It is the entrained processes that play central roles in regulating circadian cycles. Direct responses are sometimes said to be controlled by an hour-glass mechanism while entrained processes are described as being controlled by a biological clock or a circadian mechanism. The former type of process responds directly to light, while the latter are under the control of an entrained clock. There is an important difference between daily cycles that respond directly to light, for example photoreception, and process that are entrained by light but continue to cycle in a circadian manner when kept in constant darkness. Photoreceptors that may synchronize light mediated responses have also been described in corals. Candidate orthologs were identified for many such genes, including bmal/cycle, clock, cryptochromes 1 and 2, nr1d1, period 1 and 2, timeless and many others, some of which have also been described in the sea anemone Nematostella vectensis. In an effort to shed light on circadian processes in corals, we and others have previously searched in the coral transcriptome for potential orthologs of genes involved in regulating circadian processes in other animals –. Other processes in corals may also be under the control of biological clocks, for example the carefully controlled timing of gamete/planula release during sexual reproduction (e.g. Like many daily cycles, the extension and retraction of tentacles is an entrained biological process that continues in a rhythmic manner in corals that are kept in constant darkness. At night, when no photosynthesis occurs, corals extend tentacles and actively feed on drifting prey. For example, in daylight most scleractinian corals retract their tentacles and rely on photosynthesis within endosymbiotic zooxanthallae to produce energy. Like other animals, corals exhibit major transitions between daytime and nighttime. Light plays a major role in entraining most biological clocks operating on a daily cycle, but other factors, such as nutrient intake, can also drive clocks (e.g. Biological clocks regulate many diverse aspects of animal behavior and physiology.
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