A popular aquarium pet, the red-claw crayfish has made its way into the wild.
It is among a growing number of alien species taking over reservoirs in Singapore. But unlike most invasive fish which cannot leave the water, it has crawled up into freshwater streams, home to our precious native crabs found nowhere else in the world.
Since the crayfish can outgrow and out-eat the local crustaceans, it could well wipe them out.
"Native species likely play important ecological roles in our local natural ecosystems that non-native species may not necessarily be able to replace," said Assistant Professor Darren Yeo, a freshwater ecology expert at the National University of Singapore's department of biological sciences.
"They are part of our natural national heritage, so they deserve our appreciation and effort, as custodians of the land, to protect them."
Prof Yeo, 43, and his graduate student, Mr Zeng Yiwen, 28, have therefore stepped up to study how extensive the crayfish are, and the potential damage they could cause.
Mr Zeng is pitting crab against crayfish in the laboratory, to see which emerges the victor.
He places the crayfish, which are more than double the size of lowland crabs - the most common of the country's freshwater crabs - and the lowland crab in tanks containing only one burrow.
"The crab usually retreats into the burrow slightly, before both the crayfish and the crab start jostling for ownership of the burrow," said Mr Zeng.
Preliminary data shows the larger crayfish is more likely to secure the shelter, but its status as an invasive species, one that harms native populations, is still not fully known.
Said Prof Yeo: "A competition experiment under controlled laboratory conditions is one thing; the real test is in the field where other factors might affect the outcome differently.
"The invasive effects (of the crayfish) could be seen in other ways, such as modifying or disturbing the habitat by burrowing or foraging in the bottom sediment."
This is why Mr Zeng is also trying to find out where the crayfish have spread to, and to understand the environmental conditions of the streams where they can be found.
Also known by the scientific name Cherax quadricarinatus, the crayfish, which can grow up to 25cm from head to tail, are native to freshwater habitats in northern Australia and Papua New Guinea.
The first record of this species from a reservoir in Singapore dates back to 2001, and it was most likely to have been released illegally by people who bought them from the aquarium trade.
"Now it seems that they're spreading into the streams, with reproducing populations in several streams," said Mr Zeng.
Prof Yeo, who is supervising Mr Zeng's research, added: "A lot of non-natives are in the reservoirs while a lot of the natives are in the forest streams and freshwater swamps. They tend not to mix and we tend to assume that it's the environmental differences that keep them apart.
"But now, one of the things that we are learning is that it may not be just environmental differences. Other factors such as interactions between native and non-native species also need to be considered."
Unlike fish which cannot swim out of the reservoirs into streams, these crayfish are able to make their way there.
This despite challenging conditions such as higher acidity levels and lower calcium concentrations in streams, which could hinder their ability to create strong shells, grow and reproduce.
So far, the crayfish have been spotted in four streams in the Central Catchment Nature Reserve which, at 3,000ha, is the largest of its kind in Singapore, encompassing the Nee Soon Swamp Forest.
These streams contain native crabs such as the lowland freshwater crab and the Johnson's Freshwater Crab, listed as a vulnerable species by the International Union for Conservation of Nature.
Like the swamp forest crab, the Johnson's Freshwater Crab is found in Singapore and nowhere else.
The National Parks Board lists 88 freshwater fish species in local reservoirs alone.
Among them, 52 are alien species, a marked increase from the 31 recorded in 2007, out of 68 species in total. These include large predators such as the Peacock Bass and Giant Snakehead or Toman.
However, the fish list is by no means complete.
NParks, national water agency PUB and the Agri-Food and Veterinary Authority of Singapore said in a joint reply to The Straits Times: "This list is not exhaustive as it includes only records reported from reservoirs and does not account for alien species in other freshwater water bodies, such as forest streams and urban canals.
"With increasing ornamental fish trade and cases of public release of unwanted fish, it is expected that there will be more of such non-native species out there."
Prof Yeo has a word of warning: "The more alien species there are in the reservoirs, the higher the chances that eventually one or more might be able to penetrate natural forest streams and potentially cause impacts."
Invasion of the aliens: The marine chapter
The red-claw crayfish (above) is adaptable.PHOTOS: ZENG YIWEN, TAN HEOK HUI
Straits Times 23 Jul 15;
Singapore's native fish are no longer masters of their inland domains - they have been overwhelmed by invasive species, which make up over half the country's freshwater denizens now.
And few local fish stand a chance against voracious predators lurking in reservoirs such as giant snakeheads, arowana and peacock bass.
Of 88 freshwater fish species here, 52 are alien, compared to 31 alien species out of a total 68 in 2007.
"If a native fish encounters any of these large predatory non-native fish, they will not stand a chance," said ichthyologist (fish specialist) Dr Tan Heok Hui, museum officer at the Lee Kong Chian Natural History Museum.
Alien species refer to animals or plants which are not native to a particular ecosystem.
Many of the fish that first made their way into reservoirs here were released by aquarium hobbyists.
And the figures may represent the tip of the iceberg.
The National Parks Board, national water agency PUB and the Agri-Food and Veterinary Authority of Singapore say the list is by no means exhaustive - it does not account for aliens in other freshwater bodies such as forest streams and urban canals.
Examples of alien fish species include the highly adaptable African sharptooth catfish, which was first spotted in the 1990s and seems to have displaced smaller native catfish in habitats such as ponds, reservoirs and canals.
There is also the freshwater stingray, which is capable of injecting venom through a sting at the base of its tail. The authorities say its establishment in Singapore was confirmed in 2010 when a qualitative survey was conducted of the Upper Seletar Reservoir.
Most alien species are known to be hardy and are capable of establishing quickly in new environments, the three agencies say.
They multiply in large numbers and compete with the native species for food and shelter.
These alien species can also upset the ecosystem balance by altering the vegetation structures and aquatic community composition, and posing a threat to the endemic creatures which tend to have "more specific niches".
For instance, freshwater streams with a large number of the invasive Barcheek goby have significantly fewer of the native Bigmouth Stream goby.
Experts say regular monitoring of freshwater habitats would be useful in detecting newly introduced species quickly, so that management measures, for instance, can be implemented fast.
Beyond their numbers, there is also a need to consider other factors like the "persistence" of alien species, the scientists add.
Assistant Professor Darren Yeo of the National University of Singapore's department of biological sciences pointed out: "In some cases, certain alien species may disappear or decline after a few years for any number of reasons... They may get replaced by another alien species, or become less popular as pets and so fewer are released, or simply not be able to thrive here in the longer term."
Others, such as the golden apple snail, seem to be thriving.
The authorities have for years been promoting public awareness of this issue. An annual campaign "Operation No Release", for instance, aims to discourage the release of animals or unwanted pets into parks and nature reserves.
First-time offenders caught releasing animals may be charged under the Parks and Trees Act and could be fined up to $50,000, jailed up to six months, or both.
Carolyn Khew
IN THEIR WATERY ELEMENT
RED-CLAW CRAYFISH
First spotted in Singapore in the 2000s, this crayfish can grow up to 25cm in length and is found in places such as reservoirs, ponds and, more recently, natural streams. It is established here and seems adaptable. The northern Australian and Papua New Guinea native is omnivorous and, while it looks similar to a lobster, it is from a different family. Crayfish are freshwater species and are usually smaller than saltwater-loving lobsters when fully grown. (Lobsters are marine species.)
GOLDEN APPLE SNAIL
First spotted in the 1980s, it can be found in habitats such as reservoirs, ponds and canals.
It feeds on algae and aquatic plants, is highly adaptable and could have displaced native apple snails around South-east Asia.
There are reports overseas of such snails carrying disease-causing parasites, but they are not harmful to humans unless eaten raw or improperly cooked.
GIANT SNAKEHEAD
This large, ferocious fish species can reach over 1m in length. It can be found in many reservoirs - which resemble its native habitats in large lakes and rivers in parts of Africa and the surrounding region.
It is very protective of its young and will attack anything that it perceives as a threat.
FRESHWATER AND INVASION BIOLOGY LABORATORY, NATIONAL UNIVERSITY OF SINGAPORE
•For more information on other alien species, look up the Web special on http://str.sg/ZNtu