Killifish are a group of small oviparous (egg-laying) ray-finned fish that possess some pretty remarkable abilities. Nothobranchius furzeri,the African turquoise killifish, is the shortest living vertebrate that can be bred in captivity, having a natural lifespan ranging between 4 and 9 months. As a result, this short-lived little species will potentially help to advance our understanding of the fundamental processes of ageing.
“Annual” killifish, the term given to an organism that lives less than a year, lives at two speeds, either in pause or fast-forward. Living in the seasonal pools of African savannah is unpredictable, water can be scare and to do so killifish have evolved some fascinating physiological adaptations. Similar to the seeds of some plants, killifish predominantly exist as diapausing embryos in dry river bed sediments. Diapause a is a period of suspended development usually during unfavorable environmental conditions. When the rains hit, rivers and ponds form temporarily. Because time is short, these tiny fish must hatch, grow, mature sexually and reproduce before their resident water dries up, while avoiding predation and competing for both resources and mates.
Recent research in Current Biology by Martin Reichard and his colleagues of the Institute of Vertebrate Biology, The Czech Academy of Sciences, has delved into the fast-juvenile growth and rapid sexual maturation shown by N. furzeri in captivity and they have shown that it is actually an underestimate of their natural developmental rate. "We guessed that some populations of this species could achieve very rapid growth and sexual maturation under particular conditions," says Reichard, "but we have found that this rapid maturation is the norm rather than a rare exception."
The research team surveyed natural populations of N. furzeriacross its range in southern Mozambique from January to May 2016. Fish were collected from eight separate pools within a period of three weeks after the pools first filled with rainwater. They estimated the age of N. furzeri in these natural populations by counting daily-deposited increments in the otoliths and performing histological analysis of gonads. Otoliths are the small ear bones of fish, which produce growth rings essentially just like trees. Histological analysis involves removing the fish gonads (ovaries and testis), dehydrating them, embedding in wax, taking sections micrometers in thickness, mounting on slides, staining them and examining the developmental stages of the gonads under the microscope. As these fish are not very large (typically <7 cm), removing otoliths and gonads would have been quite the task.
First, they determined that the fish hatched within three days of rain by comparing the timing of the pools' filling and the estimated age of the fish. Second, microscopic analysis of male and female gonads found that individuals of both sexes were fully mature within 14 or 15 days.To their knowledge this is the fastest rate of sexual maturation on record for a vertebrate.Previous studies have shown under optimal laboratory conditions, N. furzeri reached sexual maturity in as few as 18 days.Finally, they determined that growth from 5 mm up to 54 mm occurred over a two-week period. Such rapid juvenile development is likely to be adaptive since some pools were entirely desiccated 3–5 weeks after filling.
Having two very contrasting life-cycle phases, which are seemingly very flexible and adaptable is very cool if you ask me. But what can studying killifish ageing tell us about human ageing?
For the full text please see https://www.sciencedirect.com/science/article/pii/S0960982218308224
Vrtílek M, Žák J, Pšenička M, Reichard M. Extremely rapid maturation of a wild African annual fish. Current Biology. 2018 Aug 6;28(15):R822-4.
