When we think of fish, there are certain assumptions we make. Fish breathe using their gills. Fish also live in water. However, there is one group of fish that does not follow any of these rules. Known as killifish, these small innocuous-looking creatures measure just a few inches long. There are a number of killifish species living across Africa and South America. A few can claim to be the world’s most extreme. That’s because some killifish don’t reproduce like other fish. Killifish don’t like to grow old – well, not old in the conventional sense. They don’t mature like other fish, and many killifish species will live and die within a year. One species survives for less than three months, making it one of the shortest-lived of all known vertebrates.
They have other ways of breathing, apart from using their gills. And swimming isn’t really their thing. They prefer to live in puddles and pools, and have evolved their whole life strategy to cope without water, rather than with it. Some species can survive out of water for more than two months. They will leave it to go for a walk and they even hunt on land. Some have even been found living in trees. Most are adapted to living in places where water is scarce, or where water levels change drastically from season to season. They leave behind eggs in the soil, and these eggs represent the entire surviving populations of killifish. The return of the rains causes eggs to hatch and the cycle begins anew.During the wet season, when pools are full, the fish hatch. They quickly grow to maturity and start spawning, which they keep doing until the pools start to dry. At that point all the fish perish. This cycle of life can be so quick that one species, the turquoise killifish (Nothobranchius furzeri) of equatorial Africa, lives for an average of just 10 weeks.
The mangrove killifish (Kryptolebias marmoratus), for example, is a hermaphrodite, possessing both male and female sex organs. But the mangrove killifish is the only known vertebrate hermaphrodite that can fertilise itself, producing genetically identical clones. And just to mix it up a bit, a few male mangrove killifish do exist, which mate in a more conventional sexual fashion with other hermaphrodites, which act as females. Most fish breathe with their gills, passing water over them to extract oxygen. The mangrove killifish however, can also breathe through its skin. Essentially, the fish have moved onto land as the water recedes, and survive by breathing air through their skin. They even excrete waste through their skin and tests have shown that mangrove killifish can survive out of water for up to 66 days.