2.1. Design concepts

2.1.1. Basic principles

Individuals’ bioenergetics are described according to a biphasic growth model [Andersen, 2019, Boukal et al., 2014, Quince et al., 2008] in which body mass-dependent energy fluxes are allocated between competing processes —namely maintenance, somatic growth and gonadic growth— thus accounting for physiological trade-offs that constrain both phenotypically plastic and evolutionary responses of life-history traits to selective pressures [Roff, 1922, Stearns, 1992]. Sexual maturation of individuals relies on the concept of maturation reaction norms that depicts how the process of maturation responds plastically to variation in body growth [Heino et al., 2002, Stearns and Koella, 1986]. This combination mechanistically describes the processes of somatic growth, sexual maturation and reproduction as they emerge from energy fluxes sustained by food intake resulting from size-based opportunistic predation.

On top of the biphasic growth model, individuals’ energy mobilization and maintenance energetic costs depend on dissolved oxygen concentration and temperature in a way that the resulting metabolic rate (the net energy available for new tissue production) and thus somatic and gonadic growth conform to the oxygen- and capacity-limited thermal tolerance theory (OCLTT; Pörtner [2001]) and more generally to thermal performance curves (TPC; Angilletta Jr and Angilletta [2009]). In short, oxygen delivery to mitochondria is a limiting factor of Adenosine triphosphate (ATP) synthesis and thus of the mobilization of the energy contained in chemical bonds of assimilated molecules [Clarke, 2017]. Therefore, dissolved oxygen saturation in water sets up an upper limit to energy mobilization. Moreover, beyond a temperature range within which energy mobilization increases with temperature due to chemical reaction rate acceleration, the limitation in ventilation and circulation capacity of individuals as temperature increases is such that oxygen uptake and delivery for energy mobilization saturates or even decreases at high temperatures, potentially due to temperature dependence of the rate of enzyme-catalyzed chemical reactions [Arcus et al., 2016] or enzyme denaturation [Pawar et al., 2015]. In contrast, maintenance energetic costs increase with temperature exponentially according to the Arrhenius law [Brown et al., 2004, Gillooly et al., 2002, Kooijman, 2009]. It results that when temperature increases above the individual preferred temperature, energy mobilization increases slower than maintenance costs resulting mechanistically in a dome-shaped relationship between net energy rate for tissue production (which results from energy mobilization minus maintenance costs) and temperature that conforms to OCLTT and TPC. These bioenergetic responses to ambient oxygen and temperature translate into plastic responses of the life-history processes (growth, maturation and reproduction) that emerge from the energy allocation scheme described by the biphasic growth model but also into selective pressures as individuals will incur “energetic starvation” mortality when net energy rate is negative and cannot be covered by energy reserves contained in gonads.

Finally, individuals exhibit adaptive behavior in their movements by avoiding foraging in locations of adverse conditions of dissolved oxygen and temperature [Townhill et al., 2017] that would impair their net energy rate to the point of certain energetic starvation, thus conforming to the hypothesis of metabolic constrains on marine habitat and biogeography [Deutsch et al., 2015, Deutsch et al., 2020]