The fish is found throughout South America (Sands, 1997), from the East coast to the base of the Andes: 3,000 feet above sea level (Burgess, 1989). Although no reports were found offering evidence of Oxydoras niger outside of the Amazon River system, Burgess (1989) states that the doradids' range includes the Orinoco and the Parana systems as well. Therefore, it is possible that this fish also exists there.
Oxydoras niger prefers large rivers (Sands,1997). Silvano, et. al., (2000) were able to catch them only in the main channel of the Jurua River, and not in tributaries or neighboring lakes. Here, they were only captured during the dry season and were not observed in the wet season. However, this finding contraticts the opinions of the local fishermen that were interviewed. They stated that O. niger was usually caught in the wet season as opposed to the dry season (Begossi,1999).
While many doradids were found to occupy either a blackwater or whitewater habitat exclusively, O. niger was found in both river types (Saint-Paul et. al., 2000). In this study, O. niger was observed as the ninth most common species caught in gill nets in white water (Lago do Prato of Rio Solimoes) out of 148 species. Out of 172 species caught in blackwater habitat, they were not among the fifty most common. In the whitewater habitat, they were caught with nearly equal frequency in flooded forests as in open lake habitat (adjacent to the main river channel).
According to Sands (1997), maximum total length is 600 mm (24 inches). However, I have personally observed specimens much larger than this: approximately 1 m total length. Similar large sizes have also been reported by Nomura (1m)(1984) and by Burgess (1.2 m)(1989).
The genus contains only one or two species besides O. niger, and can be differentiated by possessing a body width at the clavicles that is less than the head length. Eyes are located on the back half of the head and on the sides - not on the top. The adipose fin is a keel: long and very low and connected to the dorsal body surface throughout its length. It greatly resembles the genus Oxydoras and differs by possessing between 17 and 25 (Nomura, 1984) or between 16 and 23 (Burgess, 1989) scutes on each side, while Oxydoras has up to 40 (Burgess, 1989). In addition, Pseudodoras has fleshy appendages hanging from the roof of its mouth, presumably to taste food particles amoungst river muck (Burgess, 1989).
Color is variable and can range from jet black dorsally with a lighter underbelly to a uniform light gray.
No information could be found regarding the breeding of this animal, except that inferred from its migration pattern (see 'Behavior').
Oxydoras niger engage in an annual fish run known as the piracema. Many species spawn in the lower reaches of the Amazon Basin. Every year after spawning, around June when the water level begins to drop, they begin a massive migration uptream. This migration is performed by at least forty species and lasts about four months (Smith, 1981).
Sixty three percent of O. niger individuals were caught during the daytime, and 27% at night (Saint-Paul, et. al.). This statistic contradicts direct observations of several separate groups of aquarium captives that I have observed between 1999 and 2000. These individuals were always under 1 m in length and would always remain hidden instead of swimming. At no time have I ever observed these specimens venturing out from cover, regardless of how dim the light. This behavior is characteristic of healthy individuals. One of these specimens has been alive in captivity for over one year and always appears healthy when periodically inspected. It is possible that the behavior varies with age, I have also observed a few adults in captivity between 1980 and 2000. These larger individuals were commonly observed cruising their enclosures under "fully illuminated" conditions. Burgess (1989) has reported some daylight activity in some specimens. (Burgess, 1989; Smith, 1981)
These catfish eat crustaceans, snails, and other inverebrates, seeds, and fruits (Sands, 1997).
Lowe-McConnell (1987) has taken data from Marlier and classified O. niger as an insectivore. Indeed, chironomid midges, mayflies, and small crustaceans have been reported as stomach contents (Burgess, 1998). However, so have mud and decomposing leaves (Burgess, 1998), which would classify the fish as a detritivore.
In the aquarium, it has been observed eating boiled oatmeal, trout and koi pellets, rabbit pellets, frozen peas, flake foods, beef heart and liver, brine shrimp, tubificid worms, earthworms (Burgess, 1989). (Burgess, 1989; Lowe-McConnell, 1984)
Parasitism: O. niger is commonly host to parasitic nematodes. Five new species were recently described (Kritsky et. al, 1986), resulting from a revision of helminths. A new species living in its intestine was also described two years later (Ferraz, 1988). (Ferraz and Thatcher, 1988; Kritsky, et al., 1986)
Zaret (1984) lists O. niger as being a "main food fish species" in the Central Amazon, but not in the Orinoco, Guyana, Mogi Guacu, or Parana Rivers. They have been commonly observed in the fish markets of Santarem, a Brazilian city on the lower Amazon (Ferreira, 1996).
They are occasionally kept in aquariums by fish enthusiasts.
No negative effects on humans could be found regarding this fish.
Physiology: Maximum temperature range tolerated is 18C to 30C, but appears to prefer 20C to 27C. They become lethargic at higher temperatures (Burgess, 1989).
Genetics: Oxydoras niger is known to have 58 chromosomes and recent work has been performed on it to identify nucleolus organizer regions (Fenocchio, et. al., 1993)
Pollution: Oxydoras niger and six other fish species found in the Rio Madeira were analyzed for selenium and mercury contamination. These chemicals were found in all specimens. Only herbivores showed significant ratios of selenium and mercury needed to be used as bio-indicators. In this study, O. niger was classified as an omnivore and is therefore not suitable to be used (Doria et. al., 1998). (Burgess, 1989; Sands, 1997)
William Fink (editor), University of Michigan-Ann Arbor.
Ron Oldfield (author), University of Michigan-Ann Arbor.
living in the southern part of the New World. In other words, Central and South America.
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
A substance that provides both nutrients and energy to a living thing.
makes seasonal movements between breeding and wintering grounds
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats all kinds of things, including plants and animals
the business of buying and selling animals for people to keep in their homes as pets.
uses touch to communicate
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Burgess, W. 1989. An Atlas of Freshwater and Marine Catfishes. Neptune City, NJ: T.F.H. Publications Inc..
Dorea, J., M. Moreira, G. East, A. Barbosa. Dec. 1998. Selenium and mercury concentrations in some fish species of the Madeira River, Amazon Basin, Brazil.. Biological Trace Element Research, 65(3): 211-220.
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