Suboptimal Performance Can Be Key to Survival in Changing Climates

It is frequently assumed that the optimal temperatures (T_opt) for an organism's performance, particularly for critical fitness components, have evolved to align perfectly with habitat temperatures. However, organisms in temperate zones face significant seasonal temperature fluctuations, often leading them to operate under suboptimal thermal conditions. This study investigated how well aligned the thermal sensitivities (thermal performance curves, or TPCs) of development, growth, and survival are to local habitat temperatures across seasons, using the embryos of the marine gastropod Haminoea vesicula as a model.

Our findings revealed that habitat temperatures for Haminoea vesicula embryos mostly fall far below the T_opt for development and growth. This thermal mismatch results in tangible consequences, such as developmental periods that are 20% longer and hatchling sizes that are slightly (~1%) smaller on average in the cool spring compared to summer. While development and growth operate suboptimally for much of the year, the risk to survival intensifies during acute, high-temperature events in the summer. Simulations indicated that the thermal sensitivity for survival is well aligned to habitat temperature, but T_opt for development and growth would require a significant "cold shift" (11°C and 16°C, respectively) to achieve their maximal performance.

This apparent inefficiency highlights a critical evolutionary trade-off. While cold-shifting TPCs to maximize development and growth might seem beneficial, such a change would lead to sharp reductions in survival during seasonal heat spikes. Therefore, maintaining a higher T_opt, even if it means suboptimal performance for most of the year, is a crucial adaptation that enables embryos to endure these extreme temperature events. This suggests that what appears as "suboptimal" performance is, in fact, a necessary strategy for long-term survival in variable environments, providing vital insights into species resilience in the face of climate warming. As lead author Litle notes in the paper, "Sometimes 'suboptimal' is good enough."