Limited differentiation of fundamental thermal niches within the killifish assemblage from shallow temporary waters

The sources of sex differences in aging in annual fishes

Inter-sexual differences in lifespan (age at death) and aging (increase in mortality risk associated with functional deterioration) are widespread among animals, from nematodes to humans. Males often live shorter than females, but there is substantial unexplained variation among species and populations. Despite extensive research, it is poorly understood how lifespan differences between the sexes are modulated by an interplay among genetic, environmental and social factors. The goal of our study was to test how sex differences in lifespan and ageing are modulated by social and environmental factors, and by intrinsic differences between males and females. To disentangle the complex basis of sex differences in lifespan and aging, we combined comparative data from sex ratios in 367 natural populations of four species of African annual killifish with experimental results on sex differences in lifespan and aging from eight laboratory populations tested in treatments that varied social and environmental conditions. In the wild, females consistently outlived males. In captivity, sex-specific mortality depended on social conditions. In social-housed experimental groups, male-biased mortality persisted in two aggressive species, but ceased in two placid species. When social and physical contacts were prevented by housing all fish individually, male-biased mortality ceased in all four species. This outcome held across benign and challenging environmental conditions. Fitting demographic survival models revealed that increased baseline mortality was primarily responsible for a shorter male lifespan in social-housing conditions. The timing and rate of aging were not different between the sexes. No marker of functional aging we recorded in our study (lipofuscin accumulation, proliferative changes in kidney and liver) differed between males and females, despite their previously confirmed association with functional aging in Nothobranchius killifish. We show that sex differences in lifespan and aging in killifish are driven by a combination of social and environmental conditions, rather than differential functional aging. They are primarily linked to sexual selection but precipitated through multiple processes (predation, social interference). This demonstrates how sex-specific mortality varies among species even within an ecologically and evolutionary discrete lineage and explains how external factors mediate this difference.

Fluctuating temperatures extend median lifespan, improve reproduction and reduce growth in turquoise killifish

In natural populations, individuals experience daily fluctuations in environmental conditions that synchronise endogenous biorhythms. Artificial alterations of environmental fluctuations can have negative consequences for life history traits, including lifespan. In laboratory studies of aging, the role of fluctuating temperature is usually overlooked and we know little of how thermal fluctuation modulates senescence in vertebrates. In this longitudinal study we followed individually-housed turquoise killifish, Nothobranchius furzeri, from two thermal regimes; ecologically relevant diel fluctuations (20 °C - 35 °C) and stable temperature (27.5 °C), and compared their survival, growth and reproduction. Fish experiencing fluctuating temperatures had a longer median lifespan but reached smaller asymptotic body size. Within-treatment variation indicated that extended lifespan in fluctuating temperatures was not causally linked to decreased growth rate or smaller body size, but occurred solely due to the effect of thermal fluctuations. Male body size was positively associated with lifespan in stable temperatures but this relationship was disrupted in fluctuating thermal regimes. Females exposed to fluctuating temperatures effectively compensated egg production for their smaller size. Thus, there was no difference in absolute fecundity between thermal regimes and body-size corrected fecundity was higher in females in fluctuating temperatures. Overall, despite a brief exposure to sub-optimal thermal conditions during fluctuations, fluctuating temperature had a positive effect on survival and reproduction. These results suggest that the expression of life history traits and their associations under stable temperatures are a poor representation of the relationships obtained from ecologically relevant thermal fluctuations.

Diel schedules of locomotor, reproductive and feeding activity in wild populations of African annual killifish

Diel patterns of different activities arise from adaptations to periodic cycling of environmental parameters and may involve trade-offs between acquiring benefits and minimizing associated costs. In this study, we provide fundamental baseline data on diel activity of natural populations of Nothobranchius fishes, model organisms in laboratory studies, including links between diurnal rhythms and ageing. Initially, we quantified the diel change in activity in wild populations of three African killifish species (Nothobranchius furzeri, Nothobranchius orthonotus and Nothobranchius pienaari) and compared average activity between sexes. In all species, males were more active than females, probably as a result of their active pursuit of females. Swimming activity peaked at midday. In N. furzeri, the only species occurring at all sites, oocytes were ovulated in the early morning, and most spawning events had occurred by the early afternoon. Gut fullness and diet richness increased before spawning activity and peaked in the morning. Daytime diet was dominated by chironomid larvae, whereas notonectid bugs were the dominant prey at night, perhaps as a result of different prey detectability over the diel cycle. Finally, no loyalty to any particular pool section was detected in N. furzeri. Collectively, these data provide the first empirical description of diel activity in three wild populations of African killifish.

Nothobranchius furzeri, an 'instant' fish from an ephemeral habitat

The turquoise killifish, Nothobranchius furzeri, is a promising vertebrate model in ageing research and an emerging model organism in genomics, regenerative medicine, developmental biology and ecotoxicology. Its lifestyle is adapted to the ephemeral nature of shallow pools on the African savannah. Its rapid and short active life commences when rains fill the pool: fish hatch, grow rapidly and mature in as few as two weeks, and then reproduce daily until the pool dries out. Its embryos then become inactive, encased in the dry sediment and protected from the harsh environment until the rains return. This invertebrate-like life cycle (short active phase and long developmental arrest) combined with a vertebrate body plan provide the ideal attributes for a laboratory animal.