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Abstract

Anthropogenic climate change is causing rising average temperatures and increased thermal variability across aquatic environments. These impacts may be particularly concerning for early life-stages of fishes as their thermal windows are thought to be narrower than those of adults. However, relatively little is known about how these early-life stages will respond to predicted temperature changes with global warming. Therefore, I investigated the effects of different temperature regimes on early life-stages and how they respond to these environments both acutely and by utilizing developmental plasticity in two subspecies of Fundulus heteroclitus, a topminnow that inhabits intertidal saltmarshes along the Atlantic coast of North America. I generated thermal performance curves (TPC) for development in embryos of two F. heteroclitus subspecies reared at a series of constant temperatures and found evidence consistent with both local adaptation and countergradient variation between the subspecies. I also showed that F. heteroclitus reared at different temperatures had altered hypoxia tolerance and hif1α mRNA transcript abundance, but I observed no change in thermal tolerance. This finding demonstrates that developmental cross-tolerance can occur in F. heteroclitus. However, these differences did not persist at the age of 1 year, highlighting reversible plasticity. I then examined how fluctuating thermal regimes during development affected embryonic and larval phenotypes. I demonstrated development under fluctuating temperatures can alter performance in ways that cannot always be predicted based on performance generated at constant temperatures. Furthermore, I showed the fish reared under fluctuating temperatures had altered growth, thermal tolerance, and hypoxia tolerance, which were associated with long-lasting transcriptomic effects that persisted even in a common environment. However, high thermal variability during development had lasting negative consequences on phenotypes as the result of deleterious plasticity. Taken together, my research demonstrates that F. heteroclitus utilize developmental plasticity as a mechanism to cope with changing temperatures during early development. However, there are limitations to this plasticity which are highlighted in the reversible and deleterious plasticity I detected.