What if you were born into a world that only blinks into existence once a year, and lasts for mere weeks or months before disappearing again? How would you live your life?
We can find the answer in East Africa, where a fish lives out its entire lives in rapidly shrinking pools of water.
During the rainy season, water fills small depressions in the savannah, creating temporary ponds. It’s the cue that the turquoise killifish (Nothobranchius furzeri) has been waiting for. Its eggs, encased in mud and lying dormant within the soil, finally hatch. Right from the start, the baby fish are on borrowed time. They have a couple of months before their puddle dries out. Before that unpredictable deadline, they’ve got to grow to sexual maturity, mate, and lay the next generation.
The turquoise killifish has adapted to this precarious existence by evolving the shortest lifespan of any back-boned animal. In the wild, they live for a few months and they fare little better in captivity. Back in 2003, Italian scientists Stefano Valdesalici and Alessandro Cellerino showed that groups of captive killifish start dying after just six weeks. On average, they survive for nine weeks, and none of them make it past eleven. For comparison, other related killifish live for around a year, as do tiny mammals like shrews.
If they die young, they’ve got to live fast. By studying the turquoise killifish and a related species, Nothobranchius kadleci, Czech scientist Radim Blažek showed that their body length increases by up to a quarter every day in their first two weeks of life.
By days 11 to 13, the males are already wearing their bold red adult colours. By days 17 to 19, the females are sexually mature and start to release eggs, which the males lace with sperm. Again, these are record-breaking figures for vertebrates. Female laboratory mice take at least 23 days to become sexually mature, as do the tiny wild infantfishes. The killifishes beat that by almost a week.
At first, the females lay a few dozen eggs a day, but they start producing hundreds once they stop growing and start channelling all their energy into reproduction. One particularly enterprising female managed to lay 583 eggs in a day. The first of these fertilised eggs develop so quickly, that if there’s enough water left, they can hatch in just 15 days. In as little as a month, the next generation is born.
The killifishes show how animals can adapt to extreme environments by evolving extreme lifespans. Another example comes from Madagascar. In response to the country’s harsh and highly seasonal environment, Labord’s chameleon spends the majority of its life within its own egg. An entire generation hatches during rainy November, matures by January, breeds by February, and dies by April. Meanwhile, their eggs stay underground until the following November. This unusual cycle means that at any given time, there’s only one generation of Labord’s chameleon on the planet and they’re all the same age.
Reference: Blazek, Polacik & Reichard. 2013. Rapid growth, early maturation and short generation time in African annual fishes. EvoDevo, citation tbc.
Valdesalici & Cellerino. 2003. Extremely short lifespan in the annual fish Nothobranchius furzeri. Biology Letters http://dx.doi.org/10.1098/rsbl.2003.0048