Carcinoscorpius rotundicauda
The mangrove horseshoe crab, Carcinoscorpius rotundicauda, is a marine chelicerate 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...

. Despite its name, it is more closely related to spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s and scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

s (all are in the subphylum Chelicerata) than to crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s. It is the only species in the genus Carcinoscorpius. The mangrove horseshoe crab occurs only in Asia and can be found in India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

, Malaysia, the Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...

, Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...

, Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...

 and Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...

. There are four extant species of horseshoe crab. The biology, ecology and breeding patterns of C. rotundicauda and the two other Asian horseshoe crab species, Tachypleus gigas
Tachypleus gigas
Tachypleus 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:...

and Tachypleus tridentatus
Tachypleus tridentatus
Tachypleus tridentatus is a species of horseshoe crab found in China, Indonesia, Japan, Malaysia, the Philippines, Taiwan, and Vietnam....

, have not been as well documented as those of the North American species Limulus polyphemus. All four extant species of horseshoe crabs are anatomically very similar.

Evolutionary history

Horseshoe crabs are commonly known by biologists around the world as a 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...

 because they have remained practically unchanged in terms of shape and size for millions of years. Although their physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 has changed over the years, their typical three piece exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

, consisting of a prosoma, 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...

, and telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

, has remained since the mid-Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 era. Fossils of horseshoe crabs that have been dated to over look almost identical to those species that are still alive today. The long existence of this body plan suggests its success. It is estimated that the American species of horseshoe crab diverged from the three Indo-Pacific species approximately .


The basic body plan of a horseshoe crab consists of three parts: the prosoma, the opisthosoma and the telson. The prosoma is the dome-shaped part at the front. The marginally-spined middle is called the opisthosoma, and the rear extension that looks like a spike is the telson, which is commonly described as the 'tail'. Contrary to popular belief, the telson is used to turn itself right side up when overturned. The mangrove horseshoe crab grows up to 40 centimeters in length (including the tail), and its whole body is protected by a hard, dark brown carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

.[4, 5] Uncommon for chelicerates, horseshoe crabs have two compound eyes. The main function of these compound eyes is to find a mate. In addition, they have two median eyes, two rudimentary lateral eyes, and an endoparietal eye on their carapace and two ventral eyes located on the underside by the mouth. Scientists believe the two ventral eyes aid in the orientation of the horseshoe crab when swimming. Each individual has six pairs of appendages. The first pair, the chelicerae
The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...

, is used to place food in its mouth. The next pair of legs are the pedipalp
Pedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...

s, which are used for walking. The rest of the legs are used for locomotion and are known as pusher legs. Located behind their legs are book gills
Book lung
A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...

. These gills are used for propulsion to swim and to exchange respiratory gases.

Distribution and habitat

This species occurs only in Asia around the Indo-West Pacific region where the climate is tropical or subtropical. These horseshoe crabs can be found to exist throughout the Southeast Asia region in shallow waters with soft, sandy bottoms or extensive mud flats. The mangrove horseshoe crab is benthopelagic, spending most of its life close to or at the bottom of a body of their brackish, swampy water habitat, such as mangroves. This is the habitat for which it gains its common name: mangrove horseshoe crab. Scientists have studied the distribution of mangrove horseshoe crabs in Hong Kong specifically. The researchers noted their abundance on the beaches of Hong Kong before the sharp decline in population over the past ten years. In the study, they found an uneven distribution of the horseshoe crabs throughout Hong Kong, with a greater abundance found in the western waters. They predict this unevenness is due to the estuarine
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

Hydrography is the measurement of the depths, the tides and currents of a body of water and establishment of the sea, river or lake bed topography and morphology. Normally and historically for the purpose of charting a body of water for the safe navigation of shipping...

 in the western waters, influenced by the Pearl River
Pearl River (China)
The Pearl River or less commonly, the "Guangdong River" or "Canton River" etc., , is an extensive river system in southern China. The name Pearl River is usually used as a catchment term to refer to the watersheds of the Xi Jiang , the Bei Jiang , and the Dong Jiang...

. They also found that T. tridentatus can coexist in the same habitat as the mangrove horseshoe crab.


Mangrove horseshoe crabs are selective benthic feeders, feeding mainly on insect larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e, small fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, oligochaetes, small crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s and thin-shelled bivalves
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...

. Lacking jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

s, it grinds up the food with bristles on its legs and places it in its mouth using its chelicerae. The ingested food then enters the cuticle-lined oesophagus and then the proventriculus
The proventriculus is part of the digestive system of birds, invertebrates and insects.-Birds:The proventriculus is a standard part of avian anatomy...

. The proventriculus is made up of a 'crop' and a 'gizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

'. The crop can expand to fit the ingested food, while the gizzard grinds the food into a 'pulp'. Studies have found that mangrove horseshoe crabs have a strong preference for insect larvae over the other organisms on which it also feeds.

Mating biology

In the spring, horseshoe crabs migrate from the deeper water to the shallow, muddy areas. Nesting usually follows the cycle of the high tide
High Tide
High Tide was a band formed in 1969 by Tony Hill , Simon House , Peter Pavli and Roger Hadden .-History:...

 of the full
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

 and new moon
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...

s. During the mating period, the males will follow and cling to the backs of their potential mates using modified prosomal appendages for long periods of time before the egg-laying has occurred. Horseshoe crab species with low spawning densities and 1:1 sex ratio
Sex ratio
Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....

s, such as the mangrove horseshoe crab, are found to be monogamous. In addition, the female does not choose her mate. Males find their female mates with the use of visual and chemoreceptive signals. Once a mate is found, the female digs a hole and lays the eggs while the male externally fertilizes
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....

 them. Once the eggs are laid, the male and female head back to the ocean, and the eggs develop on their own. Their eggs are large, and after a couple weeks, the eggs hatch into miniature versions of the adults.

Mangrove horseshoe crabs in Singapore breed from August to April. Juveniles grow about 33% bigger each time they moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...

, and it takes the juveniles about five moults to grow from 2 centimetre (0.78740157480315 in) to adult size.

Human impact

Thousands of the horseshoe crabs are killed by local fishermen. In some areas, the crab is cooked, and the unlaid eggs are eaten. However, some reports show that the crabs are toxic. In addition, horseshoe crabs are prized for their blue blood, as it is widely used in biomedical sciences
Biomedical scientist
A biomedical scientist is a scientist trained in biology, particularly in the context of medicine...

 for the development of drugs for diseases like mental exhaustion and gastroenteritis
Gastroenteritis is marked by severe inflammation of the gastrointestinal tract involving both the stomach and small intestine resulting in acute diarrhea and vomiting. It can be transferred by contact with contaminated food and water...

. The blood contains a chemical called Limulus amoebocyte 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...

 (LAL) that can be used to detect pathogens and their endotoxins.
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