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

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 the 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 the TSR and suggest a roadmap to move the field forward.