Limulidae
Encyclopedia
Horseshoe crabs are arthropod
s that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin
may be present in the roe
of species inhabiting the waters of Thailand. Horseshoe crabs are considered living fossil
s.
Limulidae is the only recent family
of the order
Xiphosurida and contains all four living species
of horseshoe crabs:
or predation.
Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, which are their main food. They may also feed on crustaceans and even small fish.
Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 centimetres (23.6 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.
One synapomorphy
for the order Xiphosura is the fusion of opisthosomal tergites behind opercular tergite (free abdominal segments) to form a thoracetron. Fusion of opisthosoma
l tergites to form a thoracetron has previously been considered a characteristic of the xiphosuran superfamilies Euproopoidea and Limuloidea, but evidence was presented by Anderson & Selden that fusion also occurs in Bellinuroidea. Giribet et al. used the character in their matrix and found it to be in a synapomorphy
for Limulus and Carcinoscorpius
.
It has proven to be difficult to raise horseshoe crabs in captivity. There is reason to believe that mating only takes place in the presence of the sand or mud in which the horseshoe crab eggs were hatched. It is not known with certainty what in the sand is being sensed by the crabs nor how they sense it.
in their blood, but instead use hemocyanin
to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocyte
s, which play a role similar to white blood cells for vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate
, which is used for the detection of bacterial endotoxins.
Harvesting horseshoe crab blood involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation. Estimates of mortality rates following blood harvesting vary from 3% to 15%.
(mostly in the United States) and whelk
. However, fishing horseshoe crab is temporarily forbidden in New Jersey
(moratorium on harvesting) and restricted to only males in Delaware
. A permanent moratorium is in effect in South Carolina.
It is hypothesized that low horseshoe crab populations in the Delaware Bay endangers the future of the red knot
. Red knots are long distance migratory shorebirds that feed on the protein-rich eggs during their stopover on the beaches of New Jersey and Delaware. There is an ongoing effort to develop adaptive management plans to regulate horseshoe crab harvests in the Bay in a way that protects migrating shorebirds.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin
Tetrodotoxin
Tetrodotoxin, also known as "tetrodox" and frequently abbreviated as TTX, sometimes colloquially referred to as "zombie powder" by those who practice Vodou, is a potent neurotoxin with no known antidote. There have been successful tests of a possible antidote in mice, but further tests must be...
may be present in the roe
Roe
Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...
of species inhabiting the waters of Thailand. Horseshoe crabs are considered living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s.
Limulidae is the only recent family
Family
In human context, a family is a group of people affiliated by consanguinity, affinity, or co-residence. In most societies it is the principal institution for the socialization of children...
of the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Xiphosurida and contains all four living species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of horseshoe crabs:
- Carcinoscorpius rotundicaudaCarcinoscorpius rotundicaudaThe mangrove horseshoe crab, Carcinoscorpius rotundicauda, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders and scorpions than to crabs. It is the only species in the genus Carcinoscorpius...
, the mangrove horseshoe crab, found in Southeast Asia - Limulus polyphemus, the Atlantic horseshoe crab, found along the northwest Atlantic coast and in the Gulf of Mexico
- Tachypleus gigasTachypleus gigasTachypleus gigas is one of the four extant species of horseshoe crab. It is found in shallow water in South East Asia at depths of up to . It grows up to long and wide, and is covered by a sturdy carapace, with a long caudal spine .-Description:...
, found in South and Southeast Asia - Tachypleus tridentatusTachypleus tridentatusTachypleus tridentatus is a species of horseshoe crab found in China, Indonesia, Japan, Malaysia, the Philippines, Taiwan, and Vietnam....
, found along East Asian coasts
Anatomy
The entire body of the horseshoe crab is protected by a hard shell. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are turned upside down, so a horseshoe crab with a broken tail is more susceptible to desiccationDesiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
or predation.
Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, which are their main food. They may also feed on crustaceans and even small fish.
Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 centimetres (23.6 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.
One synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
for the order Xiphosura is the fusion of opisthosomal tergites behind opercular tergite (free abdominal segments) to form a thoracetron. Fusion of opisthosoma
Opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...
l tergites to form a thoracetron has previously been considered a characteristic of the xiphosuran superfamilies Euproopoidea and Limuloidea, but evidence was presented by Anderson & Selden that fusion also occurs in Bellinuroidea. Giribet et al. used the character in their matrix and found it to be in a synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
for Limulus and Carcinoscorpius
Carcinoscorpius
The mangrove horseshoe crab, Carcinoscorpius rotundicauda, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders and scorpions than to crabs. It is the only species in the genus Carcinoscorpius...
.
Breeding
During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. Many shore birds eat the eggs before they hatch. The eggs take about 2 weeks to hatch. The larvae molt six times during the first year.It has proven to be difficult to raise horseshoe crabs in captivity. There is reason to believe that mating only takes place in the presence of the sand or mud in which the horseshoe crab eggs were hatched. It is not known with certainty what in the sand is being sensed by the crabs nor how they sense it.
Blood
Unlike mammals, horseshoe crabs do not have hemoglobinHemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
in their blood, but instead use hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...
to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocyte
Amebocyte
An amebocyte or amoebocyte is a mobile cell in the body of invertebrates such as echinoderms, mollusks or sponges. They move by pseudopodia. Similarly to some of the white blood cells of vertebrates, in many species amebocytes are found in the blood or body fluid and play a role in the defense of...
s, which play a role similar to white blood cells for vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate
Limulus Amebocyte Lysate
Limulus amebocyte lysate is an aqueous extract of blood cells from the horseshoe crab, Limulus polyphemus. LAL reacts with bacterial endotoxin or lipopolysaccharide , which is a membrane component of Gram negative bacteria...
, which is used for the detection of bacterial endotoxins.
Harvesting horseshoe crab blood involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation. Estimates of mortality rates following blood harvesting vary from 3% to 15%.
Fishery
Horseshoe crab are used as bait to fish for eelsEel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
(mostly in the United States) and whelk
Whelk
Whelk, also spelled welk or even "wilks", is a common name used to mean one or more kinds of sea snail. The species, genera and families referred to using this common name vary a great deal from one geographic area to another...
. However, fishing horseshoe crab is temporarily forbidden in New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
(moratorium on harvesting) and restricted to only males in Delaware
Delaware
Delaware is a U.S. state located on the Atlantic Coast in the Mid-Atlantic region of the United States. It is bordered to the south and west by Maryland, and to the north by Pennsylvania...
. A permanent moratorium is in effect in South Carolina.
It is hypothesized that low horseshoe crab populations in the Delaware Bay endangers the future of the red knot
Red Knot
The Red Knot, Calidris canutus , is a medium sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia. It is a large member of the Calidris sandpipers, second only to the Great Knot...
. Red knots are long distance migratory shorebirds that feed on the protein-rich eggs during their stopover on the beaches of New Jersey and Delaware. There is an ongoing effort to develop adaptive management plans to regulate horseshoe crab harvests in the Bay in a way that protects migrating shorebirds.