The catchily named Nothobranchius kadleci is the fastest-maturing vertebrate species known to science. In just over two weeks, this fish can reach sexual maturity and start laying eggs — and those eggs only need another 15 days before they are ready to hatch. So why does this species need to cram an entire generation into a single month?
The reason is that this species, native to southern Mozambique, can't count on permanent access to water, which tends to be a pretty big deal if you're a fish. (Or most any vertebrate species, really, but fish really need water if they plan on surviving for long.) N. kadleci and its cousin species live in pools in the Mozambique savanna that don't exist year-round; rather, the pools form as the rainy season deposits clumps of water into the various natural, shallow depressions found throughout the region. Yes, this is a species evolved to live in what are basically just glorified puddles, and such a tenuous existence makes for some pretty incredible adaptations.
Because the pools tend to disappear as soon as the rainy season ends, the fish need to get through an entire life cycle as fast as possible. According to Dr. Martin Reichard and his team at the Czech Republic's Institute of Vertebrate Biology, the fish are capable of adding growing an additional 25% of their mass every single day, which is what allows them to reach sexual maturity after only about 17 days. Of course, even the ability to reproduce so quickly wouldn't mean much if the fish couldn't survive the times when the water disappears completely. While the end of the rainy season spells doom for the specific fish, their eggs are adapted so that they can remain tucked away in the soil throughout the dry part of the year, just waiting for the return of water to hatch and begin the super-charged life cycle anew. That supply of dormant eggs means the species can potentially even survive entire years without rain.
Speaking to the BBC, Dr. Reichard says that his team's lab-reared fish might actually be underselling just how fast these fish can reproduce:
"It is biologically very relevant for these fish to be able to sexually mature very fast because their habitat may dry out in three to four weeks. If they mature very fast, they can produce a new generation... I'm pretty sure if conditions are good, they would be able to sexually mature even faster in the wild. If conditions are inferior - food is less abundant, there is a high density of fish - it would take them longer but they can still complete their lifecycle."
Via BBC News. Image from the paper.