Indo-pacific horseshoe crab, Tachypleus gigas
Indo-Pacific Horseshoe Crab,
Tachypleus tridentatusPhoto: Dr Tang Qian
In creationist mythology, horseshoe crabs have remained unchanged for 400 million years, so they prove there is no such thing as evolution and Earth is only 6-0,000 years old. Apart from the typical lack of joined-up thinking in that non sequitur, there are a couple of fallacy at work here.
Firstly, evolution does not necessarily involve major changes in the phenotype because it happens at the genetic level, so the genome is where we need to look to find evidence of it (or lack of it); secondly there is no requirement for a species that is already adapted to its environment to change unless its environment changes in a way that requires adaptation. This might include increased predation, depletion of food resource, major changes in temperature or oxygenation, increased competition from another species, etc.
With their child-like understanding of evolution, creationists assume a species must change into something else for evolution to happen.
In fact, as a team a research team led by Associate Professor Frank Rheindt from the Department of Biological Sciences at the National University of Singapore (NUS) Faculty of Science have now shown, there are four different species of horseshoe crab which have all diversified from a common ancestor.
Tell me about the evolution and distribution of the horseshoe crabs, please.
Horseshoe crabs (order Xiphosura, class Merostomata) are marine arthropods that have existed for over 450 million years, making them one of the oldest living groups of animals. They are often called "living fossils" because their body plan has remained largely unchanged since the Ordovician period.
Evolutionary History
Horseshoe crabs belong to the subphylum Chelicerata, making them more closely related to arachnids (such as spiders and scorpions) than to true crabs or other crustaceans. Their evolutionary history includes several now-extinct relatives, such as Eurypterids (sea scorpions), which were once dominant marine predators.
- Ordovician Period (~450 mya): Early xiphosurans appear in shallow marine environments.
- Paleozoic Era (~300 mya): More advanced forms develop but remain morphologically similar to modern species.
- Mesozoic Era (~150 mya): Horseshoe crabs survive mass extinctions, adapting to various shallow marine environments.
- Cenozoic Era (Present day): Four extant species remain, showing remarkable evolutionary stability.
Modern Distribution
Today, only four species of horseshoe crabs survive, all found in coastal and estuarine environments:
- Limulus polyphemus (Atlantic Horseshoe Crab) – Found along the eastern coast of North America, from the Gulf of Mexico to the northeastern United States.
- Tachypleus tridentatus (Tri-spine Horseshoe Crab) – Found in coastal waters of China, Japan, and Southeast Asia.
- Tachypleus gigas (Indo-Pacific Horseshoe Crab) – Inhabits the waters of the Bay of Bengal and Southeast Asia.
- Carcinoscorpius rotundicauda (Mangrove Horseshoe Crab) – Found in shallow waters and estuaries from India to Indonesia.
Ecological Role & Conservation
Horseshoe crabs play a crucial ecological role. Their eggs provide an important food source for migratory shorebirds, and they contribute to benthic (seafloor) ecosystems. They are also vital to biomedical research because their blue blood contains limulus amebocyte lysate (LAL), used to detect bacterial contamination in medical applications.
However, populations are declining due to:
- Overharvesting (for bait and biomedical use)
- Habitat loss (coastal development)
- Climate change (affecting spawning conditions)
Conservation efforts focus on protecting nesting beaches, regulating harvesting, and exploring synthetic alternatives to LAL for medical testing.
Fossil Record of Horseshoe Crabs (Order Xiphosura)
Horseshoe crabs have an extensive fossil record dating back to the Ordovician period (~450 million years ago). Their morphology has remained remarkably stable, making them classic examples of "living fossils." The fossil record reveals several extinct relatives and a gradual adaptation to different marine environments over geological time.
Key Fossil Discoveries and Evolutionary Timeline
- Ordovician Period (485–443 mya) – Oldest Known Horseshoe Crab Ancestors
- The earliest xiphosurans appeared in shallow marine environments.
- Fossils of Tetraspidella from the Upper Ordovician suggest primitive horseshoe crab-like arthropods already existed.
- Silurian Period (443–419 mya) – Emergence of True Xiphosurans
- More recognizable horseshoe crab relatives evolved.
- Kasibelinurus (Late Silurian) is considered one of the earliest known xiphosurans.
- Devonian Period (419–359 mya) – Diversification and Expansion
- This period saw the emergence of several marine genera with more developed carapaces and feeding structures.
- Bellinurus and Euproops are well-known genera from the Devonian.
- Carboniferous Period (359–299 mya) – Peak Diversity
- This was the golden age of xiphosurans, with numerous species thriving in marine, brackish, and even freshwater environments.
- Genera like Euproops, Bellinurus, and Paleolimulus flourished, some resembling modern horseshoe crabs.
- Some species, such as Euproops danae, had spines and unique adaptations to different ecological niches.
- Permian Period (299–252 mya) – Decline and Extinctions
- The Permian saw a decline in diversity, possibly due to changing environments and competition with other arthropods.
- Many species perished during the Permian-Triassic mass extinction (~252 mya), the most severe extinction event in Earth's history.
- Mesozoic Era (252–66 mya) – Survival and Adaptation
- Xiphosurans survived the mass extinction but were less diverse than in the Carboniferous.
- Fossil genera like Mesolimulus (Jurassic) and Limulitella (Cretaceous) were similar to modern horseshoe crabs.
- Some species adapted to shallow coastal waters, much like their modern counterparts.
- Cenozoic Era (66 mya–Present) – Modern Horseshoe Crabs
- Fossil records show that Limulus polyphemus, the modern Atlantic horseshoe crab, has remained almost unchanged for at least 20 million years.
- Their ability to survive drastic environmental changes has made them one of the longest-surviving arthropod groups.
Key Fossil Finds
- Kasibelinurus (Silurian) – One of the earliest known true xiphosurans.
- Euproops (Carboniferous) – Small, with a reinforced carapace and spines for protection.
- Mesolimulus (Jurassic) – Nearly identical to modern Limulus, showing little evolutionary change.
- Paleolimulus (Permian) – A transitional form between early and modern horseshoe crabs.
Why Have Horseshoe Crabs Changed so Little?
- Stable ecological niche: They have occupied similar coastal and estuarine environments for hundreds of millions of years.
- Efficient body plan: Their tough exoskeleton, simple yet effective feeding structures, and ability to tolerate low-oxygen conditions have helped them persist.
- Survivors of mass extinctions: They have outlived dinosaurs and many other ancient marine species due to their resilience and adaptability.
The results of their study are published open access in the journal
Conservation Letters and is the subject of a news item from NUS>
Preserving Asian horseshoe crab populations through targeted conservation strategies
NUS biologists conduct the first comprehensive population study of all three Asian horseshoe crab species, mapping their population distribution, evolutionary histories and vulnerabilities to climate change to propose customised conservation strategies
The photo shows the moult of a young tri-spine horseshoe crab,
Tachypleus tridentatus – an endangered species – found on the beach at Beihai, China. The genomic data gathered by NUS researchers on this species provides a launchpad for conservation strategies.
Photo: Dr Tang Qian.
Among the four species, only the Atlantic horseshoe crab (Limulus polyphemus), found along the Atlantic coast of the United States and the Gulf of Mexico, has been extensively studied. In contrast, scientific information about the three Asian species is so scant and scattered that the IUCN Red List, which tracks the extinction risk of species around the world, listed two of them (the mangrove horseshoe crab and the coastal horseshoe crab) as “data deficient”. This designation indicates insufficient data to assess their extinction risk. On the other hand, the tri-spine horseshoe crab is considered endangered.
Understanding our planet’s living fossils
to help fill in these knowledge gaps, a research team led by Associate Professor Frank Rheindt from the Department of Biological Sciences at the NUS Faculty of Science conducted the first comprehensive population genomic study of all three Asian horseshoe crab species: the mangrove horseshoe crab (Carcinoscorpius rotundicauda), coastal horseshoe crab (Tachypleus gigas), and tri-spine horseshoe crab (Tachypleus tridentatus).
The study underscores the importance of Southeast Asia’s Sunda Shelf, a shallow-marine region, as a critical coastal marine habitat. Importantly, this region has sustained the survival of these ancient arthropods for millennia and could continue to act as a refuge for Asian horseshoe crabs amid accelerating anthropogenic climate change.
The researchers have also established the first-ever genomic baseline dataset for these species, which lay the groundwork for targeted conservation planning. Their findings, which propose different conservation strategies for each species, were published in Conservation Letters on 16 December 2024.
Back to the basics: Filling data gaps to advance conservation efforts
To protect and conserve these species, it is crucial that we first cover the basics — understanding their population structure, evolutionary histories and climate-change-driven vulnerabilities. This foundational knowledge will enable us to develop targeted conservation strategies and prioritise habitats critical for their survival.
Associate Professor Frank E. Rheindt, co-corresponding author
Department of Biological Sciences
National University of Singapore, Singapore.
Tracking and monitoring Asian horseshoe crabs is in and of itself a challenging feat. They spend most of their lives on the seabed, making them difficult to observe, and they take 14 years to mature — too long to assess population changes effectively through traditional surveys. To overcome these challenges, the researchers turned to population genomic approaches, where they analysed DNA from 251 horseshoe crabs collected across 52 sites in 11 countries.
Using this data, NUS researchers created the first genomic baseline dataset for Asian horseshoe crabs. This dataset enabled the team to map population structures and delineate genetic boundaries among the three species.
Such distinctions are important, as they highlight populations that harbour unique genetic traits essential for adapting to specific local environments. Genomic data also helps us pinpoint coastal hotspots that should be prioritised for conservation.
Dr Tang Qian, first author.
Department of Biological Sciences
National University of Singapore, Singapore.
The study also revealed how horseshoe crabs have responded to environmental fluctuations over time. The Sunda Shelf emerged as a vital refuge for horseshoe crabs during periods of past climate change. By reconstructing the species’ evolutionary histories, the researchers found that the region has not only preserved genetic diversity but also served as a migratory corridor, which allowed populations to remain connected despite environmental changes.
Tailored conservation strategies needed
The study highlighted that future climate change poses varying levels of risk to the three species of Asian horseshoe crabs. While all are vulnerable, their ability to adapt differs. For instance, the mangrove horseshoe crab, with its limited dispersal capacity, faces higher threats of local extinction compared to the more mobile coastal and tri-spine horseshoe crabs.
Based on these findings, the researchers have proposed tailored conservation strategies to support each species in adapting to climate change:
Mangrove horseshoe crabs
- Protect and restore mangrove habitats, which are essential for the species’ survival and ability to migrate southward in response to rising temperatures.
- Prioritise the conservation of populations in the Gulf of Tonkin and South China as they face the highest evolutionary pressures from climate change.
Coastal horseshoe crabs
- Protect the Sunda Shelf region, which serves as a critical refugial habitat, particularly around the Bay of Bengal, the Malacca Strait and Southern Vietnam.
- Maintain connectivity between populations by safeguarding coastal corridors to mitigate the species’ vulnerability to habitat fragmentation.
Tri-spine horseshoe crabs
- Implement sustainable fishery regulations and restore coastal habitats, especially in areas with a history of intensive development, such as Japan, Taiwan and China.
- Focus conservation efforts on reducing human-driven threats like harvesting and habitat loss as these currently pose greater risks than climate change.
Next steps
Our study provides an important impetus and the necessary baseline data for the preservation of key habitats for horseshoe crabs’ future survival. As an important caveat, however, our work is only based on environmental factors and does not take into account future human activities that may directly alter habitats, such as coastal development. The survival of horseshoe crabs will therefore critically depend on interventions based on local contexts.
Dr Tang Qian.
Looking ahead, the researchers plan to further explore the evolutionary potential of Asian horseshoe crabs. This includes studying how specific functional genes contribute to their ability to adapt to local environments and changing climates.
We have established the Horseshoe Crab Global Biorepository, with its physical collection located at the Lee Kong Chian Natural History Museum at NUS, to support ongoing and future research. Through this resource, we hope to foster collaborations and secure funding to advance genomic research on horseshoe crabs. We are currently working with the Chinese University of Hong Kong on genomic research specifically focused on the tri-spine horseshoe crab.
Associate Professor Frank E. Rheindt.
ABSTRACT
Horseshoe crabs are unique living fossils that have remained almost unaltered through 400 million years of global change. They face rapid worldwide declines under increasing anthropogenic pressure. Using comprehensive geographic and genomic sampling combined with approaches that integrate DNA with environmental and climatic datasets, we assessed the population genetic structure, demographic histories, and vulnerability to future climate change in three out of four extant horseshoe crab species, all centered in Asia. Our study highlights that the Sunda Shelf, a complex and dynamic shallow-marine landscape, has been the sole repository of most genetic diversity among all three Asian species, and therefore crucial to the long-term survival of horseshoe crabs. Our study not only provides the first genomic baseline data for the evaluation of Asian horseshoe crabs’ conservation status but also identifies core habitats that potentially act as refugia and corridors for Asian horseshoe crab populations with impending anthropogenic global warming.
1 Introduction
Horseshoe crabs are one of the planet's foremost “living fossils”. They are known for their high morphological conservatism and a consistently slow evolutionary rate over hundreds of millions of years (Bicknell and Pates 2020). Although they are a fundamental building block of coastal marine ecosystems where they occur (Botton 2009), only four geographically restricted species remain today (Sekiguchi and Shuster 2009.1): the Atlantic horseshoe crab (Limulus polyphemus) along the Atlantic coast of the United States and the Gulf of Mexico, and three Asian horseshoe crabs, the mangrove horseshoe crab (Carcinoscorpius rotundicauda), coastal horseshoe crab (Tachypleus gigas), and tri-spine horseshoe crab (Tachypleus tridentatus), in coastal East, Southeast, and South Asia (Figure 1). Although anecdotal reports suggest declines of horseshoe crabs worldwide (John et al. 2018; Wang et al. 2020.1), important baseline population data remain scant and are mostly only available in economically advanced regions. Hence, the Atlantic horseshoe crab is more intensively studied compared with its Asian cousins (Luo et al. 2020.2), two of which (C. rotundicauda and T. gigas) are currently classified as data deficient in the IUCN Red List, whereas T. tridentatus is considered endangered.
FIGURE 1
Distribution and genetic divergence of Asian horseshoe crabs. Top left: map of the sampling localities; colored coastal lines (see legend) highlight the natural range of the three Asian horseshoe crab species; dotted lines delimit seven geographical regions (A–G) identified in genetic analysis; sampling localities are color coded according to each of the seven geographical regions. Top right: Principal component analysis (PCA) plots of horseshoe crab individuals (based on SNP dataset with 10% missing data and linked loci filtered). Colors in PCA plots correspond to colors of sampling localities on the map. Bottom: consensus population assignments (thick lines above the bar plots) and individual population assignments using ADMIXTURE (bar plots based on SNP dataset with 10% missing data and linked loci filtered) and discriminant analysis of principal components (membership assignments across SNP datasets displayed as lines under the bar plots). The consensus populations’ pairwise Fst and Dxy values (calculated based on the SNP dataset with 10% missing data and linked loci filtered) are depicted on the right of the corresponding population assignments.
The biology of all four extant horseshoe crab species has been tightly adapted to ephemeral coastline habitat and shallow-marine conditions since the late Paleozoic (Blażejowski et al. 2017; Bicknell and Pates 2020). The three Asian species’ ranges broadly overlap across the Southeast Asian Sunda Shelf, which is known as one of world's biodiversity hotspots for terrestrial (Myers et al. 2000), freshwater (He et al. 2018.1), and coastal marine (Hoeksema 2007; Schumm et al. 2019) flora and fauna. Owing to accelerated diversification rates fueled by habitat dynamics across glacial cycles, the Sunda Shelf has become a cradle of terrestrial and freshwater endemism following its first submergence at ∼400 ka (Cros et al. 2020.3; Husson et al. 2020.4; Salles et al. 2021; Sholihah et al. 2021.1; Garg et al. 2022). The region also exhibits high marine biodiversity but relatively low endemism (Costello et al. 2017.1), suggesting niche filling by dispersion (Ludt and Rocha 2015; Pinheiro et al. 2017.2), which may have turned the Sunda Shelf into a sanctuary for coastal marine biodiversity during interglacial periods.
To understand population structure, evolutionary histories, and climate change-driven vulnerability, we comprehensively sampled the three Asian horseshoe crab species across their natural range. Our results provide baseline data for conservation action geared toward the continued existence of a species group that has survived global change almost unaltered since before the age of the dinosaurs.
Far from being the 'living fossils' that 'prove evolution never happened', these four species of horseshoe crab turn out to be evidence that it has happened, driven by environmental changes that produced the present distribution and diversity, and then stabilised, resulting in a prolonged period of evolutionary equilibrium because the species were optimally adapted to their environments in warm, shallow seas with sandy beaches on which to spawn.
To anyone with the courage to lean about evolution and what causes it, which of course excludes creationists, the horseshoe crabs are not a problem for the Theory of Evolution but a vindication of it.
