Elucidating rhythmic gene expression in the coral Euphyllia paradivisia

We utilize the mesophotic coral Euphyllia paradivisa to examine temporal patterns of gene expression in symbiotic and aposymbiotic morphs exposed to natural light: dark cycles and constant dark conditions. Comparative transcriptomic analyses of 138 individuals sampled with intervals of four hrs in each lighting treatment revealed circadian and circatidal cycles of gene expression with a predominant diel pattern in both symbiotic and aposymbiotic corals. We discovered a significant number of transcripts consistently rhythmic under both light conditions, including genes likely involved in the cnidarian circadian clock, implying E. paradivisa comprises an endogenous circadian clock that can oscillate independently from the algae clock. Additionally, our results revealed several genes that change from a 24 to 12-hrs period depending on the symbiotic state, indicating that the presence of the photosynthetic algae is associated with changes in rhythmic gene expression. Moreover, the circadian rhythmicity in the host demonstrates that the algae's presence determines the timing of molecular and cellular processes in the coral biorhythmicity. Overall, this work demonstrates the significant impacts the Symbiodiniacea presence has on the host's lifestyle and illustrates the complexity of biological rhythms in early metazoans.