killifish

Killifish are a diverse group of small, often colorful freshwater fishes in the order Cyprinodontiformes. They inhabit tropical and subtropical regions, including Africa, the Americas, and parts of Asia, in a range of habitats from permanent rivers to temporary ponds and marshes. Many species tolerate brackish water, and some are adapted to isolated wetlands.

A distinctive feature is their life history variation. Some species, known as annual killifish, live in ephemeral ponds that dry out seasonally; their eggs survive the dry period in the mud and hatch when rains return. Non-annual killifish live in more stable aquatic environments and have shorter diapause periods or none at all. Reproductive behavior varies; males are often more colorful and may display elaborate courting displays.

Killifish are typically egg-layers. Eggs are laid on firm substrates or vegetation and may require incubation, especially in annual species where desiccation-resistant eggs develop. In captivity, they are popular among aquarists for their vivid patterns and relatively small size, though some species require specific water conditions, such as soft, acidic freshwater, or higher hardness for others.

Taxonomy is complex and varies by authority. Families commonly referred to as killifish include Aplocheilidae, Nothobranchiidae, Rivulidae, and Fundulidae. With hundreds of species described, they range from tiny 2 cm micro species to larger ones approaching 10 cm. Conservation concerns arise from habitat destruction and pollution, particularly for specialty freshwater habitats and isolated wetlands.