Animal Diversity Web

Geographic Range

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.

• Biogeographic Regions
• neotropical
  • native

Habitat

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).

• Aquatic Biomes
• benthic
• rivers and streams

Physical Description

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.

Images of Pseudodoras: one, two. (Burgess, 1989; Nomura, 1984)

• Other Physical Features
• ectothermic
• bilateral symmetry

• Range length

  1.2 (high) m

  3.94 (high) ft

Reproduction

No information could be found regarding the breeding of this animal, except that inferred from its migration pattern (see 'Behavior').

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)

• Key Behaviors
• natatorial
• diurnal
• nocturnal
• motile
• migratory

Communication and Perception

• Perception Channels
• tactile
• chemical

Food Habits

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)

• Primary Diet
• omnivore

• Animal Foods
• insects
• mollusks
• aquatic or marine worms
• aquatic crustaceans

• Plant Foods
• seeds, grains, and nuts
• fruit

Ecosystem Roles

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)

Economic Importance for Humans: Positive

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.

• Positive Impacts
• pet trade
• food

Economic Importance for Humans: Negative

No negative effects on humans could be found regarding this fish.

Conservation Status

• IUCN Red List

  No special status

• CITES

  No special status

Other Comments

This fish is also known as the mother of snails catfish (Sands, 1997), although Burgess (1989) states that this is the local's name for Megalodoras uranoscopus. It also is known as Oxydoras niger.

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)

Contributors

William Fink (editor), University of Michigan-Ann Arbor.

Ron Oldfield (author), University of Michigan-Ann Arbor.

References

Begossi, A., R. Silvano, B. do Amaral, O. Oyakawa. 1999. Uses of fish and game by inhabitants of an extractive reserve (Upper Jurua, Acre, Brazil). Environment, Development, and Sustainability: 73-93.

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.

Fenocchio, A., L. Jorge, P. Venere, L. Bertollo. 1993. Karyotypic characterization and nucleolus organizer regions in three species of Doradidae (Pisces, Siluriformes).. Revista Brasileira de Genetica, 16(4): 1097-1101.

Ferraz, E., V. Thatcher. 1988. Bacudacnitis-grandistomis new-genus new-species Nematoda Cucullamidae an intestinal parasite of the catfish Pseudodoras-niger Valenciennes of the Brazilian Amazon.. Amazoniana, 10(3): 249-254.

Ferreira, E., J. Zuanon, G. dos Santos. 1996. A list of commercial fish species from Santarem, State of Para, Brazil. Naga, the ICLARM Quarterly, 19(3): 41-44.

Goulding, M., M. Carvalho, E. Ferreira. 1988. Rio Negro, Rich Life in Poor Water: Amazonian Diversity and Foodchain Ecology as seen through Fish Communities. The Hague, The Netherlands: SPB Academic Publishing.

Kritsky, D., V. Thatcher, W. Boeger. 1986. Neotropical monogenea 8. Revision of Urocleidoides Dactylogyridae Ancyrocephalinae. Proceedings of the Helminthological Society of Washington, 53(1): 1-37.

Lowe-McConnell, R. 1987. Ecological Studies in Tropical Fish Communities. Cambridge: Cambridge University Press.

Lowe-McConnell, R. 1984. The status of studies on South American freshwater food fishes. Pp. 139-156 in T Zaret, ed. Evolutionary Ecology of Neotropical Freshwater Fishes. The Hague, The Netherlands: Dr. W. Junk Publishers.

Nomura, H. 1984. Dicionario dos Peixes do Brasil. Caixa, Brazil: Editerra Editorial.

Saint-Paul, U., J. Zuanon, M. Villacorta Correa, M. Garcia, N. Fabre. 2000. Fish communities in Amazonian white- and blackwater floodplains. Environmental Biology of Fishes, 57: 235-250.

Sands, D. 1997. Back to Nature Guide to Catfishes. Jonsered, Sweden: Fohrman Aquaristic AB.

Silvano, R., B. do Amaral, O. Oyakawa. 2000. Spatial and temporal patterns of diversity and distribution of the Upper Jurua River fish community (Brazilian Amazon). Environmental Biology of Fishes, 57: 25-35.

Smith, N. 1981. Man, Fishes, and the Amazon. New York: Columbia University Press.