List of freshwater fauna of Sri Lanka
Sri Lanka is an island close to the southern end of India with a tropical environment. The freshwater fauna is as large as it is common to other regions of the world. There are about two million species of arthropods found in the world, and still it is counting with many new species still being discovered. It is very complicated and difficult to summarize the exact number of species found within a certain region. Freshwater habitats. Freshwater animals are important to the economy of a country. The animal life in freshwater comprises a  great diversity. The organisms ranging from tiny protozoa to large mammals in size. Sri Lanka is a hydraulic civilization. First civilizations in Sri Lanka were originated closer to main rivers in Sri Lanka such as Mahaweli, Kelani, Malwathu and Gin Ganga. Rivers in Sri Lanka flow in a radial pattern, from central hills where there are numerous waterfalls, the rivers flowing to the west, east and south being shorter than thoe flowing to north., north west and north east. Several streams join these rivers. Some of the streams in the hill country and mid country are quick flowing streams known  as torrential streams. Sri Lankan freshwater habitats can be divided into 6 major types - river, stream, lake, pond, villu and paddy field - according to size, depth, rate of flow and type of bottom. Medium-sized rivers are called "Oya" and fast flowing streams torrential streams are called "Dola" in Sinhalese. The following list provide the freshwater fauna of Sri Lanka. Adaptations of freshwater fauna. The life history of any animal is divisible into 3 phases, initially a period of slow development, followed by a rapid growth with a voracious apetite and finally to an adult which breeds at regular intervals  and continue to grow slowly. Most freshwater animals breed during or after monsoon rains where water is available for the young to spread over a wide area where crowding is less and there is a plentiful supply of food in the form of minute plants and animals. During dry season, conditions in temporary habitats become less and less favourable for aquatic animals. Those cut off in small habitats are quickly killed off, others in larger habitats survive longer. During this period, food become scarce and water level reduces. Animals become weakened or diseased and die or are eaten by birds  and terrestrial animals. Many freshwater animals show adaptations to drought conditions. Fish with air breathing ability, utilize atmospheric oxygen to moisten gills, mud burrowing ability are some of them. Smaller crustaceans, produce eggs with hard resistant covering capable of surviving even a severe drought. These cysts are blown by wind and the species is dispersed. Freshwater crabs and prawns carry their eggs until they hatch out, while leeches carry the young in a pouch. Researches on Sri Lankan freshwater fauna. Knowledge of Sri Lankan freshwater fauna has been considerably enhanced in the last twenty years or so in two ways . One is through the publication of monographs. The other way is through Swedish Expedition and Smithsonian Institution. Several animals groups were identified extensively from Sri Lanka, such as fish, amphibians, odonates and arthropods. Other freshwater fauna of Sri Lanka need more detailed work, which are currently based on the notes of way back in British period. According to the National Red Data List in 2012, many freshwater habitats are degraded at an alarming rate due to rapid human interference. Habitat destruction, fragmentation and loss of forest cover has increased in past decade rapidly. The result becomes severe, where many amphibians  have been not recorded after their discovery, which led the scientists to categorize Sri Lanka as a biodiversity hotspot. Protozoa. Family: Amoebidae. Phylum: Amoebozoa Class: Tubulinea Order: Tubulinida Family: Actinophryidae. Phylum: Ochrophyta Order: Actinochrysophyceae Family: Arcellidae. Phylum: Amoebozoa Class: Tubulinea Order: Arcellinida Family: Centropyxidae. Phylum: Amoebozoa Class: Tubulinea Order: Arcellinida Family: Ceratiaceae. Phylum: Dinoflagellata Class: Dinophyceae Order: Gonyaulacales Family: Clathrulinidae. Phylum: Cercozoa Class: Granofilosea Order: Desmothoracida Family: Codonellidae. Phylum: Ciliophora Class: Oligotrichea Order: Choreotrichida Family: Difflugiidae. Phylum: Amoebozoa Class: Tubulinea Order: Arcellinida Family: Epistylidae. Phylum: Ciliophora Class: Ciliatea Order: Sessilida Family: Euglenaceae. Phylum: Euglenozoa Class: Euglenoidea Order: Euglenales Family: Euglyphidae . Phylum: Cercozoa Class: Imbricatea Order: Euglyphida Family: Glaucomidae. Phylum: Ciliophora Class: Oligohymenophorea Order: Tetrahymenida Family: Hyalospheniidae. Phylum: Amoebozoa Class: Tubulinea Order: Arcellinida Family: Ichthyophthiriidae. Phylum: Ciliophora Class: Oligohymenophorea Order: Hymenostomatida Family: Oxytrichidae. Phylum: Ciliophora Class: Spirotrichea Family: Parameciidae. Phylum: Ciliophora Class: Oligohymenophorea Order: Peniculida Family: Pelomyxidae. Phylum: Amoebozoa Class: Archamoebae Order: Pelobiontida Family: Peridiniaceae. Phylum: Myzozoa Class: Dinophyceae Order: Peridiniales Family: Trichodinidae. Phylum: Ciliophora Class: Oligohymenophorea Order: Peritrichida Family: Volvocaceae. Phylum: Chlorophyta Class: Chlorophyceae Order: Chlamydomonadales Family: Vorticellidae. Phylum: Ciliophora Class: Oligohymenophorea Order: Sessilida Moss animals. Phylum: Bryozoa Class: Phylactolaemata Order: Plumatellida Hairybacks. Phylum: Gastrotricha Order: Chaetonotida Sponges. Phylum: Porifera  Class: Demospongiae Order: Spongillida Cnidarians. Phylum: Cnidaria Class: Hydrozoa Order: Anthoathecata Wheel animals. Phylum: Rotifera Sri Lankan freshwaters are home for 124 species of wheel animals. Flatworms. Phylum: Xenacoelomorpha Order: Acoela Family: Convolutidae. Phylum: Platyhelminthes Order: Rhabdocoela Family: Typhloplanidae. Phylum: Platyhelminthes Class: Trematoda Order: Plagiorchiida Family: Pleurogenidae. Phylum: Platyhelminthes Class: Cestoda Order: Bothriocephalidea Roundworms. Phylum: Nematoda Class: Enoplea Order: Dorylaimida Family: Dorylaimidae. Class: Chromadorea Order: Rhabditida Gordian worms. Phylum: Nematomorpha Class: Gordioida Order: Chordodea Segmented worms. Phylum: Annelida Family: Aeolosomatidae. Class: Clitellata Order: Haplotaxida Family: Almidae. Order: Rhynchobdellida Family: Glossiphoniidae. Order: Arhynchobdellida Molluscs. Phylum: Mollusca Class: Gastropoda Order: Cycloneritida Family : Neritidae. Order: Caenogastropoda