Is oxygen limitation in warming waters a valid mechanism to explain decreased body sizes in aquatic ectotherms?

Abstract
Aim
The negative correlation between temperature and body size of ectothermic animals (broadly known
as the temperature-size rule or TSR) is a widely observed pattern, especially in aquatic organisms.
Studies have claimed that TSR arises due to decreased oxygen solubility and increasing metabolic
costs at warmer temperatures, whereby oxygen supply to a large body becomes increasingly difficult.
However, mixed empirical evidence has led to a controversy about the mechanisms affecting
species’ size and performance under different temperatures. We review the main competing genetic,
physiological and ecological explanations for TSR and suggest a roadmap to move the field forward.
Location
Global
Taxa
Aquatic ectotherms
Time period
1980 – Present
Results
We show that current studies cannot discriminate among alternative hypotheses and none of the
hypotheses can explain all TSR-related observations. To resolve the impasse we need experiments
and field-sampling programs that specifically compare alternative mechanisms and formally consider
energetics related to growth costs, oxygen supply and behaviour. We highlight the distinction
between evolutionary and plastic mechanisms, and suggest that the oxygen limitation debate should
separate processes operating on short, decadal and millennial timescales.
Conclusions
Despite decades of research, we remain uncertain whether TSR is an adaptive response to
temperature-related physiological (enzyme activity) or ecological changes (food, predation, other
mortality), or a response to constraints operating at a cellular level (oxygen supply and associated
costs). To make progress, ecologists, physiologists, modellers and geneticists should work together
to develop a cross-disciplinary research program that integrates theory and data, explores time scales
over which TSR operates, and assesses limits to adaptation or plasticity. We identify four questions
for such a program. Answering these questions is crucial given the widespread impacts of climate
change and reliance of management on models that are highly dependent on accurate representation
of ecological and physiological responses to temperature.

Keywords: adaptation, alternative mechanisms, climate change, growth, poikilotherm, energy
budget, geometric biology, temperature size rule