“Temperature dependent growth and fission rate plasticity drive seasonal and geographic changes in body size in a clonal sea anemone”
Will H. Ryan (Feb 2018)
Fission rate plasticity allows clonal animals to track rapid changes in optimal body size through time and space
Phenotypic plasticity, or the ability of individuals to continuously refine their morphology in response to changing conditions, presents a major challenge when predicting the ecological and evolutionary effects of environmental change. The “temperature-size rule” is a commonly observed pattern in animals where individuals grow to be smaller as adults when conditions are warmer. In aquatic animals, this pattern may reflect an increasing risk of oxygen limitation as water warms, favoring smaller bodied animals with a higher surface area to volume ratio. Clonal animals may be able to change size throughout their lives by adjusting the rate at which they grow versus divide. This flexibility may allow them to track a changing optimal body size as the environment changes, but can also influence patterns of investment in asexual and sexual reproduction.
Using lab experiments and field observations, Will H. Ryan demonstrated that changes in the fission rate and average body size of the clonal sea anemone, Diadumene lineata, follow seasonal temperature changes. He grew anemones collected from across the species’ US Atlantic range (Florida, Georgia, and Massachusetts) in chambers mimicking seasonal temperature patterns found at these sites. The year-long experiment showed that annual patterns of growth, size, and clonal investment are highly dependent on local temperature patterns, leading to a gradient in the degree of asexuality expressed across the species range. Northern populations stay large and divide infrequently, whereas southern sites stay small and divide all year. Intermediate sites alternate between phases of individual growth and clonal proliferation. Variation in the response among individuals from different sites suggests that natural selection may be able to shape these patterns.
For organisms with complex life cycles, like these sea anemones, environmental variation not only alters the speed of growth and development, but can also change the timing and nature of major life history events, such as fission.
The temperature-size rule (TSR) is a commonly observed pattern where adult body size is negatively correlated with developmental temperature. In part, this may occur as a consequence of allometric scaling, where changes in the ratio of surface area to mass limit oxygen diffusion as body size increases. As oxygen demand increases with temperature, a smaller body should be favored as temperature increases. For clonal animals, small changes in growth and/or fission rate can rapidly alter the average body size of clonal descendants. Here I test the hypothesis that the clonal sea anemone Diadumene lineata is able to track an optimal body size through seasonal temperature changes using fission rate plasticity. Individuals from three regions (Florida, Georgia and Massachusetts) across the species’ latitudinal range were grown in a year-long reciprocal common garden experiment mimicking seasonal temperature changes at three sites. Average body size was found to be smaller and fission rates higher in warmer conditions, consistent with the TSR pattern. However, seasonal size and fission patterns reflect a complex interaction between region-specific thermal reaction norms and the local temperature regime. These details provide insight into both the range of conditions required for oxygen limitation to contribute to a negative correlation between body size and temperature and the role that fission rate plasticity can play in tracking a rapidly changing optimal phenotype.