A piece of crabby news comes in the form of a recent paper in the journal Biological Invasions, where Shane T. Ahyong and Darren C.J. Yeo discuss the feral populations of Cherax quadricarinatus that have invaded Singapore's water catchment areas. Native to the warmer parts of its home continent of Australia and Papua New Guinea, C. quadricarinatus has proven adept at establishing itself in other balmy regions such as Puerto Rico, Jamaica, Mexico and South Africa. The animal is gaudily coloured, with "red and maroon highlights on a blue-green to green body", a feature that probably adds to its appeal as a novel pet for hobbyists bored with fortune-telling cichlids. Unfortunately, few retailers are likely to warn buyers that this crayfish, which also boasts large red claws, reaches nearly 25 cm in length and a weight of up to 600 g. Captive animals fortunate enough to attain this size would also have demonstrated an unpleasant habit of tearing apart unwary tankmates, prompting many owners to take the unfortunate path of false mercy by releasing the crayfish into local waterways.
In Singapore, the authors found feral populations in the Kranji, Lower Peirce and Upper Seletar reservoirs. C. quadricarinatus is believed to be a relatively recent introduction, dating back to the late 1990s. Anecdotal evidence suggests that anglers and casual fishermen are becoming increasingly familiar with the crayfish, with some using protein-rich bait to lure the animals into dipnets or home-made traps. The crayfish are said to make good eating, with a sweet-flavoured flesh and convivial to soups and bisques. Other authorities suggest buttering them for a barbeque, grilling or pan-frying.
Non-native crayfish have proven to be an ecological menace in some places, threatening to drive native species into extinction by direct competition or serving as vectors for pathogens to which native species have little or no resistance. Singapore has no native crayfish, the closest ecological equivalent being some freshwater shrimps in the genus Macrobrachium. For the moment, the authors note that though locally abundant, C. quadricarinatus appears to be restricted to the open-water areas of the reservoirs, which harbour few natives. The forested streams and freshwater swamp that surround the reservoirs, however, are the last strongholds for many local freshwater crustaceans, including three species of endemic crabs (one of which, Parathelphusa reticulata, is found only in the Nee Soon Swamp Forest), 7 species of Macrobrachium and 6 species of Caridina shrimps. A good number of threatened aquatic insects (water beetle and semi-aquatic bugs) and fishes also occur in the reserves' dwindling arteries. "Should C. quadricarinatus invade shaded acid-water forest streams and swamps, the dire consequences for native decapods could be dire," is the authors' concluding warning, and it is hoped that just as these acidic, nutrient-challenged pools have repelled alien cichlids and foreign turtles, so will they prove resilient against predacious invasive decapods.