Coastal Horseshoe Crab

Coastal Horseshoe Crab
Coastal Horseshoe Crab

  • Horseshoe crabs are not true crabs! They are more related to scorpions, spiders and mites.
  • Fossils that resemble horseshoe crabs date back to 445 million years ago.
  • The horseshoe crab has blue blood.


Coastal Horseshoe Crab

Binomial name: Tachypleus gigas (Muller, 1785)

Common name: Indo-Pacific horseshoe crab, Indonesian horseshoe crab, Indian horseshoe crab, Chinese horseshoe crab

IUCN redlist status: Data deficient

Distribution: India, Indonesia, Malaysia, Philippines, Singapore, Thailand (Figure 1)

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Fig. 1: Global distribution of horseshoe crab (Picture by University of Delaware Sea Grant College program)


Horseshoe crabs are in the taxa Merostomata. In addition to T. gigas, there are three other species of horseshoe crab, Tachypleus tridentatus, Carcinoscorpius rotundicauda and Limulus polyphemus. T. gigas is grouped together with the two other Asia-Pacific species Tachypleus tridentatus and Carcinoscorpius rotundicauda. Limulus polyphemus that is distributed in North America, is in another group of its own. Refer to phylogeny for more information.


Fig. 2: Photo of T. gigas (Photo by Wang Luan Keng)
Fig. 2: Photo of T. gigas (Photo by Wang Luan Keng)

T. gigas is shaped like a horseshoe and its body is dorsal-ventrally compressed.

Size: Average carapace width of 250mm; males are generally smaller than females
Colour: Yellowish grey exoskeleton
Telson: Triangular cross section

Interesting facts

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Horseshoe crabs VS true crabs

Horseshoe crabs have a misleading name. Despite being called crabs, they do not possess the characteristics of true crabs.
Instead, they are more closely related to spiders, ticks and mites.

Ancient creatures

Fig. 3: Exhibition of horseshoe crab death march in Houston Natural History Museum

The oldest horseshoe crab fossil was found recently in Canada and dates back to 445 million years ago (Upper Ordovician period)¹. Death tracks of a horseshoe crab, dating between 150 and 145 million years ago were also found in Germany in 2002.

Blue blood

The iron molecule used to transport oxygen in our blood, hemoglobin, is red. This gives rise to the colour of our blood. Unlike humans, horseshoe crabs do not use hemoglobin to transport oxygen. Instead, they make use of a copper compound, hemocyanin, that is blue.This causes the horseshoe crabs' blood to be blue. The horseshoe crab blood is also special because it clots upon detecting bacteria endotoxin.

Horseshoe Crab blood ² ³

In the 1960s, Dr. Frederik Bang, a researcher working in a Marine Biology laboratory in Massachusetts, United States, realized that upon injecting bacteria into the blood stream of Limulus polyphemus, clotting occurred. It was later found out that the clotting was due to the endotoxin of bacteria reacting with a reagent in the horseshoe crab's blood. This reagent was named Limulus Amebocyte Lysate (LAL).

Function of 'bacteria-detecting blood' in Horseshoe Crabs.

Horseshoe crabs live in coastal areas and seawater where bacteria thrive and thus, are prone to infections. Unlike us, horseshoe crabs do not have an immune system to develop antibodies and fight bacteria. Instead, it has components in its blood (LAL is one of them) that will bind to nasty things such as bacteria, viruses and fungi and inactivate them. Furthermore, clotting also forms a physical barrier and further prevents more bacteria from entering its blood stream.

Applications for Man

Bacteria endotoxin can cause infection in Man and in severe cases, result in death. Furthermore, endotoxin is resistant and remains active after steam sterilization. This poses a problem for pharmaceutical companies that need to produce sterile drugs, vaccines and medical devices. Care must be taken to ensure that these medical equipment are sterile so that they will not cause infections when used. LAL clots upon contact with endotoxins and is thus, indicative of sterility of the medical equipment.

Before the discovery of LAL, solutions were injected into rabbits to test for sterility of the solution. If the rabbits injected with the solution developed fever, the solution was deemed to be in sterile and is rejected for human use. With the discovery of LAL, the process of ensuring sterility became less complicated.

General anatomy of Horseshoe Crabs

Basic body plan

The body of a horseshoe crab is divided into three parts, the prosoma, opisthosoma and the telson (Figure 4).

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Fig. 4: Basic body plan of horseshoe crab

The prosoma contains the digestive system, nervous system, the heart, excretory glands and base of all the appendages.

The opisthosoma holds the muscles that are needed to work the book gills and the telson. The hinge between the prosoma and opisthosoma allows the horseshoe crab to bend its body.

The main purpose of the telson is to help the horseshoe crab flip itself over when it gets overturned. People often mistake it as a weapon and are wary of overturned horseshoe crabs flailing their telsons. However the horseshoe crab is quite incapable of inflicting harm.

Dorsal view

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Fig. 5: Dorsal view of horseshoe crab .

Both simple (ocelli) and compound eyes are located dorsally. Horseshoe crabs are the only living chelicerates with compound eyes, making them unique. Several other eyes (median eyes, endoparietal eye and lateral eyes) are also positioned on the dorsal side of the horseshoe crab.

Ventral view

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Fig. 6: Ventral view of horseshoe crab.

There are six pairs of appendages (Figure 6) on the ventral side of the horseshoe crab. The first pair of appendages are known as the chelicerae. They are used to manipulate food and to place it near the mouth. The second pair of appendage is called the pedipalp. The swelling of both the pedipalps and third appendage are used to differentiate matured males from the females (Figure 7). The third, fourth and fifth appendages are known as walking legs. The last pair of appendage, the pusher, looks slightly different from the other appendages and is flatter at the distal end. Its purpose is to help the horseshoe crab move over soft sediments.

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Fig. 7: Male with swollen appendages and female without swollen appendages.


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Fig 8: Close up of gnathobases of horseshoe crab

Gnathobases are tough and bristly structures attached to the base of appendages. These mouth parts give rise to the taxa that horseshoe crabs are in, Merostomata. They line the opening of the mouth and act as external molars. During feeding, the horseshoe crab buries into the substrate using the dome of its prosoma, the chelicerae are used to gather food such as worms and mollusc before they are directed to the mouth opening. As the horseshoe crab walks, the appendages move and the gnathobases that are connected to the appendages produce a grinding motion thus crushing its prey.

Book gills
Book gills are named as such because they resemble pages in a book. They need to be kept moist in order to extract oxygen. Fanning of the book gills also helps to propel the horseshoe crab during swimming.


As metioned, there are four species of horseshoe crabs and they may look similar to a layman. However, they can be quite different in terms of biology. Thus, it should not be assumed that the biology of a certain species of horseshoe crab generalized to all horseshoe crabs. As such, subtle morphological differences can be used to diagnose T. gigas from the other species.

The morphological differences of horseshoe crabs are shown in Figure 9 .

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Fig. 9: Morphological traits used to differentiate the four species of horseshoe crabs.

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Fig. 10: T. gigas: triangular cross section of telson.

To summarize the table, T. gigas has a round, complete frontal margin and its carapace is less convex as
compared to L. polyphemus and T. tridentatus.T. gigas also has a triangular telson cross section (Figure 10).

Biology of T. gigas


T. gigas are found in sandy to muddy areas. Intertidal zones such as creeks and estuaries are used for breeding purposes and they seem to prefer nesting sites with mean grain size of 63–125 µm in a study conducted in India . Nesting site in India were shifted when the monsoon resulted in sediments of that size shifting to another part of the coast . There have been cases of both T. gigas and C. rotundicauda occurring sympatrically in India and Singapore.

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Fig. 11: Mating pair of horseshoe crab.

Life cycle

During mating, the male latches onto the female using its specialized second and third appendages (Figure 11). The female carries the male and heads to higher shores where she will bury into the substrate and deposit her eggs. Eggs are fertilized externally.

Fig. 12: Horseshoe Crab eggs.
Fig. 12: Horseshoe Crab eggs.

For T. gigas, incubation last for about 40 days after which trilobite larvae emerge from the eggs . The exoskeleton of the horseshoe crab is hard and cannot expand. Thus, horseshoe crab juveniles need to moult to increase in size. Freshly hatched trilobite larvae has an average total length of 8mm and are able to swim actively . The first moult was recorded 30 days after the eggs hatched and they moult up to four times in 6 months. By this time, the body length is 45mm . Because it is difficult to rear horseshoe crabs in laboratory conditions, there are no studies that determine when horseshoe crabs reach sexual maturity even though researchers believe that it takes about 10 years. Extrapolation from the growth of L. polyphemus estimates that T. gigas juveniles undergo 12 moults before becoming fully grown for males and 13 moults for females . After each moult, the new exoskeleton is soft and will take some time to harden. Thus, horseshoe crab juveniles bury into the substrate for protection after moulting ¹⁰.


Horseshoe crabs are selective benthic feeders. A study conducted on T. gigas in the Bay of Bengal, India reported that they feed on mollusc, decayed organic material and polychaetes in order of prevalence. Other components of its diet included insects, crustaceans, copepods, and cirripeds ¹¹.


Why are horseshoe crabs important?

Horseshoe crabs are part of a food web. Horseshoe crab eggs are an important source of food for migratory birds such as the Red Knot in America. They are also thought to have significant impacts on the infauna biodiversity because they prey on worms and other infauna .

Even though threats that all horseshoes crab faces are similar, some threats may affect one species more than the other.

1. Loss of habitat

Alteration of coastal areas such as mangroves and beaches for anthropogenic uses have resulted in a loss of habitat for horseshoe crabs. These areas serve as nesting sites for horseshoe crabs to lay their eggs. However, they are also areas of high productivity and have been converted for agriculture and human settlement especially in developing countries ¹². In India, nesting beaches are suffering from anthropogenic effects such as maintenance work by fishing trawlers on the beach and shifting of beach sands for construction purposes . As such, the habitat may no longer be suitable as nesting sites nor accessible to horseshoe crabs and it has been reported that number of T. gigas mating pairs have decreased . Loss of habitat is viewed as the main threat for T. gigas.

Similarly, in Hong Kong and Singapore, reclamation has resulted in the natural shoreline changing. This has affected horseshoe crabs and in particular, the population of C. rotundicauda in Hong Kong has declined to dangerously low numbers ¹³.

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Fig. 13: Horseshoe crabs entangled in a gill net at Mandai Besar mangrove.

2. Pollution

Juveniles are reported to be found in well-aerated waters ¹³ and low water quality may affect horseshoe crabs.

Horseshoe crabs get entangled by gill nets left out by fishermen. A single gill net may trap up to 300 horseshoe crabs ¹⁴. When the tide recedes, horseshoe crabs are unable to burrow or return to the sea and are hence, prone to dessication. Additionally, they are also unable to forage for food and may die of starvation.

3. Overharvesting

Horseshoe crabs are harvested for several purposes.

L. polyphemus are collected for use as eel baits in the United States. Due to the high demand of bait and legislation that controls the number of horseshoe crabs that can be harvested in the United States, the supply chain has turned to Asia for horseshoe crabs. This indirectly affects the Asia-Pacific horseshoe crabs such as T. gigas as they are being exported to the United States to meet this demand and this number is also expected to rise ¹⁵.

Horseshoe crabs, mainly L. polyphemus, are also harvested for the medicinal properties of its blue blood. To obtain the blue blood, horseshoe crabs are harvested from the wild and are bled in laboratories. About 30% of the horseshoe crab's blood is collected before they are released back to the wild. The value of horseshoe crab blood was estimated to be at $222 million annually ¹⁶). Since then, a synthetic substitute to LAL was invented in 2011 ¹⁷ and this will hopefully take off some pressure on horseshoe crabs.T. gigas are not known to be commercially exploited for LAL but there has been talk going on in Malaysia on harvesting horseshoe crabs in general for pharmaceutical purposes.

Regionally, T. gigas are collected for consumption of the eggs. It is considered as a delicacy in Thailand and Malaysia. However, there are no known records of shops selling horseshoe crabs for consumption in Singapore. T. gigas and C. rotundicauda are also harvested for use as traditional medicine in India where it is known to treat joint pains .

Singapore got horseshoe crab meh?

T. gigas is found in Singapore and is listed as 'endangered' in the Singapore Red Data book ¹⁸. Its distribution (purple stars) is shown in Figure 14.
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Fig. 14: Distribution of horseshoe crabs in Singapore. Red stars: C. rotundicauda; Purple stars: Tachypleus gigas. (Map adapted from Yee et al., 2010)

Observations suggested that T. gigas is now seldom seen on the main island. Those found on the main island were either dead during an island wide survey in 2010 or were in fishermen's nets ¹⁹. Raffles Museum of Biodiversity and Research reports that they are only found at Changi and Marine Parade on the main island ²⁰.

In addition to T. gigas, C. rotundicauda (red stars) is also found in Singapore. Both species of horseshoe crabs are believed to have declined and loss of habitat is probably the main threat to horseshoe crabs in Singapore. Our coastlines have changed rapidly due to reclamation; mangrove cover has decreased from 89km² in 1819 to 6.9km² in 2010 due to conversion to prawn ponds, the building of reservoirs and reclamation ²¹. Similarly, most of our sandy shoreline has been artificially modified and may no longer be suitable for T. gigas to nest. This may have resulted in T. gigas being more common on our off-shore islands where there is less disturbance.

How can we help?

1. Help out with horseshoe crab rescue work

Nature Society Singapore has an ongoing horseshoe crab rescue project that anyone can sign up for. Volunteers are brought out to Kranji Bund to free horseshoe crabs entangled in nets. Horseshoe crabs are also recorded and measured for various studies. Nature Society has since produced three research papers from data collected by volunteers.

2. Stop littering
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Fig. 15: Rubbish at Mandai mangrove.

All rubbish eventually ends up in our water bodies. Coastal areas such as beaches and mangroves are especially prone to litter at the high tide line. Depending on the size and nature of our rubbish, horseshoe crabs can either get trapped by the rubbish or have difficulties accessing the coastal areas for nesting. In the case of abandoned gill nets, large number of horseshoe crabs may get trapped. As rubbish degrades, toxic chemicals may be produced and harm horseshoe crabs.

3. Flip overturned horseshoe crabs.

Overturned horseshoe crabs are prone to dehydration and may risk the soft parts of their body being attacked by predators such as birds. You can help by flipping them over if you see overturned horseshoe crabs!

Just flip em'! is a program initiated by Ecology and Research Development Group in the United States to encourage people to help horseshoe crabs stranded on the beach by flipping them over if they are overturned. As part of this project, a song was created to educate people on horseshoe crabs.

Just flip ‘em!
(copyright Janine Kelly 2001)

If you take a little walk down by the sea
You just might find some horseshoe crabs washed up on the beach.
And if theyʼre stranded upside down and if their legs are in the air,
Tell yourself, “Iʼm gonna help them out; Iʼm gonna walk right over there.

Just flip ʻem, flip ʻem over. Flip ʻem over, let them live.
Fig. 16: Overturned horseshoe crabs at Mandai Kechil mangrove (Photo by Germaine Leng)

Just flip ʻem, flip ʻem over. They need all the help we have to give them.
Flip ʻem, flip ʻem over and very soon youʼll see
Those horseshoe crabs will be making their way back home to the sea.

You know, horseshoe crabs are really not crustaceans; theyʼre not crabs!
Theyʼre called Merostomata, kind of related to a spider. How ʻbout that!
And theyʼre gentle and theyʼre harmless. And youʼd be such a help–
If you see them stranded upside down, gently flip them by their shell.
Now donʼt flip them by their tail ʻcause that will hurt that horseshoe crab.
It hurts them the way it hurts a little puppy or a cat.
So to flip them over properly, flip them by their shell
And as the horseshoe crabs go home, theyʼre gonna thank you for your help!


The mommies lay their eggs on the beach in holes. These are her nests.
She lays a hundred thousand eggs and then she needs a rest.
And then the trilobites, or babies hatch and then they make their way
To the intertidal waters where theyʼll grow up in the bay.
Horseshoe crabs were here a hundred million years before the dinosaur
And theyʼre very hardy creatures in their homes along the ocean floor.
But they canʼt survive a day stranded on the beach upside down
So if you see any flipped onto their backs please help them turn around.


Horseshoe crabs have ten eyes; not just two like you and me.
They use them for more than seeing things while their swimming in the sea.
Theyʼve been such a help to science ʻcause research on those eyes
Helped to win the 1967 Nobel Prize!
These quite amazing creatures have saved millions of lives
The horseshoe crab protects us from toxins that might hide
In our vaccines and medicines. Theyʼve been such a gift!
So if you see they need your help on the beach, wonʼt you offer them a little FLIP?



Taxonomic history

Tachypleus gigas was first described by O. F Müller in the book Entomostraca seu insecta testeacea in 1785.


The common name (horseshoe crab) was derived from the horseshoe shape of its body.
Horseshoe crabs are in the taxa Merostomata. Merostomata has greek origins with 'meros' meaning legs and 'stoma' meaning mouth in reference to horseshoe crabs having their mouth surrounded by its legs.
The species name gigas means giant in greek.


A holotype is the reference specimen that was used to describe and name a species.
Holotypes are
Fig. 17: Location holotype was collected from
important because they allow taxonomist to refer to the actual specimen that the species described was based on if the description of the species is unclear.

However, given the long history of taxonomy, catalogs of types have gone missing and the location of the holotype for T. gigas is unknown.

The only information about the holotype that is available are records by Müller indicating that the holotype was collected from East India.


Synonyms are different names given to the same species. The oldest name is recognized and the other names are called synonyms. The following are synonyms of T. gigas.

Limulus gigas (Muller, 1785)
Limulus heterodactylus (Latreille, 1802)
Limulus latreili (Leach, 1819)
Limulus macleaii (Leach, 1819)
Limulus moluccanus (Latreilie, 1802)
Limulus viriscens (Latreilie, 1806)
Tachypleus heterodactylus (Latreilie, 1802)
Tachypleus hoeveni (Pocock, 1902)


Phylogeny describes the evolutionary relationship between organisms. Several phylogeny analysis were performed in the 1980s and it is generally concluded that L. polyphemus is a sister taxon to the other three species of horseshoe crabs. Serological analysis suggest that L. polyphemus diverge from the three Asian-Pacific species 130 million years ago ²². The early separation of L. polyphemus is also supported when artificial hybridization experiments were conducted. None of the Asian Pacific species were able develop the eggs of L. polyphemus ²³. The phylogeny of the Asian-Pacific species are still being debated. It is thought that T. gigas separated from the other two Asian-Pacific species 20 million years ago and C. rotundicauda and T. tridentatus diverged 10 million years ago (Shishikura et al., 1982). In 2000, two mitochondrial genes (16S ribosomal rRNA and cytochrome oxidase subunit 1) and one nuclear gene (coagulogen) were analyzed to determine the relationship between the 4 species of horseshoe crabs to find out the phylogenetic relationship between the Asian Pacific species and the conclusion of all three genes were that T. gigas and C. rotundicauda are in the same monophyletic taxon (Xia, 2000)²⁴.

Useful horseshoe crab links

E-Book on Horseshoe crab research papers: Biology and Conservation of horseshoe crabs
Homepage of Ecology and Development Group: Ecology and Research Development Group
Homepage of Maryland Department and Natural Resources: Maryland Department of Natural Resources
Nature Society link to volunteer for Horseshoe crab research: Nature Society Singapore
Information of T. gigas in Singapore: Wildsingapore


1. Rudkin, D. M., G. A. Young & G. S. Nowlan, 2008. The oldest horseshoe crab: A new Xiphosurid from late Ordovican Konservat-Lagerstatten deposits, Manitoba, Canada. Paleantology 51: 1-9.

2. ERDG, 1999. Ecological Research and Development Group, medicinal uses. <>. Last accessed: 4 November 2013.

3. Shuster C. N., R. B. Barlow & H. J. Brockmann, 2003. The American Horseshoe Crab. Harvard University Press, Cambridge. Pp:310-340.

4. Fox, R., 2007. Invertebrate Anatomy OnLine, Limulus polyphemus. <> Last accessed: 3 November 2013.

5. Shuster C. N., R. B. Barlow & H. J. Brockmann, 2003. The American Horseshoe Crab. Harvard University Press, Cambridge. Pp:133-151.

6. Sekiguchi, K. & K. Nakamura, 1979. Ecology of the extant horseshoe crabs. Biomedical Applications of the Horseshoe Crab (Limulidae). Cohen E, New York. Pp 37–45.

7. Mishra, J. K., 2009. Horseshoe crabs, their eco-biological status along the north-east coast of India and the necessity for ecological conservation. In: J. T. Tanacredi, M. L. Botton, D. R. Smith & S. A. Earle (eds.). Biology and conservation of horseshoe crabs. Springer Science and Business Media, Heidelberg. Pp 89–96.

8. Mishra, J. K., 2009. Larval culture of Tachypleus gigas and its molting behavior under laboratory conditions. In: J. T. Tanacredi, M. L. Botton, D. R. Smith & S. A. Earle (eds.). Biology and conservation of horseshoe crabs. Springer Science and Business Media, Heidelberg. Pp 513-519.

9. Sekiguchi, K., H. Seshimo & H. Sugita, 1988. Post-Embryonic Development of the Horseshoe Crab. Biology bulletin 174: 337-345.

10. ERDG, 1999. Ecological Research and Development Group, life cycle. <>. Last accessed: 4 November 2013.

11. Chatterji, A., J. K. Mishra, A. H. Parulekar, 1992. Feeding behaviour and food selection in the horseshoe crab Tachypleus gigas (Muller). Hydrobiologia 246: 41-48.

12. Liow, L. H., 2000. Mangrove conservation in Singapore: A physical or a psychological impossibility? Biodiversity and Conservation 9: 309–332.

13. Morton, B. & C. N. Lee, 2011. Spatial and temporal distribution of juvenile horseshoe crab (Arthropoda: Chelicerata) approaching extirpation along the northwestern shoreline of the New Territories of Hong Kong SAR, China. Journal of Natural History 45: 227-251.

14. Sivasothi, N., 2009. 300 entangled horseshoe crabs rescued at Mandai Besar mangrove. Habitatnews. < > Last accessed: 4 November 2013.

15. Atlantic States Marine Fisheries Commission, 2013. Review of importation of Asian Horseshoe Crab.<>. Last accessed: 5 November 2013.

16. Manion, M., R. A. West & R. E. Unsworth, 2000. Economic assessment of the Atlantic horseshoe crab fishery. Industrial Economics.

17. Du, R. J., B. Ho & Ding. J. L, 2011. Application of cell-free hemolymph of horseshoe crab in antimicrobial drug screening. Current Pharmaceutical Design 17: 1234-1239.

18. Davison, G.W.H., P. K. L. Ng & H. C. Ho, 2008. The Singapore Red Data Book (2nd Edition). Nature Society (Singapore). Pp. 285.

19. Cartwright-Taylor, L., V.B. Yap, H.C. Chi & L.S. Tee, 2011. Distribution and abundance of horseshoe crabs Tachypleus gigas and Carcinoscorpius rotundicauda around the main island of Singapore. Aquatic Biology 13: 127-136.

20. Raffles Musuem of Biodiversity and Research, 2013. The DNA of Singapore, Tachypleus gigas. <>. Last accessed: 4 November 2013.

21. Yee, A. T. K., W. F. Ang, S. Teo, S. C. Liew & H. T. W. Tan, 2010. The present extent of mangrove forests in Singapore. Nature in Singapore 3: 139–145.

22. Shishikura F, Nakamura S, Takahashi K & Sekiguchi K.,1982. Horseshoe crab phylogeny based on amino acid sequences of the fibrino-peptide-like peptide. Experimental Zoology 223:89–91.

23. Sekiguchi, K., H. Seshimo & H. Sugita, 1980. Systematics and Hybridization in the Four Living Species of Horseshoe Crabs. Society for the Study of Evolution 34: 712-718.

24. Xia, X., 2000. Phylogenetic Relationship Among Horseshoe Crab Species: Effect of Substitution Models on Phylogenetic Analyses. Systematics. Biology 49: 87–100.