About Oxydoras niger (Valenciennes, 1821)
Oxydoras niger, commonly called the ripsaw catfish or cuiu cuiu, is a species of thorny catfish. It is native to the Amazon, Essequibo and São Francisco basins across Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru and Venezuela, and it is the largest member of the Dorididae family. This species reaches a maximum length of 100 centimetres (39 in) in standard length, and can weigh up to 13 kilograms (29 lb). It is a minor part of local commercial fisheries. It has lateral thorns that can damage potential predators or anyone handling it. It feeds by shifting through sand, and detects edible items with taste receptors on the roof and floor of its mouth. Oxydoras niger has been found in waters with temperatures from 24 to 29.8 °C (75.2 to 85.6 °F), a pH range of 5 to 9, and an alkalinity range of 42 to 142. Its diet includes detritus, chironomid and ephemeropteran larvae, and crustaceans. This species is known to form schools. It typically lives in muddy streams and lakes. During the rainy season, ripsaw catfish often migrate to large freshwater lakes and flooded basins, returning to their main river when floods recede. It is native to the Amazon River and has also been found along the east coast of South America. The large size of the Amazon River and rainy season expansion of its habitat have led to the species being found across wide areas, which has allowed it to survive successfully despite reproductive challenges. O. niger takes part in an annual upstream migration run called the piracema. An estimated 40 species participate in piracema, which lasts 4 months. Even though this run is extremely long and dangerous, the fish complete it to help their offspring survive. O. niger ranges from the east coast of South America all along the Amazon River, with large populations found in the Jurua River. Some individuals have even been found in the Andes Mountains at elevations of 3,000 feet (910 m). At the beginning of the dry season, when water levels start to drop (usually around June), the fish begin a massive upstream migration that covers 100 to 400 km (62 to 250 mi). The start and end of the run fall in completely different seasons. This migration is thought to allow the species to disperse over larger areas and improve survival chances. By occupying flooded areas and traveling to other rivers, the species is safer from natural disasters that could destroy an individual river. The piracema migration is currently threatened by the increasing number of dams in the Amazon. These fish are adapted to follow a consistent migration route, and a dam along the route can prevent them from reaching spawning grounds. Their spawning season begins in the rainy season when water levels are at their highest, which provides the greatest safety for the eggs. This lowers the chance that predators will find the eggs or the young offspring. Females can lay up to 250,000 eggs, and individuals reach sexual maturity at a length of about 54 centimetres (21 in). Even with this high number of eggs, very few survive to adulthood.
