(South American knifefishes and electric eels)
Class: Actinopterygii
Order: Gymnotiformes
Number of families: 5
Evolution and systematics
South American freshwaters are dominated by fishes in the superorder Ostariophysi: the siluriforms, characiforms, and gymnotiforms together comprise more than 75% of all the known freshwater species in South America. Recent data based on cladistic methodology strongly indicate that Gymnotiformes and Siluriformes are sister groups within the Ostariophysi. Of the three Neotropical ostariophysan orders, the gymnotiforms have the smallest number of species. It is also the least investigated group as far as systematics and ecology are concerned. This is largely because many species are difficult to distinguish, hard to catch, especially in their preferred deep riverine benthic habitats, and finally yet importantly are of little commercial interest. The first known gymnotiform fish, now known as Gymnotus carapo, was described in 1648. In 1758 Linnaeus described four species, today known as Electrophorus electricus, Gymnotus carapo, Rhamphichthys rostratus, and Apteronotus albifrons. Our main understanding of the systematics of these fishes is based on work performed during the past 20 years. Science now recognizes 105 described and at least 32 undescribed species, grouped within 29 genera and 5 families. The family Gymnotidae includes 17 described species, the family Rhamphichthyidae 13, and the family Hypopomidae 15 species. The fishes of these families have retained the plesiomorphic pulse-type electric organ discharge. Twenty-four species are grouped in the family Sternopygidae and 43 in the family Apteronotidae; these knifefishes produce the derived tone-type electric organ discharge.
Sternopygus astrabes most closely resembles the ancestral gymnotiform phenotype. Gayet and Meunier in 1991 described the first fossil gymnotiform fish, Ellisella kirschbaumi, from the Upper Miocene (about 10 million years ago) from Bolivia. However, the morphological characteristics of E. kirschbaumi—the absence of dorsal and pelvic fins and the replacement of the caudal fin by a long bony cartilaginous rod (as seen in sternopygids) in the regenerated caudal part—indicate that knifefishes are much older than 10 million years.
Physical characteristics
The Gymnotiformes constitute a group of rather specialized fishes. In most species, the body is compressed laterally, very elongated and slender, with long caudal appendages and a long anal fin tht resembles a knife (thus the name knifefishes). The fishes lack pelvic and dorsal fins; the dorsal filament of the Apteronotidae is sometimes interpreted as a rudiment of an adipose fin, but it might also be modified muscle. The caudal fin is replaced by the caudal appendage except in the apteronotids, which possess a reduced caudal fin that is interpreted by some authors as being derived from the caudal appendage and not representing an intermediate state in the loss of a caudal fin. All gymnotiform fishes possess an extremely well-developed ability to regenerate the caudal, or hind, parts of their bodies. In the apteronotids, however, this feature is less pronounced.
The caudal appendage represents an extension at the end of the anal fin, including the axial skeleton (a bony rod in most cases), the spinal cord, the remaining soft tissues, and the electric organ. A derived feature of gymnotiform fishes is their head morphology, which apparently reflects an adaptation for specialized feeding. For example, some knifefishes possess large mouths (Gymnotus, some Apteronotus) and feed on large prey; some possess terminal mouths (Rhabdolichops, Eigenmannia) and feed on insect larvae and on plankton; some posses external teeth (Oedemognathus) and feed on scales; some posses a long curved mouth (Sternarchorhynchus) and search for insect larvae in holes and crevices. The same variations in head morphology, related to feeding, are found in the African mormyrid fishes.
The most outstanding feature of the knifefishes is the possession of electroreceptors and electric organs in the skin. In some Rhabdolichops species, the large transparent electric cells in the tail region can be easily seen by external inspection. Electrophorus electricus is characterized by strong electric discharges up to 700 volts (V) at more than 1 ampere, or amp(A); in addition, this species can emit weakly electrical discharges for orientation. All the remaining knifefishes produce weakly electrical discharges around 1 V. The electric organs are derived from muscle, except in apteronotids, where modified spinal axons constitute the electric organ. Some species, such as Steatogenys elegans and Hypopygus lepturus, possess accessory electric organs in the head region. Larvae of some species (families Sternopygidae, Apteronotidae) differentiate larval electric organs early during their development.
Distribution
Gymnotiform fishes occur in all Neotropical river systems, from the Rio Salado in the pampas of Argentina to the Rio San Nicolas of Chiapas in Mexico. Species diversity is highest in the Amazon river system (89 species), followed by the Orinoco river system (61 species), rivers found in the Guyanas (35 species), and the Parana-Paraguay river system (26 species). Fourteen species occur in northwestern South America, twelve in southeastern Brazil and in Uruguay, nine in northeastern Brazil, seven on the Pacific slope of South America, seven in Central America, and just one species in the Rio Sali-Dulce of northwestern Argentina and on the island of Trinidad.
Largely because of their poorly understood systematics, the distribution of gymnotiforms at the species level is not well known. The most widely distributed species seem to be Gymnotus carapo, Brachyhypopomus pinnicaudatus, Apteronotus albifrons, Eigenmannia lineata, and Sternopygus macrurus. Not surprisingly, these are the species most commonly encountered in the tropical fish trade.
Habitat
Most rivers in tropical South America can be classified according to water conditions: white water (e.g., the Amazon), black water (e.g., the Rio Negro) or clear water (e.g., the Rio Tapachos). All three types are characterized by low pH values (from 7 down to 3.5) and low levels of salts and nutrients. The freshwater systems of Central America have higher levels of mineral content due to their different geology. Gymnotiform fishes occur in all three major water types. In the extremely acidic and nutrient-poor black waters of the Rio Negro, 36 gymnotiform species have been found. Knifefishes occur in small streams, large rivers, lakes, and various types of backwaters. The enormous abundance of gymnotiforms that occur in deep, main-river channels was only recently discovered (1980s): in the Orinoco River system, for example, 86% of the fish species are gymnotiforms. It is still not well understood why this predominance occurs in this type of habitat.
Some knifefishes show adaptations to habitats with low oxygen. The electric eel (Electrophorus electricus; modified buccal cavity) and the banded knifefish (Gymnotus carapo; swim bladder) can take air from the water surface, as can several species of Brachyhypopomus. The electric eel is an obligatory air breather and drowns if denied access to atmospheric air. However, as long as its skin is kept moist, it can survive for several hours out of water. A large embryonic fin fold of the gymnotiform free embryos serves as a respiratory organ during early development.
Most gymnotiform fishes tolerate temperatures between 68 and 92°F (20 and 35°C); a few species, however, have been found in colder waters. Apteronotus sp. and the glass knifefish (Eigenmannia lineata) occurred in considerable quantities in the Huallaga River (about 64.4°F or 18°C in the tropical mountain rainforest near Tingo Maria, Peru). Under experimental conditions, Sternarchorhynchus sp. did not survive below 77°F (25°C), but tolerated temperatures up to 98.6°F (37°C). For Sternopygus sp., the range was 66.2–86°F (19–30°C).
In the field, gymnotiforms can tolerate pH values of 7 (Amazon) to below 4 (black water). In captivity, the glass knifefish tolerates a pH range between 3.5 and 8.4!
Behavior
Knifefishes are nocturnal, hiding during daytime between plants, in floating meadows, in crevices and holes, and under various kinds of shelter. During the day some species, such as Rhamphichthys rostratus and Steatogenys elegans, lie flat and motionless on the bottom, imitating marbled leaves. Species of the genus Gymnorhamphichthys burrow in the sand during daytime.
Gymnotiforms move by undulating the elongate anal fin that extends along most of the ventral part of the body. In as sociation with this form of locomotion, the anal fin rays articulate directly with proximal pterygophores, providing the fin rays unrestricted mobility. The caudal, elongate portion of the body is maintained in a rigid posture by numerous intermuscular bones. This facilitates the use of the integument as a sensory sheet due to the presence of numerous electroreceptors. This mechanism is used for object location. The presence of an individual nearby with an electric organ discharge frequency of similar magnitude disturbs the object location performance of the fish: a shift in frequency occurs, which is termed the jamming avoidance response.
Feeding ecology and diet
The food of knifefishes is of indigenous origin, mostly insect larvae, annelid worms, and crustaceans. The banded knifefish predominantly feeds on shrimp and smaller fishes, as do some larger knifefishes. Electric eels are also piscivorous and include their own weakly electric relatives in their diet. Regardless of seasonal changes, Rhamphichthys marmoratus does not change its diet, which consists mostly of chironomids (Diptera) and polymitarcids (Ephemeroptera).
Ontogenetic change in the diet is documented for the banded knifefish: up to 7.9 in (20 cm) the principal food items are aquatic insects, predominantly chironomid larvae; beyond 7.9 in (20 cm) the fish prefer large insect larvae (Odonates), shrimp, and fish, preferably small characids (genera Ctenobrycon and Curimata). This change in diet coincides with the time when the banded knifefish females reach first maturity (9.8 in/25 cm, 2 years old).
Species of the genus Rhabdolichops possess well-developed gill rakers and are known to be effective plankton feeders. The knifefish Oedemognathus exodon feeds on scales of fishes. The knifefish Magosternarchus duccis, from the murky channel of the Amazon, chiefly eats the tails of other knifefishes (these do regenerate afterward).
The importance of gymnotiforms as part of the food chain is demonstrated in the case of top predators of the pimelodid family, the catfishes; fifteen commercially important pimelodid species of the Rio Apure in Venezuela include gymnotiforms in their diet. Most pimelodids feed on only one or two species; however, some pimelodids, such as Brachyplatystoma rousseauxii, feed on a maximum of five species.
Reproductive biology
Reproduction in most species occurs during the highwater season. Experimental studies have shown that gonad maturation is triggered mainly by decreasing water conductivity and increasing water level. Information on reproductive biology has been gathered from a limited number of species (8-9), but has consistently revealed diverse reproductive strategies. The electric eel breeds during the dry season in small ponds. The male builds a foam nest at the water surface between the adventitious roots of the plant Montrichardia arborescens. The eggs are deposited in the foam nest, and at the beginning of exogenous feeding, the larvae feed on eggs of subsequent ovipositions. Juveniles are guarded up to a length of 3.9 in (10 cm). The males of the banded knifefish are mouth breeders and guard the fry afterward. The longtail knifefish (Sternopygus macrurus) is a substrate spawner and guards the eggs until hatching. Other knifefishes hide the eggs in plants, in between roots, or in crevices. All species are fractional spawners, with spawning intervals of a few days up to several weeks. Egg diameter varies between 0.08 and 0.1 in (0.2 cm and 0.3 cm). Some species live in pairs during reproduction; others form complicated social hierarchies that are based on size, motor components of aggressive behavior, and seasonal conditions. Best studied in this respect is the glass knifefish (Eigenmannia lineata).
During courtship behavior, the electrical discharges serve various purposes: males and females often produce different electrical discharge frequencies, and frequency modulations of the male discharge can trigger oviposition. Large amplitudes characterize dominant males, differences in the form of the discharge occur between males and females.
During the breeding season the males of the Rosen knifefish (Sternarchorhynchus roseni) develop external teeth; these knifefishes are very territorial and rather aggressive. The males of gymnotiforms often grow larger than the females. Males of several species of the genus Apteronotus develop longer heads than the females.
Conservation status
No gymnotiform species are listed by the IUCN.
Significance to humans
The strongly electric Electrophorus electricus was for decades a preferred animal for the study of basic bioelectric phenomena. The physiology, anatomy, and behavior of weakly electric knifefishes have been studied intensively, particularly in the context of orientation (object location) and electrocommunication. Some larger knifefishes, such as Rhamphichthys rostratus and the longtail knifefish are locally of some economical importance in fisheries.
Species accounts
Black ghostSternarchorhynchus curvirostris
Electric eel
Banded knifefish
Brachyhypopomus pinnicaudatus
Bandfish
Glass knifefish
Longtail knifefish
Resources
Books:Albert, J. Species Diversity and Phylogenetic Systematics of American Knifefishes (Gymnotiformes, Teleostei). Ann Arbor: Museum of Zoology (University of Michigan), 2001.
Bullock, T., and W. Heiligenberg. Electroreception. New York: John Wiley & Sons, 1986.
Gayet, M., and F. J. Meunier. Première découverte de Gymnotiformes fossiles (Pisces, Ostariophysi) dans le Miocène supérieur de Bolivie. C. R. Acad. Sci. Paris, t. 313, ser. 2, II, 1991.
Kirschbaum, F., and L. Wieczorek. Entdeckung einer neuen Fortpflanzungsstrategie bei südamerikanischen Messerfischen (Teleostei: Gymnotiformes: Gymnotidae): Maulbrüten bei Gymnotus carapo. Vol. 2. In Verhalten der Aquarienfische, edited by H. Greven and R. Riehl. Bornheim, Germany: Birgit Schmettkamp Verl., 2002.
Mago-Leccia, F. Electric Fishes of the Continental Waters of America. Vol. 29, Biblioteca de la Academica de Ciencias Fisicas Matematicas y Naturales. Caracas, Venezuela: FUDECI, 1994.
Moller, P., ed. Electrical Fishes: History and Behavior. Fish and Fisheries Series 117. London: Chapman & Hall, 1995.
Periodicals:da Silva Assunção, M. I., and H. O. Schwassmann. "Reproduction and Larval Development of Electrophorus electricus on Marajó Island (Pará, Brazil)." Ichthyological Exploration of Freshwaters 5 (1995): 1–10.
Kirschbaum, F. "Reproduction of the Weakly Electric Fish Eigenmannia virescens (Rhamphichthyidae, Teleostei) in Captivity." Behavioral Ecology and Sociobiology 4 (1979): 331–355. ——. "Electric Fishes." Aqua Geographia 1 (1992): 59–70.
Kirschbaum, F., and F. J. Meunier. "Gymnotiform Fishes As Model Systems for Regeneration Experiments." Arch. Anat. Mic. Morph. Exper. 75, no. 4 (1986): 307.
[Article by: Frank Kirschbaum, PhD]
