Characins are members of the order Characiformes, an extremely diverse group of fish containing well over 1,500 species. Ranging in size from the ⅔-inch pygmy blue characin (Xenurobrycon polyancistirus) to the 4½-foot tigerfish (Hydrocynus goliath), characins are found throughout Africa and in Central and South America.

Many species of characins are important to people as a food source and as popular aquarium fish. Within this group are found the tetras, many species of which are among the most popular and commonly kept fresh-water aquarium fish. Also numbered among the members of this group is the infamous piranha, much feared throughout South America as a voracious maneater. As usual, the truth is far less dramatic, with the well-publicized piranha “feeding frenzies” occurring far less regularly than people may think. The species most associated with this phenomenon is the red-bellied piranha (Pygocentrus nattereri). (See color insert for photo.) The razor-sharp teeth of this predator and its habit of hunting in large schools does render it dangerous under certain conditions, particularly during the dry season when the water levels drop, crowding the fish and diminishing food supplies. Blood in the water can trigger a mass attack, and reportedly, fairly large animals can be stripped of flesh in a relatively short time. It is also said that piranhas that live in the vicinity of wading bird rookeries are more prone to aggressive behavior, accustomed as they are to feeding on nesting birds that fall into the water.

Although famous for their meat-eating tendencies, in actuality, most members of the subfamily to which piranhas belong are peaceful vegetarians. One, the Amazonian tambaqui (Colossoma macropomum) is an important source of food throughout its range. Another, the silver dollar (Metynnis argenteus), is a well-known and fairly peaceable aquarium fish.

Fascinating in appearance, habits, and life history, the blind cave fish does not get nearly the attention it warrants. Native to only one stream near the town of San Lois Potosi in Mexico, individuals that had been swept into a cave by the stream's current have undergone a remarkable series of adaptations to their new environment. All those just outside the cave entrance appear quite normal in all respects, but the fish isolated within the cave are white in color and lack eyes. Deprived of light and, therefore, the need for eyesight, eyes have degenerated and become covered with skin, and the body color has been lost as well. They navigate entirely by the use of the lateral line organs, and it is quite amazing to see how well they get around objects and how alert they are to the presence of food, despite not be able to see. Blind cave fish are actually fairly good community tank members, despite their unique habitat. Growing to a length of 5 inches, this fish does best in hard water but is some-what adaptable in that regard. They will consume all matter of aquarium fish foods and compete well at feeding time with their sighted relatives. The young are born with what appear to be fully functional eyes, but they degenerate rapidly and are soon covered by a fatty pad of skin.

This and the other ten or so species of hatchetfish might well be dubbed “freshwater flying fish.” Although they lack the advanced aerial abilities of their saltwater namesakes, hatchetfish are specifically adapted to gliding above the water surface for short distances to escape predators. This feat is accomplished through the use of powerful muscles that propel the long pectoral fins. The unique, very thin body form also appears to lessen water resistance as the fish breaks the water's surface.

Hatchetfish range throughout the area from Panama to Paraguay in Central and South America. Most have silver coloring, with the marbled hatchetfish possessing a very attractive black pattern as well. They are specifically adapted to feeding upon surface-dwelling insects and flying insects that fall into the water. The uniquely upturned mouth suits the hatchetfish well in this regard and allows it to exploit a food source denied to fish with more generalized mouthparts.

Hatchetfish will take a variety of floating aquarium foods but should be offered small insects whenever possible. Their reaction to crickets placed on the water's surface is fascinating to behold — they go into immediate action and really do seem to enjoy it.