08 August 2011

Have a Crabby National Day!

Singapore freshwater crab;
(Photo by David Maitland, from A Guide to Freshwater Life in Singapore)

It's National Day, and I thought that it would be appropriate to highlight some very special fellow Singaporeans.

Where it comes to food, chilli crab has got to be one of our favourite local dishes. And given that it was created here, it certainly is a serious contender for Singapore's national dish.

(Photo by ladyironchef)

The crabs that we most commonly consume in Singapore are the various species of mud crabs (Scylla spp.), with flower crabs (Portunus pelagicus) coming in a distant second. Both kinds of crab are regularly encountered on many of our shores, especially in shallow waters with muddy or sandy seabeds.

Mud crab, Pulau Pawai;

Flower crab, Pulau Sekudu;
(Photos by Ria Tan)

However, did you know that we have some very special crabs? They're way too small to be eaten, but that doesn't mean that they don't deserve attention from fellow Singaporeans.

Crabs Conquering Creeks

Crabs are quite unique among the crustaceans in being one of the few groups to have successfully invaded the land, with terrestrial species being found in many parts of the world. Even though many of these so-called land crabs still depend on damp environments to keep their gills moist, and need to return to the water in order to reproduce, their ability to survive for long periods of time on dry land has allowed them to stake a claim in a realm otherwise dominated by their cousins, the insects and arachnids.

Because some of these crabs have become so well-adapted to inland habitats, shunning the coasts in favour of forests, freshwater swamps and streams, over time, they have become unable to cross marine barriers. Place a freshwater crab in the sea, and it will die. Similarly, many land crabs have completely lost the ability to swim and breathe underwater, and will drown if submerged. As a result, fluctuations in sea levels have led to populations of freshwater and land crabs being cut off from each other, resulting in the evolution of new species due to reproductive isolation from their counterparts elsewhere.

Surprisingly enough, despite her small size, Singapore is home to six species of freshwater crabs, which are found mostly in streams and swamps in our central forests. Three of these species are endemic to Singapore, meaning that as far as we know, these crabs are found only in Singapore and nowhere else! How's that for something that's 'Uniquely Singapore'?

The Crab-Man

Professor Peter Ng examing specimens in the lab;
(Photo by habitatnews)

It is impossible to discuss the freshwater crabs of Singapore without mention of an important icon in Singapore's natural history and conservation scene, Professor Peter Ng. Our knowledge of our own freshwater crabs is in part due to the efforts of Professor Ng, who described four of the six species occurring here. Currently the director of both the Raffles Museum of Biodiversity Research and Tropical Marine Science Institute, he has helped to inspire generations of students and volunteers, many of whom have gone on to play an active role in research and conservation.

In these photos taken in the late 1980s to early 2000s, the much-younger Peter Ng is shown in the field, sampling in freshwater streams in Malaysia;
(Photos scanned in from Private Lives: An Exposé of Singapore's Freshwaters)

An internationally recognised scientific authority on freshwater crabs in Southeast Asia, Professor Ng has been a part of several field expeditions to many parts of the region, uncovering new species of freshwater crustaceans and fishes formerly unknown to science. In fact, in December last year, Professor Ng was recognised as the most prolific freshwater decapod taxonomist to date, having described 262 species of freshwater decapod crustaceans (crabs, prawns, shrimps, crayfish etc.). Today, he also fronts fundraising and outreach efforts for the upcoming Lee Kong Chian Natural History Museum.

Freshwater Crabs of Singapore: A Who's Who

Singapore freshwater crab;
(Photo by Choy Heng Wah)

The Singapore freshwater crab (Johora singaporensis) was described by Peter Ng in 1986. Found in clean hill streams in Bukit Timah and surrounding areas, this species has become Critically Endangered in recent years, due to degradation of its habitat; it has actually vanished from Bukit Timah Nature Reserve, and is currently known only from two sites, one of which is a single stream near Bukit Gombak. This stream is affectionately known as Polunin Stream, since it flows past the home of the late Dr. Ivan Polunin, a naturalist who also captured many rare colour images of Singapore in the 1950s and 1960s.

Polunin Stream;
(Photo by N. Sivasothi)

Singapore freshwater crab;
(Photo by Choy Heng Wah)

Why did the Singapore freshwater crab disappear from Bukit Timah Nature Reserve? The forests are protected, and there is no development destroying the streams which this crab depends on. Acidification of the streams has been suggested to be a factor. As for one of the final refuges of this unique species, which straddles both private property and military land, development at the headwaters in the military area led to the flow downstream being cut off. A team of volunteers and researchers, together with the Polunins and the Ministry of Defence, stepped in to help salvage this population. For the time being, the Singapore freshwater crab still survives, but just barely.

Singapore freshwater crab in natural habitat;
(Photo by Daniel Ng, from ARKive)

Johnson's freshwater crab;
(Photo by Choy Heng Wah)

Another endemic, the Johnson's freshwater crab (Irmengardia johnsoni), is somewhat more widespread (although still listed as Vulnerable), being found in the Bukit Timah Nature Reserve and various parts of the Central Catchment Nature Reserve. Here it inhabits slow flowing waters with dense leaf litter and mud in well-shaded swamps and forest streams. It too is a relatively recent discovery, being identified and named by Peter Ng and Yang Chang Man in 1985. This species is named after the late Professor Desmond S. Johnson, a prominent researcher on freshwater science and crustaceans at the then University of Singapore.

Johnson's freshwater crab;
(Photo by Kelvin Lim, from Private Lives: An Exposé of Singapore's Freshwaters)

Swamp forest crab;
(Photo by Kelvin Lim, from A classification of living and fossil genera of decapod crustaceans)

The third and final endemic, the swamp forest crab (Parathelphusa reticulata) is the most recent discovery, being described by Peter Ng in 1990. Previously, it had been confused with a more common relative, but the unique colour patterns and closer examination of specimens revealed that it was a different species, and one that was found only within a tiny five-hectare patch within the Nee Soon Swamp Forest. Here it lives in well-shaded swamps, where the water is tea-coloured and acidic, with a pH of 5.0 to 5.5. Naturally, due to its highly restricted range, it is listed as Critically Endangered.

Swamp forest crab;
(Photo by Peter Ng, from The Singapore Red Data Book: Threatened Plants and Animals of Singapore [2nd Edition])

The story of how Peter Ng came to realise that this crab was something special, and the challenges of finding more specimens to study, are recounted in this article about Singapore's natural history. Ever since the days of the Victorian explorers, Sir Stamford Raffles, and Alfred Russel Wallace, we are still continuing to make new and amazing discoveries about the other lifeforms found here in Singapore.

The other three species of freshwater crab are found not just in Singapore, but also elsewhere in Southeast Asia.

Lowland freshwater crab;
(Photo by Choy Heng Wah)

The lowland freshwater crab (Parathelphusa maculata) is the most common of our freshwater crabs. It lives in lowland streams, and is capable of surviving in muddy waters and stagnant pools. Hence, it can tolerate a degree of siltation, like that caused by logging and clearing of surrounding forests. Besides forest streams in the Central Nature Reserves of Singapore, it has also been recorded in Peninsular Malaysia and Sumatra, with the species being named by Johannes Govertus de Man in 1879, based on specimens collected in Sumatra.

Lowland freshwater crab in natural habitat;
(Photo by Teo Siyang)

Peracca's land crab, Macritchie;
(Photo by Tan Heok Hui & Peter Ng)

Peracca's land crab (Geosesarma peraccae) is semiterrestrial, digging deep burrows in soft mud along the banks of slow streams and swamps. It is known from the Bukit Timah Nature Reserve, the nearby Greenbank Park area, as well as the Nee Soon Swamp Forest and forests around Macritchie Reservoir in the Central Catchment Area. It is listed as a Vulnerable species in Singapore. Elsewhere, it has been recorded in Peninsular Malaysia, in Johor and southern Pahang, and was named in 1903 by Giuseppe Nobili in honour of Dr. Mario Giacinto Peracca, who had first obtained the type specimens from Singapore. This crab exhibits very interesting and unusual behaviour, entering the cups of the narrow-lidded pitcher plant (Nepenthes ampullaria), possibly in search of food, or to wet its gills during dry periods.

Peracca's land crab inside pitcher, Macritchie;
(Photo by Tan Heok Hui & Peter Ng)

Little land crab, Upper Peirce;
(Photo by Cai Yixiong)

The final freshwater crab in Singapore, the little land crab (Geosesarma nemesis), was also described by Peter Ng in 1986, based on specimens collected from Bukit Timah. Its name is derived from the name of the mythical Greek goddess of divine anger and retribution, Nemesis, alluding to the bright red colours and fierce disposition of adult crabs. Endangered in Singapore, where it is found not only in Bukit Timah Nature Reserve, but also in one or two small streams in the Bukit Gombak area, it has also been recorded from Gunung Panti and Gunung Pulai in Johor. It is apparently a species that prefers highland areas, being replaced in areas of lower elevation by its relative, Peracca's land crab. However, in Singapore, it is possible to find both species together, possibly due to disturbance as a result of land development.

Little land crab;
(Photo by Choy Heng Wah)

Both of these land crab species are known to climb trees, which probably shouldn't be very surprising, since they belong to the Sesarmidae, the same family as the tree-climbing crabs (Episesarma spp.) commonly seen in mangroves.

Little land crab on leaf;
(Photo by Peter Ng, from The Singapore Red Data Book: Threatened Plants and Animals of Singapore [2nd Edition])

Leaving the Sea Behind

It goes without saying that the greatest diversity of crab species is found in the sea. Yet most marine crabs reproduce in the same way; after mating, the female broods her eggs beneath her abdomen. While they incubate, she looks as if she is carrying a mass of tiny berries.

This can be seen in this female floral egg crab (Atergatis floridus);
(Photo by James Koh)

When the young hatch, they do not look like crabs at all! Instead, this is known as the zoea stage, and the tiny larvae drift with the plankton, feeding on even tinier organisms.

Crab larva, zoea stage;
(Photo from Science Photo Library)

Subsequently, as they mature, they transform into the megalops or megalopa stage, and start to look somewhat more crab-like. These are still free-swimming in the plankton, but tend to remain close to the seabed.

Crab larva, megalops stage;
(Photo from Science Photo Library)

Eventually, the larva moults, and turns into a tiny juvenile crab, settling on the seabed.

Juvenile crabs;
(Photo by bebexvy)

Crab life cycle;
(Diagram from Smithsonian Environmental Research Center)

Although crabs are mostly marine, there are many species of crab that are able to inhabit freshwater, or have become fully terrestrial, often in habitats far from the coast. Still, many are unable to fully shake off their ancestral bond to the sea, producing young which still need to complete their development in a marine environment. Similarly, there are numerous species of crab in mangroves and river estuaries which are able to live in freshwater for some time, but ultimately still have to return to the sea to breed. As a result, they don't necessarily qualify as 'true' freshwater crabs.

For example, the paddler crab (Varuna yui) of Southeast Asia (including Singapore) is often found in freshwater far inland, but the larvae still need to develop in saltwater. You could argue that this counts as a seventh species of freshwater crab in Singapore, since it can live for an extended period of time in freshwater, but its life cycle is still tied to the sea.

Paddler crab seen in a pond at Punggol Park;
(Photo by Marcus Ng)

This is the case even for some crabs that spend nearly their entire lives on land. For example, the famous red crabs (Gecarcoidea natalis) of Christmas Island inhabit inland forests, but undertake masive annual migrations to the coast in order to breed.

What makes 'true' freshwater crabs stand out from the rest is that they are able to complete their entire life cycle in freshwater. In these crabs, the free-swimming larval stage has been lost. Instead, the females bear very large yolky eggs in comparison to those produced by marine crabs. These eggs are carried around and brooded beneath her abdomen, where they hatch into tiny fully-formed crabs. The juvenile crabs subsequently disperse, having skipped the most vulnerable larval stages and getting a headstart in life.

Singapore freshwater crab carrying eggs;
(Photo by Peter Ng, from Institute of Zoology)

Johnson's freshwater crab brooding young;
(Photo by Choy Heng Wah)

Geosesarma, on the other hand, produces free-swimming larvae, except that these do not drift with the plankton, but develop in the freshwater within their mother's burrow. They also require a much shorter span of time to mature, and do not need to feed, since they have ample yolk reserves to sustain them during their larval stages.

Little land crab with young;
(Photo by Daniel Ng)

As a result, these crabs have become capable of breeding without having to return to the sea at all. In this manner, freshwater crabs have managed to colonise rivers, streams and lakes in many parts of the world. There are even crabs so specialised for living in complete darkness, in subterranean streams that flow through caves, that they have turned virtually white and lost their eyes.

Blind cave crab (Cerberusa caeca) in Mulu Caves, Sarawak;
(Photo by Alan Cressler)

But the most extreme case has to be the inland freshwater crab (Austrothelphusa transversa) of Australia, which has even adapted to live in desert areas! During the dry season, the crab remains dormant in its deep burrow, where the humidity remains high, and the crab is able to absorb water through condensation. Only when the rains arrive and flood low-lying areas do the crabs emerge, feeding and taking advantage of this brief period to reproduce.

Inland freshwater crab, Western Australia;
(Photo by Adrian Boyle)

Singapore's own freshwater crabs are not that extreme, living in shady forest streams and freshwater swamps. Shy and retiring by nature, they are largely nocturnal, and hide under rocks, leaf litter and other debris. They also seek refuge from predators by digging burrows along the banks.

Where it comes to feeding, freshwater crabs are omnivores. A lot of their diet is made up of fallen leaves, which they shred and macerate with their pincers and mouthparts, hence playing an important role in the recycling of nutrients. They will also scavenge on carrion and other decaying animal matter, as well as actively prey on worms and snails.

Can eat or not?

Given how much we love food, the inevitable question where it comes to discussing our freshwater crabs is whether they are edible. Crab lovers racing to fish for crabs at the nearby reservoir or monsoon drain are bound to be disappointed, for two main reasons.

The first is that all of our freshwater crab species are restricted to forest streams, and nearly all are vulnerable to extinction. The bulk of the surviving populations live in places that are currently protected, whether as nature reserves or military training areas. It goes without saying that poaching of these crabs for human consumption will definitely not be condoned, especially since we are talking about highly endangered crabs found only in Singapore, and nowhere else in the world.

The other point is that compared to the crabs we find at the market or seafood restaurants, these crabs are pathetically tiny.

Singapore freshwater crab;
(Photo by Teo Siyang)

Johnson's freshwater crab and Singapore freshwater crab both measure 2 to 3 centimetres across the carapace, while Peracca's land crab and little crab are even smaller, at 1 to 1.5 centimetres.

The largest of our freshwater crabs is the lowland freshwater crab, but even so, its size isn't very impressive at all, at just 4 to 5 centimetres. The swamp forest crab reaches a similar size.

(Top photo by Teo Siyang, other photos by Daniel Ng)

I personally think that it's simply not worth the effort catching such small crabs for food. There isn't enough meat to justify going through all the trouble.

Still, freshwater crabs are regularly consumed in other countries.

The European freshwater crab (Potamon fluviatile) has been eaten since the time of the ancient Greeks and Romans;
(Photo by mauro_morando)

The Japanese freshwater crab (Geothelphusa dehaani), known to the Japanese as sawagani (さわがに), is often fried until crispy, then lightly salted. Small enough to be eaten whole, you simply pop the entire crab into your mouth like popcorn shrimp;
(Photo by Damon Bay)

(Photo by yummyinthetummyblog)

One particular species of freshwater crab is highly popular as food in many parts of Asia. This is the Chinese mitten crab (Eriocheir sinensis), which is found in rivers, lakes, and estuaries in Korea and China. It gets its name from the dense covering of hairs on its pincers, making it appear as if its wearing a pair of fuzzy mittens.

Chinese mitten crab;
(Photo by Monika Štambergová)

Like its relative the paddler crab, the Chinese mitten crab spends most of its life in freshwater, but still depends on the sea for reproduction. Mature adults migrate downstream to estuaries where they mate, after which the females will travel further offshore, overwintering in deeper waters. In the spring, the females then return to brackish water to release their larvae. After going through the larval stages, the young crabs make their way back up rivers, even travelling overland to reach reservoirs and lakes.

Chinese mitten crabs fresh and ready for the kitchen;
(Photo by me, i am zoe)

This is the famed 'hairy crab' that features as a popular delicacy in Shanghai cuisine. Large quantities are harvested and sold live in the markets, with specimens hailing from Yangcheng Lake in Jiangsu Province being the most valuable. Autumn is often thought of as the best season to eat Chinese mitten crabs, as this is when the adults have built up substantial energy reserves for their migration downstream, and their gonads have become well-developed in preparation for breeding; during this time, the females are full of roe. They are most often steamed, served with vinegar, then slowly taken apart with bare hands. Demand for hairy crabs is so great that you can now even buy Chinese mitten crabs (chilled but still alive) at vending machines!

Chinese mitten crabs ready for consumption;
(Photo by Simon Dang)

There is one other reason why freshwater crabs are not necessarily an ideal food: great care must be taken to ensure that they are well-cooked. If the crabs are eaten raw or half-cooked, there is a high risk of the person becoming host to oriental lung flukes (Paragonimus westermani). These are parasitic worms that have a relatively straightforward life cycle. Larvae first grow and mature inside a freshwater snail, before moving on and continuing to grow inside freshwater crustaceans such as crabs or crayfish, turning into cysts that lay dormant until consumed. Once the crab is eaten by a mammalian predator (humans are included), the cysts develop into worms that burrow through the intestinal wall, diaphragm and the rest of the body until they reach the lungs, where they mature. Eggs are released into the environment when the host coughs up sputum, or they are swallowed and eventually passed out in the faeces, beginning the cycle anew. While infestations may cause diarrhea, abdominal pain, fever and cough, there are incidents where the worms make a wrong turn, and instead of ending up in the lungs, reach the brain or spinal cord, with paralysis as a possible consequence. The risk of infection has prompted authorities in some places to issue health advisories after people caught flukes due to inadequate precautions.

Life cycle of the oriental lung fluke;
(Diagram by Division of Parasitic Diseases and Malaria)

Freshwater Crabs as Pets

In recent years, various species of freshwater and land crab are becoming highly sought after, not for food, but as pets. Like their counterparts in the marine aquarium hobby, many aquarists with freshwater aquariums have a preference for invertebrates, including crustaceans such shrimps, crayfishes, and crabs. Many of these crabs possess bright colours, and a number also adapt well to captivity, as long as water quality is well taken care of, and adequate hiding places are provided. Finding the right sort of food is also usually not much of an issue as well, since these crabs will usually feed on anything. Special attention should be paid where it comes to providing the right conditions when the crab is moulting, a time when it is very vulnerable.

Lowland freshwater crab with moult;
(Photo by David Maitland, from The Freshwater Crabs of Peninsular Malaysia and Singapore)

The main issue, however, is whether the trade relies almost entirely on wild-caught specimens, or if there is an active effort to get these crabs to reproduce in captivity. Like many other exotic pets, demand can far outstrip supply, and overcollection might decimate wild populations of some freshwater crab species. Considering that many species have a very small range, since they are often limited to a single drainage basin or water catchment, harvesting for the pet trade might put some of these crabs at risk of becoming extinct.

On the other hand, captive collections provide insurance against the event that some calamity befalls populations in the wild. Raising them in captivity also enable us to find out more about the biology of these crabs, and can provide an invaluable resource for us to learn possible strategies to save endangered species. You can find photo galleries of four of our native freshwater crabs in captivity on this German hobbyist website dedicated to crabs - the Singapore freshwater crab, lowland freshwater crab, Peracca's land crab, and little land crab. Some of these crabs appear to have been sent to aquarists with the experience and expertise in raising freshwater crabs in captivity, and by collaborating with researchers, these hobbyists play an active role in research on the behaviour, ecology and reproduction of these crabs, as well as in conservation, by boosting numbers through captive breeding.

Because some of these crabs still require saltwater for the young to develop, captive breeding is bound to occur only very rarely, and the odds of raising the larvae to maturity are slim. Those species which produce young capable of developing in freshwater are more likely to be bred successfully, and may hence help build up captive breeding stocks, so as to relieve pressure on wild populations.

For instance, one of the land crabs most commonly offered for sale, both here in Singapore and elsewhere, is the rainbow land crab (Cardisoma armatum). Hailing from mangroves and other coastal areas in west Africa, this crab spends a lot of time out of water, but needs to return to the ocean to breed. As a result, despite it being very common in captivity, stories of successful captive breeding are few, and most of the crabs seen are still wild-caught.

Rainbow land crab;
(Photo by Claudia Barzaeva)

Here is a selection of other freshwater and land crab species found in the pet trade:

The Halloween crab (Gecarcinus quadratus) originates from Central America;
(Photo by David Reed)

The bicolor vampire crab (Geosesarma bicolor) (Note: 'vampire' only in name - it doesn't drink blood!) is from western Java;
(Photo by Andreas Werth)

The panther crab (Parathelphusa pantherina) is found only in Lake Matano in Sulawesi;
(Photo by Jérôme Picard)


In Singapore, our freshwater crabs face a variety of threats, including pollution of their habitats, due to runoff from development projects in the area, or even spraying of oil or pesticides on water bodies to prevent mosquito breeding. There is also the risk of people encroaching on protected areas, trampling and destroying the banks of the streams where these crabs live. Poachers who enter our forests deliberately in search of native aquatic life for their aquariums (or even as feeders for predatory fish) are another threat; there are apparently incidents where people have casually come across freshwater crabs and then attempted to keep them as pets.

Lowland freshwater crab;
(Photo by Choy Heng Wah)

But the greatest danger to our freshwater crabs probably comes from habitat loss; many of our streams have been lined with concrete and turned into drains, while development of surrounding areas has either completely destroyed streams and swamp habitats, or caused them to be clogged up with silt. For instance, the status of the Singapore freshwater crab is precarious, and depends a lot on the water level and water quality in the two sites where it is still known to persist. Similarly, the continued survival of our other native freshwater crabs is dependent on continued protection of the forests they live in. Poor drainage planning, pollution, or deforestation can all very easily wipe out the habitats they are restricted to.

There is an intriguing single record of a land crab from the Paya Lebar area in 1938; The brown land crab (Discoplax hirtipes) is commonly known as the Christmas Island blue crab (due to the bluish coloration seen in the population inhabiting Christmas Island), although the species actually has a wide distribution across the Indo-Pacific, including Madagascar and Mauritius, the Andaman Islands, southern Japan and Taiwan, Australia, and Fiji all the way to the Hawaiian Islands. (EDIT: The Christmas Island blue crab was recognised as a distinct species in a 2012 paper in the Raffles Bulletin of Zoology)

Christmas Island blue crab (Discoplax celeste);
(Photo by Robin Ngiam)

If the brown land crab did indeed inhabit Singapore in the past, it is most likely to have become extinct sometime in the 20th century, as a result of extensive deforestation. However, it could also represent a record of an escapee from captivity. (EDIT: The brown land crab has been rediscovered in Singapore in 2015!)

Brown land crab, Taiwan;
(Photo by Sin Syue Li)

In July 2015, a land crab was spotted in a drain on St. John's Island, to the surprise of many people working at the Tropical Marine Science Institute (TMSI). After not having been seen for more than 70 years, it was a most unexpected sighting of what turned out to be the brown land crab!

(Photo by St. John's Island Marine Laboratory)

What are the chances of you finding something new while walking to your office?

Our colleagues did just that this morning! A terrestrial crab of the genus Discoplax was found in the drain on the island. We have yet to determine the species but it could be a new record for Singapore.

UPDATE: This specimen could be Discoplax hirtipes, which has been presumed to be nationally extinct! Apparently, the last record was that of a single female found in Paya Lebar about 77 years ago. In any case, this is the first record of Discoplax on St. John's Island.

As reported in The Straits Times, it's worth trying to find out if this crab is the only representative of her species here, or is part of a population lurking in St. John's Island or elsewhere in Singapore. It's also possible that this species didn't actually go completely locally extinct, and that a small and isolated population has managed to survive all along, only to be rediscovered as a result of increased sampling and greater awareness of Singapore's marine life.

On a global scale, freshwater crabs as a whole are threatened with extinction, with a 2009 assessment reporting that possibly up to two-thirds of all species may be at risk, with one out of six species being particularly vulnerable. The survey looked at the conservation status of 1,280 known species of freshwater crab, and determined that 227 species should be considered globally Near Threatened, Vulnerable, Endangered or Critically Endangered. For another 628 species, not enough data exists to adequately assess their population structure, which means that some of these might be highly endangered, just that there is a lack of studies and data to confirm that these crabs are under some risk of extinction. Optimistically, it means that 16% of all freshwater species are at risk, while the worst case scenario suggests the figure could be as high as 65%, or two-thirds of all species. Some species have not been seen for some time, and the original habitats where they were found have since been developed or destroyed. It's too early to officially declare them extinct, but there is always some hope that undiscovered populations linger somewhere. Habitat destruction is the main culprit here, with critical forest streams and swampland being logged and converted for agriculture and urban development. The paper also cited the Singapore freshwater crab and swamp forest crab as examples of two species on the brink of extinction, and extremely dependent on habitat protection for their continued survival.

Swamp forest crab;
(Photo by Peter Ng, from A Guide To Threatened Animals Of Singapore)

Another possible threat stems from introduced species. It is fair to say that many of our freshwater habitats are now home to a great number of non-native species, some of which may have deleterious effects on other species around them. In recent years, the Australian red-claw crayfish (Cherax quadricarinatus), popular in the pet trade, has been recorded in several of Singapore's reservoirs and other water bodies. Ecologically, it is quite similar to our freshwater crabs, and while it has yet to invade the streams and swamp forests of our Central Nature Reserves, there is always the risk that should the crayfish establish itself in these habitats, it will end up outcompeting our crabs for resources such as food and burrowing sites, or even preying on them.

Red-clawed crayfish;
(Photo by planet_b)

Red-clawed crayfish caught at Upper Peirce Reservoir;
(Photo by Tan Heok Hui)

Red-clawed crayfish caught by recreational fishermen at Kranji Reservoir;
(Photo from Shane Ahyong and Darren Yeo)

Ironically, the Chinese mitten crab has been inadvertently introduced into waterways in Europe and North America, with fears that it will negatively compete with the native crayfish species found there. One solution that has been suggested is for people to start eating this invasive; after all, since it's so highly prized by the Chinese, we might as well learn to exploit them as a resource, in the hope that this will reduce their numbers.

Other non-native species which might pose a threat to our freshwater crabs include predators such as peacock bass (Cichla spp.), or American bullfrog (Lithobates catesbeianus), which would most probably treat our native crabs as delightful crunchy little snacks.

Peacock bass caught at Bedok Reservoir;
(Photo by Sim Kian Peng)

Two non-native species of turtle might also be possible predators of our freshwater crabs should they be released into our forest streams; these are the red-eared slider (Trachemys scripta elegans) and Chinese softshell turtle (Pelodiscus sinensis), seen here at the Singapore Botanic Gardens;
(Photo by Daniel Koh)

While these invasives are still found largely in the open habitats of our reservoirs and other urban waterways, it is always possible that they may eventually invade our forest streams, the last refuge for many native aquatic species. All it takes is for one irresponsible individual to release a few bullfrogs into a forest stream, and it could wipe out an entire population of freshwater crabs.

American bullfrogs released into a stream in the Central Catchment Area;
(Photos by Shirley Ng)

One final threat that looms over not just our freshwater crabs, but also casts uncertainty over the future of myriad other endangered plants, fungi, and animals in Singapore, stems from ignorance and apathy, and the erroneous assumption that Singapore does not harbour any natural heritage worth protecting. I find it a shame that not more people are aware of the fact that despite all the development and loss of habitats over the years, Singapore still harbours a rich array of biodiversity, including species unique to our country. Yes, it is a statement that is often repeated as part of outreach and education efforts, but it is sad that many people don't seem to understand or appreciate the significance of having three species of freshwater crab found only here in Singapore.

In 1992, these three endemics were featured as part of a series of stamps, alongside the mosaic crab (Lophozozymus pictor), an Endangered marine species.

Top left: Mosaic crab;
Top right: Johnson's freshwater crab;
Bottom left: Singapore freshwater crab;
Bottom right: Swamp forest crab;
(Image from SG Stampers)

First day cover for this stamp series;
(Image from Tan Hsiao Wei)

The plight of these unique crabs has also been mentioned on several occasions, particularly by their discoverer, Professor Peter Ng. Not just in his lectures and public talks, but also in the media and several books on Singapore's biodiversity. For example, recent publications featuring the freshwater crabs of Singapore include Private Lives: An Exposé of Singapore's Freshwaters, and The Singapore Red Data Book: Threatened Plants and Animals of Singapore [2nd Edition]. The famous Science Centre Guidebook series also mentions our freshwater crabs in two of its releases, A Guide To Freshwater Life In Singapore and A Guide To Threatened Animals Of Singapore.

A very useful reference book is The Freshwater Crabs of Peninsular Malaysia and Singapore, written by Professor Peter Ng himself. Published in 1988, it provides a comprehensive list of all the known species of freshwater crab in this part of the world.

Unfortunately, there is no entry for the swamp forest crab, but that was because at that time, it was suspected to be merely a variant of the lowland freshwater crab. It is a book that provides lots of relevant background information about our freshwater crabs, such as their identifying features and ecology, and I have turned to it on many occasions in the process of writing this post.

Several students have carried on Peter Ng's legacy, taking a closer look at the ecology and conservation of these crabs; several years ago, Daniel Ng did his Honours thesis on the Singapore freshwater crab. Another student, Chua Yi Teng, carried out a study on the Johnson's freshwater crab. Through their efforts, we are uncovering more secrets about these often elusive crabs, and learning ways to protect their remaining habitats.

It is important to highlight that these three species are found only here in Singapore, and nowhere else in the world. Barring undiscovered populations in Johor, and those in captive collections, should some disaster wipe out say, that tiny patch of Nee Soon Swamp Forest where the swamp forest crab lives, that would truly be a catastrophic blow for the species. In most cases when a species becomes locally extinct, we can usually comfort ourselves by saying that it's not so dire, that there are still some populations left elsewhere in Malaysia or Indonesia. However, should the Singapore freshwater crab, Johnson's freshwater crab, or swamp forest crab disappear completely from our country, there is no such consolation.

Johnson's freshwater crab, Raffles Museum of Biodiversity Research;
(Photo by Ivan Kwan)

It would be unthinkable if we allowed these species to disappear due to a lack of commitment towards protecting these icons of Singapore's natural heritage, leaving behind nothing but museum specimens to remind us of what we once had. Just like how Singaporeans were up in arms over an attempt to claim chilli crab as a Malaysian dish, perhaps it is time more of us kicked up a fuss, and played a part in preventing our very special crabs from becoming extinct.

Peracca's land crab;
(Photo by Tan Heok Hui, from Private Lives: An Exposé of Singapore's Freshwaters)

Not all of us can be freshwater ecologists or specialists on crab taxonomy, but at the very least, tell the people you know that this small island does have invaluable pockets of nature that are worth protecting, and share about the crabs found only here in Singapore. In the end, before we can talk about what we can do to ensure that we don't lose even more species, Singaporeans need to know what natural treasures we have in the first place. Allowing an endemic species to vanish off the face of the earth simply because more of us didn't care enough would be a tragedy that Singapore could never be proud of.

Singapore freshwater crab, museum specimen;
(Photo by Peter Ng, from ARKive)

The fact that we have endemic freshwater crabs which have managed to weather the challenges and survive against all odds to the present day, actually mirrors the nation's struggles and development in some way. Perhaps, if these crustaceans could talk, they might borrow from #IamSingaporean, a very popular hashtag that has been making its rounds among local Twitter users in recent weeks, and declare how they are found only here and nowhere else on the planet. You can't get any more Singaporean than that.

1 comment:

Anonymous said...

I was looking for rainbow crab info and saw your post. Great stuff and good to know we have quite a species in sg! Interesting read

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