Body size is a fundamental trait that impacts many aspects of species biology and ecology. It is in turn influenced by a suite of environmental factors, and often decreases with warming. Although environmental conditions can also impact body shape, which is another functional trait that influences locomotion, resource acquisition, and potentially physiological processes, such responses are poorly understood and rarely quantified. 

We experimentally tested the independent and combined effects of environmental temperature, resource level, and interspecific competition on the body shape and size responses of two model protist species. We also tested the degree to which these individual-level phenotypic responses are associated with population densities and species coexistence. 

Body shape was strongly influenced by resource competition, whereas body size changes were mainly driven by environmental temperature. In both species, lower resource levels resulted in body shape elongation, suggesting that relatively more elongate individuals with potentially higher swimming speed were advantaged in the resource scarce environment. However, competition had contrasting influence on the body shape of the two species. Competition decreased the population densities of Blepharisma japonicum, which exhibited relative body shape elongation, similar to the response at low resource levels. In contrast, competition increased the population densities of Paramecium aurelia, which exhibited reduced elongation similar to body shape response at high resource levels. Hence, body shape responses could be indicative of changes in resource availability, aiding our understanding of competitive hierarchies and species interactions.

Coexistence was observed in all treatment combinations, likely because body size of both species decreased similarly under warming, potentially maintaining constant per capita competitive intensity. These findings, along with recent research on phytoplankton, diverse pelagic invertebrates, and birds highlight the importance of body shape and morphology across different taxonomic groups. Hence, we call for body size and shape to be considered in concert when investigating ecological consequences of climate warming.

Temperature column reflects the two temperature levels: high temperature (HT - 25 °C) and low temperature (LT - 22 °C)

Resource column reflects the two temperature levels: high resource (HR - 0.56 g L^-1 protozoa pellet) and low resource (LR - 0.28 g L^-1 protozoa pellet) dissolved in spring water.

Length, width, biovolume and aspect ratios are mean measurements obtained from 9-11 individuals per replicate.