Thermal variability during development facilitates enduring physiological tolerance

Many organisms in aquatic ecosystems experience physiological stress owing to hypoxic events, warm temperatures and high thermal variability. Within-lifetime phenotypic plasticity might act to buffer, or exacerbate, the negative impacts of these stressors, but long-term experiments using ecologically realistic conditions are needed to understand and predict organismal responses in adults. In this eight month experiment, we investigate responses in multiple physiological traits in response to diel thermal variability (27 ± 5°C) or a constant optimal temperature (27°C). Following zebrafish (Danio rerio) from fertilization through to sexual maturity, we use a factorial design to isolate the effects of developmental plasticity in early ontogeny (0-29 days post-fertilization (dpf)) from effects of later life acclimation (30+ dpf to testing). We show that developmental plasticity in response to thermal variability can facilitate enduring, beneficial plasticity in multiple whole-organism physiological traits, including thermal tolerance (CTmax), and, to a greater degree, hypoxia tolerance (Pcrit), resting metabolic rates and oxygen supply capacity (α). However, only thermal tolerance is additively improved by further acclimation to thermal variability later in life. These results highlight the early developmental environment as a significant modulator of adult phenotypes and emphasize the importance of testing environmentally variable treatments across life stages. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.