Critically endangered Singapore freshwater crab lives in genetically isolated populations

June 1, 2018, National University of Singapore
Singapore freshwater crab (Johora singaporensis), a critically endangered species unique to Singapore. Credit: Daniel NG Jia Jun

Researchers from NUS and National Parks Board (NParks) have established that the critically endangered Singapore freshwater crab (Johora singaporensis) does not mix or breed between different populations, and has overall low genetic diversity.

The International Union for Conservation of Nature (IUCN) considers the Singapore freshwater crab to be one of the 100 most endangered species worldwide. It is uniquely from Singapore and not found anywhere else in the world. Only a few hundred mature individuals of this pebble-sized freshwater crab are known to exist. They live in a few isolated populations in very small areas (less than 0.01 km2) within Singapore that are vital to the survival of the species. As part of the national conservation strategy formulated in 2015, one immediate concern was to establish if these are experiencing inbreeding within their own isolated populations, which may cause them to lose .

A research team led by Prof Rudolf MEIER from the Department of Biological Sciences and Lee Kong Chian Natural History Museum, NUS, partnering the NParks has found that the Singapore freshwater crabs from the remaining populations are indeed physically isolated and unlikely to move and breed between populations.

Presently, levels of genetic diversity are low. Cutting-edge genomic tools were used to analyse the genetic variation of the crabs for this study. The genetic information, together with their migration patterns, were compared with a more widespread and common species, the lowland freshwater crab (Parathelphusa maculata) that often lives in the same places as the Singapore freshwater crab. The researchers used 2,617 and 2,470 genome-wide SNPs (Single Nucleotide Polymorphisms, a common type of genetic variation) from the two groups of about 90 individuals each, with each group belonging to either of the two crab species mentioned above. The study also showed that the dispersal of individuals between populations for both crab species has been hampered by urbanisation, with the amount of genetic diversity being similar in both species despite the Singapore freshwater crab being much rarer.

This study is among the recommended actions described in the Singapore freshwater crab Conservation Strategy that was published in 2015 by the Freshwater Crab Conservation Working Group (FCCWG). The FCCWG includes members from NUS, NParks, and Wildlife Reserves Singapore. The FCCWG continues to meet regularly to discuss the various conservation efforts, and the results of this genetic study will contribute to how the populations are managed.

"The low levels of genetic diversity may reduce the adaptive potential and resilience of the Singapore freshwater crab, and render the species more vulnerable to extinction in the event of disease outbreaks or environmental changes. Coupled with poor mobility and restricted habitat range, their risk of extinction in this era of rapid and erratic climatic changes is even higher, as they are not able to retreat from sites that have become inhospitable to them," explained Dr. TAY Ywee Chieh, who is the first author of the study.

Prof Meier said, "This study found that the crab populations for both species are strongly isolated with low genetic diversity. The most genetically isolated populations for both crab species are separated from other populations by the Upper Bukit Timah Road, one of the oldest roads in Singapore. This suggests that urbanisation may have impacted stream-dwelling crab species in the same manner, regardless of ubiquity, habitat preference or the way they move between streams. Fortunately, signs of inbreeding were not detected for the critically endangered species. However, the genetic distinctiveness and low diversity of the separate populations call for genetic rescue and connecting corridors between the remaining fragments of their natural habitat."

Explore further: Crab nebula of life

More information: Ywee Chieh Tay et al. Roads to isolation: Similar genomic history patterns in two species of freshwater crabs with contrasting environmental tolerances and range sizes, Ecology and Evolution (2018). DOI: 10.1002/ece3.4017

Related Stories

Crab nebula of life

February 14, 2014

Researchers Chu,, have constructed the most complete and extensive crab sequence dataset to date. Their recalibrated crab gene tree using DNA and mitochondrial sequences from 140 species and 58 crab families provides ...

New species of tree living crab found in Western Ghats

April 3, 2017

A recent research paper in The Journal of Crustacean Biology reveals a new genus and new species of tree crab in Kerala, southern India. Known scientifically as the "Kani maranjandu," it is substantially different from other ...

Recommended for you

Researchers engineer a tougher fiber

February 22, 2019

North Carolina State University researchers have developed a fiber that combines the elasticity of rubber with the strength of a metal, resulting in a tougher material that could be incorporated into soft robotics, packaging ...

A quantum magnet with a topological twist

February 22, 2019

Taking their name from an intricate Japanese basket pattern, kagome magnets are thought to have electronic properties that could be valuable for future quantum devices and applications. Theories predict that some electrons ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.