LAUSR

The intertidal mussel Mytilus californianus is subjected to thermal and oxygen cycles due to fluctuations of the tidal cycle. Mussels that reside in high-shore microenvironments experience extended periods of aerial-exposure and greater thermal challenges than low-shore populations. In this study, we investigated the combined effect of daily extended periods of aerial-exposure and midday thermal events on the physiology of mussels by monitoring transcriptome-wide gene expression in a simulated tidal environment. Mussels were acclimated to aerial-exposure for 20 h a day combined with a simulated midday solar heating event of + 8 °C. Results revealed that 11% of the transcriptome exhibited robust patterns of rhythmic gene expression. Two broad sets of genes were induced, including one that was upregulated during the daytime thermal stress period and another following the 4-h evening high-tide. The daytime thermal stress associated gene set was enriched for genes related to cellular regulatory pathways such as those that encode transcription factors while genes expressed following evening submergence varied in functional theme. Additionally, genes related to energy metabolism were also upregulated during warming periods of the tidal cycle. Comparing these data to published data from a previous tidal simulation study revealed that acclimation to daily heat stress yielded a muted transcriptional response to heat compared to an acute response observed in mussels maintained under isothermal conditions. These results indicate that M. californianus displays phenotypic plasticity with respect to transcriptomic expression and that this trait likely promotes homeostasis in the intertidal environment.