Broadcast spawning by corals is an amazingly precise temporal process. Every year, on just one or two evenings, many of the individuals of each broadcast spawning species release their gametes in a time window that is usually approximately 30 min long. With only a simple nerve net and no specialized light sensing organs to sense and interpret their local environment these simple cnidarians somehow manage to tell the time of year, the time of the lunar month, and the time after sunset."

The month of spawning is set by local weather cycles. The key environmental variable correlated with the selection of the month of spawning is solar insolation cycles but exactly how solar radiation and the weather patterns they drive are sensed or responded to by corals remains unknown. The night of spawning is set by the lunar cycle. Lunar cycles can be used to accurately predict the broadcast spawning night in many locations and altering lunar irradiance cycles can change planula release cycles in brooding corals. Low levels of light such as moonlight can be perceived by corals and can result in changes in gene transcription. However, exactly how lunar light entrains broadcast spawning also remains unknown."

See: Coral spawn timing is a direct response to solar light cycles and is not an entrained circadian response (2009) Coral Reefs (2009) 28:677–680

A 2021 research project suggests that the spawning might be triggered not the light itself, but by the period of darkness between sunset and moonrise that occurs after the full moon.

See: Moonrise timing is key for synchronized spawning in coral Dipsastraea speciosa PNAS August 24, 2021 118 (34)