The rotation of the Earth causes predictable changes in light and temperature in the natural environment. Organisms have adapted to these cyclic variations, so most of their biochemical, physiological and behavioral events are rhythmic. Accordingly, natural selection has favored the evolution of endogenous circadian oscillators that impart a survival advantage by enabling an organism to anticipate daily environmental changes. Circadian oscillator is synchronized by the day-night cycle, allowing the organism to accommodate to daily and seasonal rhythms. A circadian system is composed of three components: Light input pathway, an endogenous circadian oscillator and an output evident rhythmis activity. Outputs are driven directly by oscillator and the oscillator is entrained by thelight input.
Cnidarians are the most primitive invetebrates to have multicellular light detecting organs, called ocelli. However, the anthozoa familiar within the cnidarians including corals, sea anemones and sea pens do not contain ocelli but still display a distinct behavior towards light quality and intensity. Recently, it was demonstrated that coral polyps show photosensitivity in the blue light region of the spectrum; hence they are capable of detecting the blue portion of lunar irradiance. Lunar illumination is an important factor for synchronizing spawning in many species of symbiotic corals. Even though, many anthozoans respond to light including phototaxis, expansion and contraction, spawning and circadian rhythmicity no specific photoreceptor cell has been identified in these animals.
We propose that photoreceptors like cryptochromes (blue-light-sensing pigments), which have an evolutionary conserved role in the entrainment of circadian rhythms in plants, insects and vertebrates can be found in corals. Finding these photoreceptors may be the key for interpreting the photobehavior responses found in those animals, in particular in the blue region of the spectrum.
