Abstract
In watersheds of equatorial West Africa, monophyletic groups of killifish species (genus Aphyosemion) occur in discrete altitudinal ranges, low altitude species (LA, sea level to ∼350 m) or high altitude species (HA, 350 to 900 m). We investigated the hypothesis that local adaptation to altitude by the LA and HA species would be revealed as divergent effects of temperature on their physiological energetics. Two species from each group (mass ∼350 mg) were acclimated to 19, 25 and 28°C, with 19 and 28°C estimated to be outside the thermal envelope for LA or HA, respectively, in the wild. Wild-caught animals (F0 generation) were compared with animals raised in captivity at 25°C (F1 generation) to investigate the contribution of adaptation versus plasticity. Temperature significantly increased routine metabolic rate in all groups and generations. However, LA and HA species differed in the effects of temperature on their ability to process a meal. At 25°C, the specific dynamic action (SDA) response was completed within 8 h in all groups, but acclimation to temperatures beyond the thermal envelope caused profound declines in SDA performance. At 19°C, the LA required ∼14 h to complete the SDA, whereas the HA required only ∼7 h. The opposite effect was observed at 28°C. This effect was evident in both F0 and F1. Reaction norms for effects of temperature on SDA therefore revealed a trade-off, with superior performance at warmer temperatures by LA being associated with inferior performance at cooler temperatures, and vice-versa in HA. The data indicate that divergent physiological responses to temperature in the LA and HA species reflect local adaptation to the thermal regime in their habitat, and that local adaptation to one thermal environment trades off against performance in another.
Highlights
Temperature is the abiotic factor which exerts the greatest influence on the physiology of ectotherms such as teleost fishes [1]
Our study indicates that local adaptation to altitude is associated with divergent thermal physiology in tropical freshwater fishes
Our data support the hypothesis that local thermal adaptation by ectotherms involves a trade-off whereby optimising performance at cooler temperatures is associated with reduced performance at warmer temperatures, and vice versa
Summary
Temperature is the abiotic factor which exerts the greatest influence on the physiology of ectotherms such as teleost fishes [1]. It has been argued that ectotherms which have evolved within a limited thermal range will become highly specialised to function within that range, with the consequence that they function poorly at temperatures outside it [8,11,12,13]. Equatorial climes are characterised by limited seasonal temperature variations but they show thermal gradients with altitude, with mean temperature cooling by between 0.5 and 1.0uC every 100 meters [11]. The thermal physiology of tropical freshwater fish species that occupy discrete altitudinal ranges has not been studied, but it seems highly probable that they have adapted to function within the thermal range of their habitat
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