Ephemeral habitats can impose challenging conditions for population persistence. Survival strategies in these environments can range from high dispersal capacity to the evolution of dormant stages able to tolerate a harsh environment outside the temporal window of favourable conditions []. Annual killifish have evolved to live in seasonal pools on the African savannah and display a range of adaptations to cope with an unpredictable environment []. For most of the year, killifish populations survive as diapausing embryos buried in dry sediment. When savannah depressions fill with rainwater, the fish hatch, grow rapidly and, after attaining sexual maturity, reproduce daily []. Nothobranchius furzeri, a model species in ageing research [], is distributed in a region where the climate is particularly dry and rains are unpredictable []. Here, we demonstrate that the fast juvenile growth and rapid sexual maturation shown by N. furzeri in captivity is actually an underestimate of their natural developmental rate. We estimated the age of N. furzeri in natural populations by counting daily-deposited increments in the otoliths and performing histological analysis of gonads. We found that N. furzeri are capable of reaching sexual maturity within 14 days after hatching, which to our knowledge is the fastest rate of sexual maturation recorded for a vertebrate. We also demonstrate that N. furzeri can grow from an initial length of 5 mm up to 54 mm over the course of a two-week period. Such rapid juvenile development is likely to be adaptive since some pools were entirely desiccated 3–5 weeks after filling, but retained a viable killifish population that reproduced before the adults succumbed to the disappearance of their pool.

From the bush to the bench: the annual Nothobranchius fishes as a new model system in biology.

From the bush to the bench: the annual Nothobranchius fishes as a new model system in biology.

From the bush to the bench: the annual Nothobranchius fishes as a new model system in biology.

Main Text

Figure 1 Adult turquoise killifish (Nothobranchius furzeri) and its natural habitat. Show full caption (A) Site ‘D’ on 27 January 2016 when it contained at least 27 adult fish estimated to be 16–17 days old. (B) Two 14-day-old male N. furzeri from site ‘A’. (C) Histological section of an ovary from a 14-day-old female from site A that contains oogonia (dotted black circle), primary oocytes at the primary growth phase, chromatin-nucleolus stage (black circle) and perinucleolus stage (white circle), oocytes in the previtellogenic (-) and vitellogenic phase (+) and fully developed oocytes (∗). (D) Histological section of the testis from a 14-day-old male from site A that includes spermatogonia (full white circle), spermatocytes (dotted white line), spermatids (broken black line) and spermatozoa (full black line). The boxed area (E) shows a magnified view of mature spermatozoa released into the spermatic duct. We surveyed natural populations of N. furzeri across its range in southern Mozambique between January and May 2016 by regular inspection of field sites ( Figure 1 A) and installation of dataloggers. We collected N. furzeri from eight separate pools (A–H, Table S1 , see Supplemental Information ) within a period of three weeks after the pools first filled with rainwater. Comparison between the timing of pool filling (recorded during our presence at sites and estimated from dataloggers) and fish age (estimated from otolith growth checks, n = 26) indicated that fish hatched within the first three days after rains flooded a pool ( Table S1 ). Histological analysis of male and female gonads of individuals aged 14–15 days (n = 6 fish from 2 populations, and 8 control fish) demonstrated that both sexes were mature. Females possessed ripe ova and males had developed spermatozoa that were present in the sperm ducts ( Figure 1 C–E). Sexually dimorphic nuptial colouration ( Figure 1 B, Table S2 ) suggested that all fish were sexually mature in four populations (A, C–E) estimated to be 14 days old. In a further three populations (B, F and G) fish were estimated to be 12–15 days old and included a combination of juvenile and sub-adult fish. A final population (H) was comprised of 14-day-old fish that were still juveniles. Mean size of fish aged 14–15 days ranged from 18.8 to 42.7 mm in four populations, with an overall range of 17–43 mm in females and 17–54 mm in males. At the age of 17 days one male reached 63 mm ( Table S2 ; n = 260 fish measured overall).

4 Blažek R.

Polačik M.

Reichard M. Rapid growth, early maturation and short generation time in African annual fishes. 3 Hu C.K.

Brunet A. The African turquoise killifish: A research organism to study vertebrate aging and diapause. 6 Polačik M.

Blažek R.

Reichard M. Laboratory breeding of the short-lived annual killifish Nothobranchius furzeri. Previous studies demonstrated that under optimal laboratory conditions, N. furzeri can reach sexual maturity in as few as 18 days (first mating attempts were observed 1–3 days earlier), followed by rapid senescence in functional traits and high intrinsic mortality [], resulting in a median lifespan of only 4–6 months []. It was questioned, however, whether such a rapid life history was a natural feature of the species, or a consequence of laboratory selection for short lifespan combined with particularly favourable housing conditions with temperature, food supply and population density optimised for rapid growth []. Indeed, juvenile growth and sexual maturation are flexible in N. furzeri and respond strongly to changes in population density and resource availability.

7 Vrtílek M.

Žák J.

Polačik M.

Blažek R.

Reichard M. Longitudinal demographic study of wild populations of African annual killifish. 7 Vrtílek M.

Žák J.

Polačik M.

Blažek R.

Reichard M. Longitudinal demographic study of wild populations of African annual killifish. Surveys of natural N. furzeri populations in 2016 revealed that pools typically desiccated 1–4 months after filling []. The shortest period for which a pool contained water and supported a killifish population that reached sexual maturity was 20 days. In that population we sampled fish at the age of 7 days (all juveniles, n = 12), 14 days (adult colouration, size not measured, n = 5) and 16–17 days old (all fish adult, 32–45 mm long, n = 16) ( Table S2 ). Two other pools held water (and fish) for just 33 days and the median pool duration was 65 days [].

2 Cellerino A.

Valenzano D.R.

Reichard M. From the bush to the bench: the annual Nothobranchius fishes as a new model system in biology. 3 Hu C.K.

Brunet A. The African turquoise killifish: A research organism to study vertebrate aging and diapause. 2 Cellerino A.

Valenzano D.R.

Reichard M. From the bush to the bench: the annual Nothobranchius fishes as a new model system in biology. 3 Hu C.K.

Brunet A. The African turquoise killifish: A research organism to study vertebrate aging and diapause. This finding demonstrates that fast juvenile development is adaptive in killifish despite an apparent tradeoff with early-onset ageing; even a pool that desiccated in three weeks permitted successful reproduction of N. furzeri and, thereby, supported a viable population that year. Nothobranchius species display two contrasting life-cycle phases. The first is a long-lasting embryonic period with high resistance to ageing and a bet-hedging strategy of three facultative diapause stages that spreads out the developmental time of individual embryos []; this strategy serves as an adaptation to promote survival during extended periods of drought. The second is an exceptionally short post-embryonic period characterised by rapid juvenile growth and sexual maturity — an adaptation to unpredictable pool longevity that comes with a cost of short post-embryonic lifespan [].