Preserving Asian horseshoe crab populations through targeted conservation strategies

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Jan 26, 2025, 23:06 ET

Biologists from the National University of Singapore 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

SINGAPORE, Jan. 26, 2025 /PRNewswire/ -- Horseshoe crabs are often referred to as the "living fossils" of our planet — the four known species, including three in Asia and one in North America, remain nearly identical to their ancient relatives from hundreds of millions of years ago. These arthropods are a fundamental building block of coastal marine ecosystems. Their eggs, for example, serve as a major food source for shorebirds, some of which have evolved to time their migrations to coincide with peak horseshoe crab spawning activity. In addition to their ecological role, horseshoe crabs are also used in biomedicine to test for harmful toxins in vaccines.

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 National University of Singapore (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," said Assoc Prof Rheindt. "This foundational knowledge will enable us to develop targeted conservation strategies and prioritise habitats critical for their survival."

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," said Dr Tang Qian, the first author of the study. "Genomic data also helps us pinpoint coastal hotspots that should be prioritised for conservation."

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," said Dr Tang. "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."

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," added Assoc Prof Rheindt. "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."

SOURCE National University of Singapore