Exploring the mechanism of how ectotherms change size with changing temperature

Abstract

The “temperature-size rule” (TSR) describes the intraspecific, phenotypically plastic response of ectotherm body size to temperature: individuals take longer to mature at cooler temperatures but do so at a larger size. The TSR is ubiquitous, affecting >83% of organisms in which it has been studied. This suggests a fundamental physiological mechanism underpinning the TSR which requires explanation. Additionally, with increasing global temperatures it is vital that we understand how this will impact on body size. Currently, we lack a description of how size changes occur, and its ontogenetic basis. Using a simple conceptual model, it is shown that adult: progeny mass determines growth and development rates which drive the TSR. Adult size changes more than progeny size in acclimated metazoans. Conversely, due to the constraints of binary fission, these changes are equal in acclimated unicells. This suggests that how rates decouple is fundamentally different in uni- and multicellular organisms: the acclimated rates are not decoupled in unicellular organisms at different temperatures but are in multicellular organisms. This is supported with a multilevel analysis of data for over 30 copepod species across multiple life stages. Experimental data for the crustacean Artemia franciscana shows temperature-size changes to increase during ontogeny; data for multiple crustacean species supports this outcome, with the temperature-size response becoming more negative through ontogeny. Experimental examination of the TSR during thermal acclimation in the ciliate Cyclidium glaucoma supports the conceptual model: growth and development are temporarily decoupled for approximately one generation. Finally, I move from investigating proximate to ultimate mechanisms. I examine the primary hypotheses used to explain why the TSR is nearuniversal. This analysis highlights that environment type (i.e. aquatic or terrestrial) and organism size are major determinants of the size of the response, suggesting the TSR is adaptive and exists to maintain aerobic scope in ectotherms.