Meet the Fish That Grows Up in Just 14 Days

The turquoise killifish, which lives in ephemeral pools in Mozambique, progresses from embryo to sexual maturity faster than any other vertebrate

These ephemeral puddles are the birthplace, home and grave of the killifish. M. Reichard

Some animals live fast and die young. That means they need to grow up fast, too. This week, researchers crowned a new record holder for quick growth: Susan Milius at Science News reports that the turquoise killifish, Nothobranchius furzeri, found in Mozambique, can reach maturity in just 14 days, the fastest of any known vertebrate animal.

That rapid maturation is an adaptation to the killifish’s habitat, according to the study published this week in the journal Current Biology. The fish spend most of their lives as tiny embryos that have been deposited in sediment in small depressions across the savannah. When rain fills the ephemeral pools, the embryos mature rapidly reaching sexual maturity and depositing their own embryos before the pool once again dries up. Not only do they make babies quickly, they bulk up fast, too—typically growing from about 5 millimeters to 54 millimeters in their lifespan.

Researchers have been aware of the turquoise killifish’s super-fast maturation for a while. In fact, the fish is used as a model animal in aging studies because of this trait. In the lab, where the fish live a relatively leisurely lifestyle, the average rate of maturation is 18 days.

The new study’s author, Martin Reichard, a biologist at the Czech Academy of Sciences in Prague, and his team suspected that in the wild the fish might reach reproductive age even more quickly. They surveyed wild populations of killifish in southern Mozambique between January and May 2016, looking at eight ephemeral pools of water. They found that the buried fish eggs hatch within three days of a rainfall. By observing the gonads of both males and females, the researchers found the fish reach sexual maturity between 14 and 15 days.

“We guessed that some populations of this species could achieve very rapid growth and sexual maturation under particular conditions,” Reichard says in a press release. “But we have found that this rapid maturation is the norm rather than a rare exception.”

Bilal Choudhry at The New York Times reports that growing up so fast comes at a cost. Their cells also deteriorate much more quickly than other fish, meaning they age much faster as well. Then again, they don’t have much to live for—eventually their muddy hole will evaporate, leaving them high and dry. That also means they don’t mess around when it comes to mating. “[These fish] do not waste time with anything,” Reichard tells Milius. “Mating does not involve much elaborate courtship.”

Typically, a male simply extends his fins, and if the female likes what she sees, she’ll drop an egg that he can fertilize. Then she’ll swim off to another male. In total, she can release 20 to 100 eggs per day, “typically before noon,” says Reichard.

Those embryos will eventually sink into the sediment as the pool dries—protected by a hard shell akin to a plant seed—waiting for the next round of rain to come, a technique known as diapause. It’s an adaptation to a backwards lifestyle. Embryonic vertebrates usually grow up protected in a womb, an egg mass fussed over by a feisty mama fish or some other safe space. The hard part of life is being a grownup, looking for food, avoiding predators and searching out a mate.

For killifish, adulthood is a quick orgy in a mud puddle and the egg stage is the dangerous part. “Usually, vertebrates cope with harsh conditions during their adult stage, like bears during wintertime. However, with the embryos, it can be observed during their early developmental stages,” Reichard tells Choudhry.

The research also shows that the speed of aging can be variable for the fish. While the wild fish aged rapidly, some killifish in the lab have taken a drawn out 10 weeks to grow up, five times as long as their wild cousins. In follow up studies, the team hopes to look at the slight differences in maturation rates for wild killifish and look into why male killifish tend to die sooner than females.

And at the end of the day, if you’re worried about your kids growing up too fast, just be glad you’re not a killifish.

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