'', a killifish of the family
Nothobranchiidae. Some members of this order are notable for inhabiting extreme environments, such as
saline or very warm waters, heavily polluted waters, rain water pools devoid of minerals and made acidic by decaying vegetation, or isolated situations where no other types of fish occur. They are typically
carnivores, and often live near the surface, where the oxygen-rich water compensates for environmental disadvantages. Scheel (1968) observed the gut contents were invariably ants, others have reported insects, worms and aquatic crustaceans. Aquarium specimens are invariably seen eating protozoans from the water column and the surfaces of leaves, however these are not apparent as stomach contents. Many members of the family
Cyprinodontidae (the pupfishes) eat plant material as well and some have adapted to a diet very high in algae to the point where one, the
Flagfish also known as American flagfish, is a renowned algae eater in the aquarium, in spite of belonging to an order of fishes that do not generally consume any plant material. In addition, killifish derive some of the
carotenoids and other chemicals required to make their body pigments from pollen grains on the surface of and in the gut of insects they eat from the surface of the water; this can be simulated in culture by the use of special color enhancing foods that contain these compounds. Although the Cyprinodontiformes are a diverse group, most species contained within are small to medium-sized fish, with small mouths, large eyes, a single
dorsal fin, and a rounded
caudal fin. The largest species is the
cuatro ojos (
Anableps dowei), which measures in length, while the smallest, the least killifish (
Heterandria formosa), is just long as an adult. ==Systematics==