Diel thermal fluctuations during embryonic development affect molecular and physiological phenotypes in later life...

Temperature varies in aquatic environments across many temporal scales, but relatively few studies have studied the effects of stable versus variable temperatures in fishes, particularly during early development. The goal of our study was to compare the lasting effects of varying extents of diel thermal fluctuation during embryonic development on whole-animal phenotypes and to examine underlying molecular mechanisms. To address this, we used Fundulus heteroclitus, a species of topminnow that inhabits intertidal saltmarshes along the Atlantic coast of North America that experience daily tidal fluctuations. We developed embryos under four different levels of diel thermal fluctuation (26 ± 0 °C, 26 ± 3 °C, 26 ± 5 °C, 26 ± 7 °C) from fertilization to hatch. Once hatched, larvae were raised at a common constant temperature of 26 °C for up to 6 months to test for lasting effects on phenotype due to developmental plasticity. We found evidence of developmental plasticity at our early sampling timepoints (1 and 3 months), as thermal tolerance (Critical thermal maximum (CT_max) and agitation temperature) and hypoxia tolerance decreased in response to fluctuating temperatures, whereas length was greater. By 6 months, length and metabolic rate did not differ between groups. Changes in physiological phenotypes were accompanied by altered mRNA levels of key regulatory genes such as insulin-like growth factors (igfs), heat-shock proteins (hsps), and hypoxia-inducible factor (hif1α). Together, these data demonstrate exposure to fluctuating temperature during early development has lasting effects for several months at both the physiological and molecular levels, which has implications for predicting the responses of fishes to climate change.