Nannostomus trifasciatus
three-lined pencilfish

Nannostomus trifasciatus is a freshwater fish native to Guyana, the Rio Negro, and the middle portion of the Amazon River (Wheeler, 1975; Sterba, 1963).

N. trifasciatus prefers slow-moving, peaty, slightly acidic water. It is also usually found in shaded, weedy areas. This species fares best at temperatures between 78-80° F. Within these conditions, N. trifasciatus is found in several different habitat types. In the Rio Negro, it inhabits the large swamps that form where tributaries meet the main branch of the river. When the river floods, it moves into the inundated rainforest. During the low water season, it often becomes trapped in small lakes that are left behind, or stays close to the wooded edges of the forest. N. trifasciatus rests near the water surface at night and during the day inhabits the middle to upper water layers (Torres et al., 2002; Masagaki et al., 1999; Goulding et al., 1988; Wheeler, 1975; Sterba, 1963; McInerny, 1958).

The three-lined pencilfish is a small fish, with adults not reaching more than 6 cm in length. It has a small terminal mouth, and an adipose fin may or may not be present. Many characiform fishes of the Amazon basin possess distinct daytime and nighttime color patterns, and N. trifasciatus is no exception. During the day, it has three dark longitudinal stripes that run the length of the body. At night, the stripes disappear and three large dark spots materialize on the sides of the fish, extending from its back to its belly. Recent research has shown that this change in coloration is due to the differential action of the pineal hormone melatonin on pigment cells in different regions of the integument. It has been suggested that the daytime color pattern may serve an aposematic or recognition function for individuals of the same species, and that the nighttime pattern may help hide the fish from nocturnal predators. In between the variable, darkly-pigmented regions, N. trifasciatus is greenish-gold in color on its back and sides and silver underneath. The gill cover and dorsal, pelvic, anal, and caudal fins possess large red blotches. Males of the species may have an additional row of red spots in the gold area between the middle and uppermost stripes (Masagaki et al., 1999; Wheeler, 1975; Sterba, 1963; McInerny, 1958; Innes, 1956).

Immediately after hatching, the young hang on to plants or rest on the bottom. They do not swim freely until their 5th day. A juvenile N. trifasciatus does not look like its parents; it is long, thin, and darkly colored. It also possesses an extended adipose fin that it uses to swim. Growth is slow in this species, and juveniles reach adulthood in seven months (Sterba, 1963; McInerny, 1958; Innes, 1956).

This species is active during the day, resting at night in the upper layers of the water. It swims with its body in a horizontal position. The fact that its daytime coloration may be an aposematic signal to members of its own species suggests that it may exhibit some degree of territoriality or dominance hierarchy in the wild. N. trifasciatus may also be migratory, as it moves from the main river channels to flooded forest during the rainy season (Masagaki et al., 1999; Goulding et al., 1988; Sterba, 1963).

Little is known about communication in N. trifasciatus. As mentioned above, its bright daytime coloration may serve either to warn or help it to recognize conspecifics (Masagaki et al., 1999).

N. trifasciatus spends much of its time near the water surface, where it feeds primarily on insects. It has also been reported to eat detritus and algae. In captivity, it will eat just about any type of standard fish food (Goulding et al., 1988; Wheeler, 1975; Sterba, 1963; Innes, 1956).

N. trifasciatus often falls prey to other fish species, such as the striped pike-characin, Boulengerella lateristriga. Its blotchy nocturnal pattern may be a means of avoiding predation at night, as the blotches serve to break up the outline of its body. (Goulding, Carvalho, and Ferreira, 1988; Masagaki and Fujii, 1999)

N. trifasciatus is a primary, secondary, and tertiary consumer, because it feeds on a variety of invertebrate prey and plant material. It also provides food for animals at higher trophic levels (Torres et al., 2002; Goulding et al., 1988).

Nannostomus trifasciatus have no known adverse effects on humans.

Due to its attractive coloration and ease of keeping, N. trifasciatus is a popular aquarium fish (Sterba, 1963; McInerny, 1958; Innes, 1956).

N. trifasciatus is fairly common throughout its home range, and because it is bred easily in captivity for the aquarium trade, it is unlikely that this species will face extinction in the near future (Torres et al., 2002; Goulding et al., 1988; Sterba, 1963; McInerny, 1958; Innes, 1956).