Order

Gymnotiformes

  Class

Actinopterygii

No. in FishBase

Genera : 15 | Species : 88 Eschmeyer's Catalog of Fishes

  Environment

Fresh : Yes | Brackish : No | Marine : No

  Division

Primary freshwater

  Aquarium

some

  First Fossil Record

     

  Remark

Species diversity and systematics of the Apteronotidae has been reviewed by Mago-Leccia (1994), Albert and Campos-da-Paz (1998), and Albert (2001). Apteronotid species are readily recognized as the only gymnotiform fishes with a caudal fin and a dorsal organ (a longitudinal strip of fleshy tissue firmly attached to posterodorsal midline). Among gymnotiforms, apteronotid species also possess the following unique combination of characters: small eye (its diameter less than distance between nares); infraorbital and supraorbital laterosensory lines connected anterior to eye; infraorbital bones ossified as slender tubes; anterior nares located outside gape; no urogenital papilla; 1-2 rows of conical teeth in both jaws; anal-fin origin at, or anterior to, isthmus; no urogenital papilla. Apteronotids also possess a high frequency tone-type electric organ discharge (more than 750 Hz at maturity). There are currently 72 names available in the literature for apteronotid genera and species. Of these, 13 generic and 52 species names are valid. There are in addition 12 manuscript names for species currently in preparation, bringing the known diversity of Apteronotidae to 64 species. Many more apteronotid species are anticipated, as the rate of description is accelerating. Apteronotids are relatively conservative in terms of overall external morphology. Like all gymnotiforms the shape is typically culteriform, with an elongate body and anal fin. Higher level apteronotid taxonomy emphasizes differences in snout length and shape, and shape and position of the mouth. Apteronotids exhibit substantial diversity in total adult body size, ranging from an undescribed Amazonian species of Adontosternarchus, and Megadontognathus kaitukaensis (each growing to about 160 mm) to Apteronotus (formerly Ubidia) magdalenese (1.3 M) (Campos-da-Paz, 1999). The Apteronotidae is confined to the Humid Neotropics, ranging the Rio de la Plata of Argentina (35° S) to the Rio Tuira of Panama (8°N). Apteronotids are known from the continental waters of all South American countries except Chile (Albert, 2001), and are most diverse - both taxonomically and ecologically - in the Amazon basin. The ecology and natural history of most apteronotid species is poorly understood. Apteronotus species are the most widely distributed, extending the full range of the family. Apteronotus species inhabit both floodplain and tierra firme (non-floodplain) streams and rivers. Like many apteronotids, Apteronotus are aggressive predators of small aquatic insect larvae and fishes. Apteronotids are most diverse in the Amazonian floodplain, where many species are specialized to inhabit deep portions of the river channel. Some main channel apteronotids are aggressive piscivores (e.g., Sternarchella), whereas other are planktivores (Adontosternarchus). Magosternarchus spp. predate the tails of other electric fishes. Other species (e.g., Sternarchorhynchus, Sternarchorhamphus) have tubular snouts and forage on the beds of aquatic insect larvae on the rivers bottoms. At least one species (Sternarchogiton nattereri) eats freshwater sponges which grow on submerged trees, stumps, and other woody debris. Orthosternarchus is perhaps the most specialized for life at the river bottom, in many ways resembling cave fishes; they are almost entirely blind, with minute, assyemtically arranged eyes; and they have almost no pigments or scales, appearing bright pink in life due to the underlying blood hemoglobin. An important aspect of apteronotid diversity is sexual dimorphism in snout size and shape. Reproductively mature males of Parapteronotus hasemani have highly elongate jaws, which are presumed to be involved in male-male competition. Mature males of Sternarchorhynchus roseni possess numeous teeth on their lower jaw. Mature male Sternarchogiton nattereri grow teeth externally over the snout, giving them an appearance to unusuall that for 40 years they have been regarded as representing an entirely different genus (Oedemognathus). Apteronotid systematics and taxonomy is rapidly changing as new species and previously undocumented phenotypes are discovered. The total expected number of species cannot currently be estimated with confidence. A large number of undescribed apteronotid species are known in museum collections and many more are likely to be captured from continued field studies, particularly in Western and Central Amazonia. There is little direct commercial exploitation of Apteronotid species. Many species are ecologically important in Amazonian floodplains, often constituting a significant fraction of the biomass (Crampton, 1996). Two species (Apteronotus albifrons and A. leptorhynchus) are common in the aquarium trade. Apteronotids are not an important food resource.

  Etymology

Greek, aptero = without fins + Greek, noton = back ( Ref. 45335).

  Reproductive guild

nonguarders

  Typical activity level

  Main Ref

Albert, J.S. 2003

  Coordinator

Albert, J.S. 2003

  Deep Fin Classification

Osteichthyes | Actinopterygii | Actinopteri | Neopterygii | Teleostei | Osteoglossocephalai | Clupeocephala | Otomorpha | Ostariophysi | Otophysa | Characiphysae | Gymnotiformes | Sternopygoidei | Apteronotidae