Fish
Killifish
Cyprinodontiformes · also called killifish, annual fish
A killifish can pack an entire life, hatching, growing, breeding, and dying, into a few months, because the pool it lives in is going to dry up completely and on schedule. When that happens, every adult dies. The species survives only as eggs buried in the drying mud, paused mid-development, waiting for the next rain to bring them back.
Overview
Killifish are not one family but an order, Cyprinodontiformes, with well over a thousand species across Africa, the Americas, Europe, and Asia. They eat mosquito larvae and other small invertebrates from the surface, and many, especially the males, are prized for colour far more intense than their small size suggests, used in courtship display.
Built to outlast a drought
Annual killifish live in seasonal African and South American pools that dry up completely every year. Their eggs can enter diapause, a genuinely arrested state of development that resists months of drying and heat, resuming only once rain refills the pool. Some species, like the turquoise killifish, complete their whole life cycle in four to six months — among the shortest of any vertebrate, which has made them a fast model for ageing research. Hobbyists trade the eggs packed dry in peat moss rather than shipping live fish.
Status
Highly variable — many widespread species are secure, but a large number of annual killifish are restricted to a single pool or small cluster of pools in seasonal African and South American wetlands, and a significant share are Vulnerable, Endangered, or Critically Endangered as those wetlands are drained and developed.
Killifish: common questions
What does it mean that a killifish is annual?
An annual killifish is a species whose entire life cycle is timed to a temporary body of water that dries up completely for part of the year, so called because generation after generation typically lasts about one year, from egg to breeding adult to death. When the pool the fish lives in dries out at the end of the wet season, every adult fish dies, and the species survives purely as eggs buried in the damp mud, which resist drying and heat until rain refills the pool and triggers the next generation to hatch, grow, and breed, often within just a few weeks to months.
How can killifish eggs survive being completely dried out?
The eggs of annual killifish are able to enter diapause, a genuinely arrested state of development in which growth essentially pauses at one of several possible checkpoints, protected inside a tough shell that resists water loss. In this suspended state the embryo can withstand conditions, including extended drying and considerable heat, that would kill an active embryo of almost any other fish. Development only resumes once the egg senses that conditions, chiefly moisture and appropriate temperature, are suitable again, which is generally triggered by the pool refilling with fresh rain.
Why do killifish only live a few months?
For annual species, a short lifespan is a direct consequence of living in a habitat that periodically vanishes. Because the pool will dry up on a fairly predictable seasonal schedule, there is no advantage to a longer adult lifespan, and instead these killifish grow explosively fast, reach sexual maturity in weeks, and spend their brief adult life spawning as much as possible before the water disappears. The turquoise killifish takes this to an extreme, completing its whole life cycle in around four to six months, which has made it valuable to scientists studying the biology of ageing because results that would take years to observe in a mouse can appear in months.
Can killifish be kept in a normal community aquarium?
Some non-annual killifish adapt well to a peaceful community tank, but many, especially the more colourful annual species, are better suited to a dedicated species tank because of their specific water and breeding needs, their sometimes short and demanding life cycle, and the fact that many are more aggressive or fin-nippy than typical community fish. Annual species are also usually bred by hobbyists specifically to collect and store the eggs through a dry period, a practice that does not fit the usual pattern of simply stocking and maintaining fish long-term, so killifish keeping tends to be a more specialised branch of the aquarium hobby.