Order Gymnotiformes
Suborder Gymnotoidei
Gymnotiform fishes are distributed exclusively in the freshwaters of Central and South America, from Guatemala to La Plata, in Argentina and Paraguay. They inhabit mainly slow-flowing or standing water bodies, in small lakes or bays densely overgrown with aquatic vegetation.
The order of gymnotiform fishes combines four families: electric eels, Gymnotidae, Rhamphichthyidae, and Sternopygidae (or Apteronotidae). The last three families are often called American knifefishes by aquarists. In total, these four families contain about 50 species.
Some representatives of the suborder are also very similar to African knifefishes (family Notopteridae), sometimes to such an extent that an inexperienced observer could assign these fishes to the same family. However, even in this case, the similarity in appearance is merely a manifestation of convergence—the development of similar features and adaptations to similar conditions, rather than true kinship.
Certain features of internal structure undoubtedly indicate that gymnotiform fishes are most closely related to Characiformes, although externally they differ significantly by having an elongated, often eel-like body and the complete absence of pelvic and true dorsal fins.
The external appearance of gymnotiform fishes is characterized by small eyes, an elongated, thin, and sometimes sharp caudal region, a very long anal fin occupying 3/4 to 4/5 of the body length, and the unusual position of the anus, which is located on the throat, always in front of the base of the pectoral fins. The body of gymnotiform fishes is usually greatly elongated and laterally compressed, becoming almost ribbon-like in some species.
Scales are either absent or very small. The caudal fin is absent (in Gymnotidae and Rhamphichthyidae) or very small (in Sternopygidae). A true fin-rayed dorsal fin is never present in these fishes, but Sternopygidae possess a long, thread-like outgrowth in the middle of the back—a peculiar adipose-like fin.
Many species of this suborder possess more or less developed electric organs. Unlike African mormyrids, the electric discharge in gymnotiform fishes is not continuous, but pulsatile, and the pulse frequency in some species (for example, the black ghost knifefish — Apteronotus albifrons) can reach 1000 per second.
Family Gymnotidae
This family is characterized by the following features: head scaleless, no barbels; the margin of the upper jaw is formed by the premaxillary and maxillary bones; body elongated, eel-like; dorsal fin absent or represented by an adipose ridge, caudal fin usually absent, tail ending in a point, anal fin extremely long, pelvic fins absent. The anus is located on the throat or a short distance behind it; gill openings rather narrow; swim bladder double.
Characteristics of the genus Gymnotus: dorsal and caudal fins absent, anal fin reaching the end of the tail, scales absent, teeth conical, in a single row, eyes extremely small. The main feature of the electric eel is the most strongly developed electric apparatus among all electric fishes, in the form of two pairs of long bodies (almost 4/5 of the total length) lying directly under the skin outside the muscles; one extends along the dorsal side of the tail, the other along the sides of the base of the anal fin. These organs are divided by connective tissue septa into numerous polygonal plates forming prisms.
The electric shock depends entirely on the will of the animal and serves it, on the one hand, as a means to stun prey, and on the other, to defend against enemies. The strength of the shocks depends on the size and condition of the fish. After a series of shocks, the fish needs rest to be able to strike again. Gymnotus species are very numerous in Brazil and Guiana. They are all secretive, predominantly nocturnal, colored in brown and black tones. They reach a length of up to 2 m and a weight of 15-20 kg.
Identification: Genus Gymnotus.
Genus Gymnotus.
— Banded Knifefish, Carapo — Gymnotus carapo [Linnaeus, 1758]. Synonym: Carapus macrourus, Gymnotus albus, brachiurus, fasciatus, omarorum, Sternopygus carapus. (Eng. Banded Knife Fish). From Mexico to Paraguay. It lives in both large rivers and swamps. 65 cm. The front part of the body is cylindrical, the rear is laterally compressed, the mouth is directed upwards, dorsal and caudal fins are absent, the body is covered with small scales.
One of the distinctive features of these fishes is the long anal fin, which begins near the pectoral fins and reaches the very tip of the tail. The coloration on the back is olive-green, the lower part is orange and red with oblique, forward-directed white stripes and spots. Fry of Gymnotus are very similar to electric eel fry.
This is a very aggressive and territorial fish, and a very large aquarium will be required. It has a large mouth, characterized by well-developed teeth on the jaws — active nocturnal predators. Gymnotus hide during the day at the bottom of water bodies, and at night they hunt small fish, worms, and insect larvae. Any small fish in the aquarium will be eaten, but they can be kept with peaceful, similarly sized fish. Considering that they grow up to 65 cm in length, they can be introduced to large South American cichlids; young fish also live with smaller fish. Large flower pots or pipes, shelters made of stones and driftwood are necessary to create hiding places for these fishes.
They move using undulating bends of the long anal fin without bending their body. Do not keep them together with other knifefishes; it is better to keep them singly or introduce them to large South American cichlids. Requires a very large and densely planted aquarium equipped with hiding places made of plants and stones. They lie on the bottom a lot and hide in shelters. Floating plants should be used to reduce lighting.
They have poor vision, so to compensate for it, they have an organ that creates a very weak electric field around their body. They are capable of emitting electric discharges. These discharges are weak; the fish use them not to paralyze prey, as electric eels do, but merely for reconnaissance, finding food, and communication. The carapo senses an obstacle or another fish by the distortion of the magnetic field surrounding it. When two Gymnotus meet, the fishes change the activity of their electric signals and communicate. If the carapo decides to chase away a smaller rival, it slightly increases the signal frequency. The frequency increases especially noticeably during an attack. If two Gymnotus are friendly towards each other, they switch to the same frequency. The fish that behaves as a dominant synchronizes its signals in such a way as to emit them during pauses of the subordinate fish.
Very sensitive to polluted water, so weekly 30-50% water changes are necessary. Feeding is the most difficult part of caring for them. Carnivorous, feeding on small fish and fleshy fresh or frozen foods. They are also fed tubifex and bloodworms. They are sensitive to certain medications containing copper and formalin. Live up to 12 years. pH 6.0-7.5; dH up to 15°; T 23-28°C.
— Zebra Knifefish — Gymnotus pedanopterus [Mago–Leccia, 1994]. (Eng. Zebra Knifefish). Distributed in the Amazon river basin in southern Venezuela, as well as in the Rio Negro in Brazil. 28 cm. The body is more rounded, has small scales and a mouth directed upwards. They do not have a dorsal fin, but have an extremely long anal fin that begins near the pectoral fin. One of the distinguishing characteristics of these fishes is the anal fin reaching the very tip of the tail. Body coloration resembles a “zebra”, alternating dark and light stripes running along the entire length of the body. Sexual differences are unknown.
They have an organ that produces a weak electric field around the body, which detects objects and movements around it. This helps in navigation and hunting, object identification in water, spatial orientation, helps identify food, and is also used as a way to communicate with other fishes.
A very aggressive fish, territorial towards its own kind, so this species is probably best kept alone. Any other fish should be of approximately the same size so they do not become food. They should only be kept by experienced aquarists. An aquarium with dense broad-leaved vegetation and shelters made of rocks and wood, clay pots, with open sandy areas. Fine sand is the best choice for substrate. Dimmed lighting with floating plants to create twilight.
They are bottom-dwelling species inhabiting the river bed and living in leaf litter and root thickets of river banks. They spend their time on the bottom or in the lower part of the aquarium. Good water quality and weekly water changes are necessary for maintenance. These are carnivorous nocturnal predators feeding on insects, worms, crustaceans, and small fish. Feeding is the most difficult part of caring for them. Live up to 15 years. T 22-28°C; pH 6.0-7.0; dH 2-12°.
Electrophorus electricus, Electric Eel.
Oryzias javanicus, latipes, woworae. 
