Aliens in water!!! Yes, it’s a surprise to hear that some aliens species are living on our planet Earth. Now a question can be built in your mind how these species survive in this planet and how they tolerate environmental conditions of our planet. A scientist and also a lecture in Sri Lanka involves to find the answers to these problems. He is Dr. U. P. K. Epa, Senior Lecturer in Zoology, University of Kelaniya and he reported us about aliens in water. Everyone knows about one of these species and it is commonly called as Tank Cleaner and the other on is Golden Apple Snail. The report of these species is as follows.
Aliens in water: Do we pay enough attention?
Alien species are organisms introduced deliberately or accidentally outside their natural habitats where they have the ability to successfully establish themselves. They become invasive when they invade, out compete native species and take over the new environments. Invasive alien species (IAS) are widespread in the world and are found in all categories of living organisms and all types of ecosystems. However, plants, mammals, fish and insects comprise the most common types of invasive alien species present in natural environments.
Introduction of species beyond their natural range has a long history. It has been cited as one of the most pervasive and ecologically damaging human activities. The introduction of species from their natural range has both desirable and undesirable effects on native species. Once established, alien species are nearly impossible to eliminate. Some introduced alien species have negatively affected the native species in the natural ecosystems thereby giving rise to conservation problems. The threat to biodiversity due to AIS is now considered second only to that of habitat loss and degradation. AIS are thus a serious impediment to conservation and sustainable use of global, regional and local biodiversity, with significant undesirable impacts on the goods and services provided by ecosystems.
Aquatic organisms are intentionally introduced into new environments in order to enhance commercial and sport fisheries and for ecological management (e.g. bio control agents). Unintentional release of farmed animals and accidental escapes from aquariums are considered as major factors that increase the number of aquatic IAS in the environment. Aquatic alien species have expanded their ranges by moving through canals and other aquatic connections created by humans. These introductions cause diverse effects by modification of the colonization to extinction equilibrium in biogeogrphical islands. Sri Lanka being an island with a rich biodiversity the threat of alien fauna should be given considerable attention. The gravity of this conservation issue increases because nearly half (48%) of the freshwaters fishes in Sri Lanka are endemic while nine species are considered globally threatened and 39 species have been identified as nationally threatened (IUCN, 2004) E.g. Danio aequipinnatus, Danio pathirana, Esomus thermoicus, Horadandiya atukorali, Labeo lankae, Puntius asoka, Puntius bandula, Puntius cumingii, Monopterus desilvai, Macrognathus aral.
Many alien species (especially plants) have been introduced to Sri Lanka during the past 500 years, during which period the island was subjected to colonial rule. Most of the deliberate introductions made during the colonial period were for agro-economic and forestry purposes. According to the FAO Database on Introduction of Aquatic Species (DIAS), 22 fish species have been introduced into the Sri Lankan fresh waters mainly for the aquaculture. First recorded alien fish introduction took place in 1880s when Rainbow trout (Oncorhynchus mykiss) was introduced in to up country streams in order to enhance sport fishery. According to observations of IUCN aquatic alien invasive fauna in Sri Lanka includes six freshwater fish species and one aquatic mollusc species. Not all the species introduced into Sri Lanka has become harmful to the natural environment and some species even could not maintain a viable population in the natural environment (E.g. Puntius gonionatus). However, no risk analysis was undertaken to assess the invasion or impacts of these alien species prior to their introduction into Sri Lanka, and a limited number of research has been conducted to predict their future spread or impacts.
Spotted Gar (Lepisosteus) is a fish native to United States of America, Mexico and Canada and it is a voracious feeder of fish and other aquatic organisms. In November 2012 fishermen accidentally caught one specimen of spotted gar in Handapanagala wewa, Moneragala and this is not the first occurrence of alien fish species in the wild environment in Sri Lanka. However, recent observation of spotted gar in Moneragala district exemplifies the alarming situation of increasing accidental introductions of fish species into fresh waters of our country. If there is a breeding population of spotted gar in Handapanagala reservoir it may impose a direct threat to the existence of other aquatic species and may rapidly spread into the other adjacent water bodies with the flood water. The country should pay attention to restrict further introductions of aquatic organisms into the natural environment and should take steps to reduce range expansion of such species before they do further harm to the native biodiversity and fisheries industry.
Majority of aquatic alien invasive species are widely distributed in the low country wet zone where the highest numbers of endemic freshwater fish species are found. Other than AIS agrochemical applications, pollution, hydrological alterations, gem mining, degradation of habitats, destructive fishing methods and capture of adult fish for the exportation are the other major factors that threaten the existence of freshwater fishes in Sri Lanka. However, threats imposed by AIS on freshwater fishes should be given more attention because reduction of native fish populations after the introduction of alien fish species is more catastrophic as no remedial measures could be planned due to time limitations. However, the linkage between the abundance of AIS and the decline of native species is surprisingly poorly documented.
Introduced piscivores have in many cases been proven strongly impact upon prey populations leading locally to their extinction. The introduction of Nile Perch (Lates niloticus) into Lake Victoria has caused great changes to the fish fauna in the lake. It has caused the extinction of more than 200 species of endemic native Cichlids through direct predation, which is a monumental loss of biodiversity for Lake Victoria. The introduction of Oscar (Astronotus ocellatus) and Red Piranha (Pygocentrus nattereri) to the north east and south east of Brazil has caused the disappearance of some native fish species and reduction of young individuals of others, coupled with reduction in mean weight of these individuals. In South Africa about 14 freshwater fish species are believed to be threatened by introduced species.
Some of the introduced fish species in Sri Lanka
Purpose of introduction
|Carassius carassius||Goldfish||Europe||Ornamental fish|
|Catla catla||Catla||India||Food fish|
|Cirrhinus mrigal||Mrigal||India||Food fish|
|Helostoma temminckii||Kissing gouramy||Thailand||Food fish|
|Hypophthalmichthys spp.||Big head carp||China||Food fish|
|Oncorhynchus mykiss||Rainbow trout||England||Sport fishery|
|Oreochromis spp.||Tilapia||East Africa||Food fish|
|Trichogaster pectoralis||Gouramy||Malaysia||Food fish|
|Osphronemus goramy||Giant Gourami||Malaysia||Food fish|
|Gambusia affinis||Western mosquito fish||–||Mosquito control|
However, accidental introductions of species are common in the world and sometimes more environmentally damaging than intentional introductions. The national list of invasive alien fauna published in 2011 is incomplete and it does not include snake skin gouramy which is a highly abundant fish in Bellanwila-Attidiya and Eriyawetiya wetlands and several reservoirs in Anurdapura and Polonnaruwa district. Therefore, the exact number of accidentally introduced aquatic species with breeding populations in the country is not known. Presently following accidentally introduced aquatic species are known to spread rapidly in the freshwater ecosystems of the country.
Aquatic species accidentally introduced by ornamental industry in Sri Lanka
|Chitala ornatus||Clown knife fish||Colombo, Kalutara|
|Clarias batrachus||Walking catfish||Gampaha|
|Pterygoplichthys pardalis||Tank cleaner||Island wide|
|Pterygoplichthys disjunctivus||Tank cleaner||Island wide|
|Poecilia reticulata||Guppy||Western and southern provinces|
|Trichogaster trichopterus||Blue gouramy||Island wide|
|Xiphophorus hellerii||Sword tail||Dambulla, Nawalapitiya|
|Atractosteus spatula||Alligator gar||Moneragala|
|Pomacea diffusa||Golden apple snail||Galle, Gampaha, Colombo, Kaluthara, Nawalapitiya, Kandy|
|Trachemys scripta||Red eared slider turtle||Kurunegala|
|Mayaca fluviatilis||Stream bog moss||Gampaha|
Among the alien invasive fish with breeding populations, four species (Rainbow trout, Clown knife fish, Shark catfish and Walking catfish) are active predators of native aquatic fauna. The Rainbow trout occurs in mainly in the Horton Plains National Park, which is also known to be the habitat of the endemic shrimp Caridina singhalensis. Rainbow trout feed on aquatic organisms such as crabs, shrimps, insects and other fish; it is possible that it has effected the populations of the above endemic shrimp as well as other endemic crabs in the streams. The Clown-knife fish is a large voracious carnivore which feeds on slow moving fish. As it is spreading rapidly in the streams, rivers, ponds and marshes of the wet zone, it may have already effected the populations of the endemic fish which are mainly distributed in this zone. Clown knife fish which has many small bones is not popular as a food fish. The tank-cleaner has been recorded in western, southern, north western, central and eastern provinces. It has been observed to attach itself by its ventral sucker to the bodies of larger fish. When it detaches, the slime layer of fish which acts as a protective covering is also removed, making the host susceptible to diseases. Diet of the tank cleaner reflects characteristics omnivorous feeding habit by having wide spectrum of food items ranging from plankton, plant matter to invertebrates showing high feeding plasticity. Therefore, the further invasion of tank cleaners in to the inland freshwaters may pose a threat to the existence of number of endemic and exotic fish species that have similar feeding habits. This fish has already negatively affected the existing fishery in the Bolgoda lake, Polgolla reservoir, Kala wewa and Hurulu wewa mainly by increasing their proportion in the fish catch and by significantly damaging the fishing nets. Tank cleaners caught in the nets are presently thrown back into the water as there is no demand for this fish in the market as a food fish. Larger fish caught by the gill nets are also not marketable in the ornamental fish trade. Therefore, fishing mortality does not play a significant role in reducing the number of tank cleaners in the reservoirs. Males of tank cleaners dig out river banks to create burrows in which an attracted female will lay and guard her eggs. In large numbers, this burrowing behavior contributes to problems with siltation. In addition, the burrows potentially destabilize the banks, leading to an increased rate of erosion. It is essential to find out ways to exploit this nuisance fish species as an animal feed or for any other use before they do further harm to freshwater fishery and river bank stability in Sri Lanka.
Spotted Gar (Lepisosteus) is a fish native to United States of America, Mexico and Canada and it is a voracious feeder of fish and other aquatic organisms. In November 2012 fishermen accidentally caught one specimen of spotted gar in Handapanagala wewa, Moneragala and this is not the latest observation of an alien fish species in the wild environment in Sri Lanka. However, this observation of spotted gar in Moneragala district exemplifies the alarming situation of increasing accidental introductions of fish species into fresh waters of our country. If there is a breeding population of spotted gar in Handapanagala reservoir it may impose a direct threat to the existence of other aquatic species and may rapidly spread into the other adjacent water bodies with the flood water. The country should pay attention to restrict further introductions of aquatic organisms into the natural environment and should take steps to reduce range expansion of such species before they do further harm to the native biodiversity and fisheries industry.
Golden apple snail (Pomacea spp.) is a notorious rice pest in other Asian countries like Malaysia, Thailand and Indonesia. The pest snail species has been identified as P. canaliculata while the species found in Sri Lanka has been identified as P. diffusa. Apple snails have been imported to many Asian countries as a food source. This may not be the case in Sri Lanka, where terrestrial and freshwater snails are not popular as food sources. Therefore, it is generally believed that apple snails have been introduced in to Sri Lanka through the aquarium fish trade. This snail is known to feed on aquatic plants. Sri Lanka exports a number of freshwater ornamental plants (Ruffled sword plant, Aponogeton, Fanwort, Cabomba, Hornwort, Ceratophyllum; Cryptocoryne, Water wisteria, Hygrophila, False loose strife, Ludwigia, Common eel grass, Vallisnaria), which earns most important foreign exchange to the country. Even though this snail has not done any economical damage to the rice farming, it might significantly affect the aquatic ornamental plant industry in future. Apple snails have recently expanded its range from low country wet zone to up country wet zone.
The ability of alien species to colonize a new environment is a complex issue and has been shown to be influenced by environmental variability, biotic interactions and abiotic disturbances. Freshwater ecosystems are more susceptible for invasions as they are particularly subjected to disturbances and have become degraded throughout the world. Sri Lanka constitutes about 153,000 ha of freshwater bodies, which include 70,000 ha of large irrigation reservoirs, 39,000 ha of minor irrigation tanks, 4,000 ha of flood lakes, 8,000 ha of upland reservoirs and 22,000 ha of Mahaweli reservoirs. There is a possibility that the availability of these aquatic habitats, which are interconnected by irrigational channels, would greatly enhance the distribution of aquatic alien invasive organisms throughout the country. Large and sudden changes of flow (floods and droughts) which could increase the habitat availability to invasive biota is a fact that also is applicable to Sri Lankan environmental conditions. Therefore it is important to introduce laws to restrict new introduction of aquatic species and to take mitigatory measures to restrict further distribution of aquatic invasive alien species with a view to conserve the native species in the country.
We would like to express the deepest appreciation and thank to Dr. U. P. K. Epa, Senior Lecturer in Zoology, University of Kelaniya, Sri Lanka
Summary of "Aliens in water! Do we pay enough attention?"
Did you know about Aliens in water? Some aliens species are living on our planet Earth.Tank Cleaner and Golden Apple Snail are two species of these aliens.