, instead being more similar to arachnids like scorpions and spiders. 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 crabs are considered living fossils due to the fact that they are present in the fossil record—in virtually unchanged form—dating to approximately 450 million years ago, predating dinosaurs.

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 eyes on the upper surface of the prosoma, which are unusual for being compound eyes. The upper shell also has five other light-sensing eye structures: a pair of rudimentary lateral eyes located behind the compound lateral eyes, a pair of median eyes in the front middle of the shell, and an endoparietal eye behind the median eyes. In addition, the underbelly has a pair of ventral eyes positioned by the mouth, thought to be used to provide orientation when swimming in an upside-down position, and the tail contains photoreceptors. This array of light-sensing organs signal variation in light and darkness, including visible light and ultraviolet light. 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. Horseshoe crabs primarily eat worms, mollusks, and small crustaceans found on the seafloor, and hunt mainly at night.

The abdomen has six pairs of appendages, five of which are modified as thin, flaplike gills. In addition to providing oxygen to the animals, these "book 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, communicating with pheromones. 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. The abundance of horseshoe crab eggs is also a critical part of the food chain along the Atlantic seaboard, with many species of fish, reptiles and amphibians, and birds relying on the eggs as a staple food; the red knot is a shorebird species that eats only horseshoe crab eggs during its migration. By the 1950s, the population of horseshoe crabs had decreased to the tens of thousands, threatening the biodiversity of the entire North American Atlantic shoreline ecosystem. Since then, controls have been put in place to protect this animal and its numbers have increased, though overharvesting as well as the development and pollution of beaches and intertidal zones remain threats. 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, especially for eels. It is also extremely valuable in biomedical research because of its unique blue, copper-based blood. An extract 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 from horseshoe crabs that may prove useful against human diseases. Horseshoe crabs are also of considerable interest to scientists because of their status as living fossils, providing a window into species development hundreds of millions of years ago.

Principal Terms

cephalothorax: the forward segment of the horseshoe crab’s body

chelate: pincerlike

chelicerae: appendages with pincers

dioecious: having separate sexes

exoskeleton: the external shell of horseshoe crabs and their relatives

gnathobase: the part of the leg closest to the horseshoe crab’s body

Bibliography

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