Biologists have carried out the first detailed population study on all three species of Asian horseshoe crabs, investigating their population distribution, evolutionary backgrounds, and susceptibility to climate change in order to propose specific conservation strategies.
Horseshoe crabs are often called the “living fossils” of Earth, as the four existing species—including the three found in Asia and one in North America—have remained almost unchanged from their ancient ancestors that lived hundreds of millions of years ago. These creatures play a vital role in coastal marine ecosystems. Their eggs, for example, are a significant food source for shorebirds, some of which have adapted their migration patterns to match the times when horseshoe crabs are spawning. Moreover, horseshoe crabs are important in biomedicine, as they’re used for testing vaccines for harmful toxins.
Of the four species, only the Atlantic horseshoe crab (Limulus polyphemus), found along the eastern United States and the Gulf of Mexico, has been thoroughly studied. In contrast, there is limited and disjointed scientific knowledge about the three species from Asia. As a result, the IUCN Red List, which monitors the extinction risk of various species globally, has classified two of them (the mangrove horseshoe crab and the coastal horseshoe crab) as “data deficient,” meaning there isn’t enough information to determine their risk of extinction. Meanwhile, the tri-spine horseshoe crab is categorized as endangered.
Understanding our planet’s living fossils
To address these knowledge gaps, a research team led by Associate Professor Frank Rheindt from the Department of Biological Sciences at the National University of Singapore (NUS) conducted the first thorough population genomic investigation of all three Asian horseshoe crab species: the mangrove horseshoe crab (Carcinoscorpius rotundicauda), the coastal horseshoe crab (Tachypleus gigas), and the tri-spine horseshoe crab (Tachypleus tridentatus).
This study emphasizes the significance of Southeast Asia’s Sunda Shelf, a shallow marine area, as an essential coastal ecosystem. This region has supported the survival of these ancient arthropods for thousands of years and has the potential to serve as a refuge for Asian horseshoe crabs in the face of rapid human-induced climate change.
The researchers also created a foundational genomic dataset for these species, which will assist in developing targeted conservation efforts. Their results, which suggest distinct conservation strategies for each species, were published in Conservation Letters on December 16, 2024.
Back to the basics: Filling data gaps to advance conservation efforts
Assoc Prof Rheindt emphasized, “To conserve these species effectively, it is essential to first understand their basic population structure, evolutionary backgrounds, and vulnerabilities related to climate change. This foundational knowledge will help us devise targeted conservation initiatives and prioritize habitats critical for their survival.”
Monitoring and tracking Asian horseshoe crabs present inherent challenges. They remain primarily on the seabed for most of their lives, making them hard to observe, and it takes them 14 years to reach maturity—too long for conventional surveys to effectively detect population changes. To tackle these issues, the research team utilized population genomic techniques, analyzing DNA from 251 horseshoe crabs collected from 52 sites across 11 countries.
This genomic data allowed NUS researchers to establish the first genomic baseline for Asian horseshoe crabs, which helped them map population structures and identify genetic boundaries among the three species. “Such distinctions are key, as they reveal populations with unique genetic traits vital for adapting to specific local conditions,” explained Dr. Tang Qian, the study’s primary author. “Genomic data also enables us to identify coastal hotspots that should be prioritized for conservation efforts.”
The research uncovered how horseshoe crabs have adapted to environmental changes over time. The Sunda Shelf has proven to be a crucial refuge for horseshoe crabs during past climate fluctuations. By piecing together the evolutionary histories of the species, the researchers discovered that the area has preserved genetic diversity and functioned as a migratory route, allowing populations to stay connected despite varying environments.
Tailored conservation strategies needed
The study indicated that future climate change presents different levels of risk to the three Asian horseshoe crab species. While all three are vulnerable, their adaptability varies. For example, the mangrove horseshoe crab’s limited ability to disperse makes it more susceptible to local extinction than the more mobile coastal and tri-spine horseshoe crabs.
Based on their findings, the researchers propose specific conservation strategies for each species to enhance their adaptation to climate change:
- Mangrove horseshoe crabs
- Protect and restore mangrove habitats, crucial for the species’ survival and their ability to migrate south as temperatures rise.
- Prioritize conservation efforts for populations in the Gulf of Tonkin and South China, as they are subject to the highest evolutionary pressures from climate change.
- Coastal horseshoe crabs
- Conserve the Sunda Shelf region, a vital refuge, particularly around the Bay of Bengal, the Malacca Strait, and southern Vietnam.
- Maintain connectivity among populations by protecting coastal corridors to reduce the species’ exposure to habitat fragmentation.
- Tri-spine horseshoe crabs
- Implement sustainable fishing regulations and restore coastal habitats, particularly in heavily developed areas like Japan, Taiwan, and China.
- Focus conservation efforts on mitigating human-induced threats such as overharvesting and habitat destruction, as these are currently more significant risks than climate change.
Next steps
Dr. Tang remarked, “Our study is a vital foundation for the conservation of essential habitats necessary for the future survival of horseshoe crabs. However, it is essential to note that our research hinges on environmental factors and does not consider future human activities, like coastal development, that may directly impact habitats. Thus, the survival of horseshoe crabs will hinge critically on interventions tailored to local contexts.”
Looking forward, the researchers aim to delve deeper into the evolutionary potential of Asian horseshoe crabs, including examining how specific functional genes help them adapt to local settings and changing climatic conditions.
Assoc Prof Rheindt added, “We have established the Horseshoe Crab Global Biorepository at the Lee Kong Chian Natural History Museum at NUS to support ongoing and future research.” Through this initiative, they hope to promote partnerships and secure funding to drive genomic research on horseshoe crabs, currently collaborating with the Chinese University of Hong Kong on research specifically focusing on the tri-spine horseshoe crab.
