The Melanotaeniidae is characterised by having their distal
premaxillary teeth enlarged. They have a
compressed body with two
dorsal fins separated by a small gap between them. There are 3–7
spines in the first dorsal fin while the second has 6–22 rays, with the first ray being a stout spine in some species, the
anal fin has 10–30 rays and, again, the first may be a stout spine in some species. The
lateral line is either weakly developed or absent. They have comparatively large scales and these number 28–60 in the lateral series. The
pelvic fins are attached to the fish's abdomen by a membrane which runs along the length of the innermost ray and this is a feature which can be used to separate rainbowfishes from silversides (
Atherinidae), although the membrane is easily torn. The majority of the species in this family demonstrate conspicuous
sexual dimorphism, with the males usually being the more colorful sex and also showing an elongated median fin ray. Most species of rainbowfish are less than in length, with some species measuring less than , while one species,
Melanotaenia vanheurni, reaches lengths of up to . They live in a wide range of
freshwater habitats, including
rivers,
lakes, and
swamps. Although they spawn all year round, they lay a particularly large number of eggs at the start of the local
rainy season. The eggs are attached to
aquatic vegetation, and hatch seven to 18 days later. Rainbowfish are generalized omnivores, feeding on small crustaceans,
insect larvae, and algae.
Melanotaenia,
Pseudomugil, and several other genera of rainbowfish include species that regularly appear in the
aquarium trade. In the wild, some rainbowfish populations have been severely affected by the aggressive
introduced eastern mosquitofish (
Gambusia holbrooki),
tilapia cichlids, and
pollution. ==Classification==