‘Aerobic scope protection’ reduces ectotherm growth under warming

Fredrik Jutfelt,Anna H Andreassen,Tommy Norin,Timothy D Clark,Ben Speers‐Roesch,Lauren E Rowsey,Eirik R Åsheim,Rachael Morgan

doi:10.1111/1365-2435.13811

Abstract

Abstract Temperature has a dramatic effect on the physiology of ectothermic animals, impacting most of their biology. When temperatures increase above optimal for an animal, their growth gradually decreases. The main mechanism behind this growth rate reduction is unknown. Here, we suggest the ‘aerobic scope protection’ hypothesis as a mechanistic explanation for the reduction in growth. After a meal, metabolic rate, and hence oxygen consumption rate, transiently increase in a process called specific dynamic action (SDA). At warmer temperatures, the SDA response usually becomes temporally compressed, leading to a higher peak oxygen consumption rate. This peak in oxygen consumption rate risks taking up much of the animal's aerobic scope (the difference between resting and maximum rates of oxygen consumption), which would leave little residual aerobic scope for other aerobic functions. We propose that water‐breathing ectothermic animals will protect their postprandial residual aerobic scope by reducing meal sizes in order to regulate the peak SDA response during times of warming, leading to reductions in growth. This hypothesis is consistent with the published literature on fishes, and we provide predictions that can be tested. A free Plain Language Summary can be found within the Supporting Information of this article.

Highlights

In this article, we compile data on the effect of temperature on appetite in aquatic ectothermic animals, with a focus on fish, and find that appetite sharply declines at temperatures above optimal
Temperature has a dramatic effect on the physiology of ectothermic animals, impacting most of their biology
We propose that water-breathing ectothermic animals will protect their postprandial residual aerobic scope by reducing meal sizes in order to regulate the peak specific dynamic action (SDA) response during times of warming, leading to reductions in growth

Summary

| INTRODUCTION

We compile data on the effect of temperature on appetite in aquatic ectothermic animals, with a focus on fish, and find that appetite sharply declines at temperatures above optimal. The term ‘functional hypoxia’ has been used to refer to oxygen limitation impacts that can arrive before hypoxia appears in the tissues, causing the animal to reduce functions such as activity and growth (Harrison et al, 2018) While these papers suggest different forms of aerobic scope conservation that resemble our hypothesis, our hypothesis differs in its emphasis on the effects of temperature on food intake and appetite, and it is readily testable through a series of predictions presented later in this paper. When animals eat a meal, their metabolic rate initially increases sharply as part of the digestive response and remains elevated for an extended period (Figure 2a), before eventually returning to baseline levels (i.e. to ṀO2min in a resting ectotherm) This increase in postprandial oxygen consumption rate is termed ‘specific dynamic action’ (SDA) and is considered a product of increased activity in the gut as well as increased biochemical nutrient processing and tissue assimilation of nutrients (Secor, 2009). Atlantic cod (Gadus morhua) Zanuzzo et al, (2019) Burbot (Lota lota) Hofmann and Fischer, (2003) Atlantic salmon (Salmo salar) Handeland et al, (2008) Koskela et al, (1997) Remen et al, (2016) Sambraus et al, (2018) Brown trout (Salmo trutta) Ojanguren et al, (2001) Bull trout (Salvelinus confluentus) Mesa et al, (2013) Brook trout (Salvelinus fontinalis) Chadwick and McCormick (2017) Greater amberjack (Seriola dumerili) Fernández-Montero et al, (2017)

Metabolic rate Aerobic scope

Findings

Peak SDA