![]() To evaluate the capacity for phenotypic flexibility, a subset of adult fish from each rearing condition were then air-exposed for 14 days before the same traits were measured. We reared fish under constant (water) or fluctuating (water-air) environments until adulthood and assessed a suite of traits along the oxygen (O2) cascade (e.g., neuroepithelial cell density and size, cutaneous capillarity, gill morphology, ventricle size, red muscle morphometrics, terrestrial locomotor performance). We used an amphibious killifish (Kryptolebias marmoratus) to test the hypotheses that organisms reared in fluctuating environments (i) will make no developmental changes to suit any one environment because fixing traits to suit one environment could be maladaptive for another, and (ii) will be highly phenotypically flexible as adults because their early life experiences predict high environmental variability in the future. The interaction between developmental plasticity and the capacity for reversible acclimation (phenotypic flexibility) is poorly understood, particularly in organisms exposed to fluctuating environments. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na ⁺ -K ⁺ -ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus ( Kryptolebias marmoratus ) when gill function ceases during the water-to-land transition. ![]() Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. Furthermore, we only found evidence for the individual quality hypothesis in one population, suggesting that patterns of phenotypic covariance are context dependent.Ĭortisol is a major osmoregulatory hormone in fishes. ![]() Overall, our data highlight that population-level trends are insufficient evidence for the existence of phenotypic trade-offs in the absence of similar within-population patterns. Notably, pond fish were in better body condition, which may mask relationships between condition and athletic performance. In pond fish, there was only a relationship between glycolytic white muscle and aquatic burst performance. In burrow fish, athletic performance was positively correlated with muscle content and body condition, consistent with the individual quality hypothesis. Within each population, however, there was no evidence of a performance trade-off. Overall, pond fish were superior terrestrial athletes but burrow fish were better burst swimmers, suggestive of a performance trade-off at the population level. We tested this overarching "context-dependence" hypothesis by comparing the aquatic and terrestrial athletic performance of the amphibious fish Kryptolebias marmoratus captured from two contrasting habitats, a large pond and small burrows. There is evidence for both ideas in the literature, suggesting that environmental context may mediate whether variation results from trade-offs or differences in individual quality. some individuals are simply better overall performers than others). improvement in one trait comes at the expense of another trait) or alternatively may reflect the intrinsic quality of an organism (i.e. ![]() Variation may be a consequence of functional trade-offs (i.e. Understanding the mechanisms that create phenotypic variation within and among populations is a major goal of physiological ecology.
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